Secondary legislation regulations and guidelines are being used as the working values against which indoor air quality (IAQ) can be assessed.

It can be regulated via health and safety regulations, such as the Control of Substances Hazardous to Health (COSHH) Regulations (2002), to which all places of work have to adhere. Regulation 6(1) of COSHH states that an employer ‘should carry out a suitable and sufficient assessment of the risks to the health of your employees and any other person who may be affected by your work, if they are exposed to substances hazardous to health’. Regulation 10 of COSHH specifies that monitoring is required ‘when measurement is needed to ensure a workplace exposure limit (WEL) or any self-imposed working standard is not exceeded’.

There are legally binding WEL for around 500 substances, listed in HSE EH40/2005. However, WEL only relate to personal (exposure) monitoring of people at work, are calibrated for a healthy working-age adult, and can’t be readily adapted to evaluate or control prolonged, continuous or non-occupational exposure. So, for most office and education settings – and all public and residential buildings – WEL cannot be used to assess building occupant exposure to IAQ.

New buildings, however, are covered by the recently updated Statutory Instrument Building Regulations (2010). Now included in Approved Document F1 is ‘Means of Ventilation’, providing statutory guidance on ventilation requirements to maintain IAQ.

Within Approved Document F1, Appendix B: Performance-based ventilation, average indoor air pollutant guideline values are set for carbon monoxide (CO), nitrogen dioxide (NO₂), formaldehyde, total volatile organic compounds (TVOC) and ozone (O₃), largely based on World Health Organization (WHO) guidelines. However, as the document relates to building performance guidance, these values are performance criteria of the assessment of ventilation, not occupant exposure assessment criteria.

For several years, regulations, standards and guidance on IAQ for school buildings have been set out in the UK government’s Education and Skills Funding Agency Building Bulletin BB101. This includes guidelines on ventilation, such as setting a maximum CO₂ in teaching spaces, and specific guidelines on IAQ, mainly based on WHO guidelines that ‘should be used for schools’.

The WHO first published air quality guidelines in 1987, which have been updated since. They ‘serve as a global target for national, regional and city governments to work towards improving their citizens’ health by reducing air pollution’.

In 2010, the WHO published guidance on risks associated with pollutants commonly found in indoor air (benzene, CO, formaldehyde, naphthalene, NO₂, polycyclic aromatic hydrocarbons, radon, trichloroethylene and tetrachloroethylene), and updated their (general) air quality guidelines in 2021 (see Table 1).

It should be noted that WHO general air quality guideline values ‘do not differentiate between indoor and outdoor exposure’, so they are applicable indoors. Some of the latest 2021 guidelines supersede its specific 2010 IAQ guidelines (eg, for CO and NO₂).

They are relevant in all countries and in all exposure settings where COSHH WEL are not appropriate. Organisations, including CIBSE, point to these guidelines for IAQ monitoring. In recent years, other organisations have published IAQ guidance.

About the author
Oliver Puddle is technical director at DustScanAQ

The post The good, bad and illegal: rules and guidelines for IAQ appeared first on CIBSE Journal.

Retrofit has also become a companion to life sciences development, mainly because of a lack of new spaces that can handle the structural and services demands of life sciences buildings. There has also been a shift in demand for city-centre locations, driven by proximity to universities, hospitals and a skilled workforce.

Cambridge, Oxford and London, deemed as the life sciences’ ‘golden triangle’, are great examples of this. Both offices and retail have seen a decrease in demand, creating an opportunity to repurpose these spaces for science laboratories, where the demand remains strong. 

The retrofit challenge

All types of laboratories need supplementary ventilation and some form of fume extraction, and this can be a particular challenge for a retrofit in a built-up area.

The higher ventilation rates required to extract fumes from laboratories means risers need to be larger than those for offices to accommodate more intensive services, and ceiling void space needs to increase by 50cm to make room for larger duct work. Existing buildings that already have high floor-to-floor space, such as shopping centres, are more easily retrofitted as laboratories.

In the past, a lot of buildings were thought to be structurally unsuitable, as external vibrations hindered the ability for optical microscopes in labs to achieve stable images. However, recent technological advancements such as active vibration damping, which operates in a similar way to noise-cancelling headphones, have helped overcome such structural issues.

Specification of ventilation starts with a suitable selection of fume cupboards and biosafety cabinets, for the specific application and chemicals that are anticipated to be used. Ducted fume hoods are typically the most effective for removing fumes.

The placement of hoods must be carefully considered to capture contaminants effectively, by ensuring that there are no obstructions blocking airflow to the hood. Computational fluid dynamics (CFD) modelling is often needed to validate the design before implementation. 

A minimum air change rate must be achieved for safety purposes in laboratories and this is typically three times more than conventional office buildings, requiring larger HVAC facilities. 

It is important to consider where the fumes are discharged and their proximity to other air intakes and receptors.

Conventionally, air intakes must be separated from discharges by at least 10 metres, and fumes are discharged vertically at least three metres above other parts of the building. However, for city centres with an abundance of developments, more detailed assessments are often required.

When direct, ducted systems cannot be incorporated, recirculating systems with activated charcoal air filters and scrubbers can be considered, although these are expensive alternatives.

Higher ventilation rates and fume-extraction systems will have a significant impact on a building’s energy use, so it is essential that buildings services pay particular attention to energy efficiency.

As a first step, it is important to work with the scientists who will be occupying the buildings in the design stages, to optimise the parameters and the airflow design, and prevent overdesign. 

Where possible, spaces should be lab-enabled, rather than fully fitted-out. This will provide end users with an adaptable blank template they can fit-out however they want. This is more attractive than receiving a fully fitted-out space that may not meet specific requirements and could put occupiers off at due diligence stage, or lead to expensive refit financial and carbon costs.

Modulating the airflow to match actual demand reduces energy consumption during low-activity periods, while still providing adequate ventilation when needed.

Variable air volume flowrate systems and demand-control ventilation can adjust the ventilation rates based on real-time occupancy and contaminant levels. Implementing scheduling controls can also optimise ventilation operation based on occupancy patterns and laboratory usage schedules.

Rob van Zyl

Natural ventilation can be used in certain situations. There are other considerations such as prioritising recirculation over full ventilation by using activated charcoal filters or liquid scrubbing to wash the air as it passes through. Heat recovery systems must also be implemented wherever practical, to capture and reuse heat or coolness from exhaust air to precondition incoming fresh air.

Typically, the requirement for safe removal of fumes is to discharge fumes at least three metres above the highest point of buildings, and this means having tall and unsightly stacks.

In the UK, planning regulations impose restrictions on the height of stacks and exhaust vents as part of the overall planning permission process. This is primarily to address concerns of air pollution, visual impact, and potential adverse effects on the environment and neighbouring properties.

Planning authorities need evidence to show that vertical stacks are tall enough to adequately control the dispersion of pollutants and they will want to see how the visual impact of stacks on the surrounding landscape have been considered.

Stack heights need to be specified to minimise impact on air quality. They will be based on factors such as the type of emissions, local air quality standards, and the proximity of sensitive receptors, such as residential areas, schools, or hospitals.

The speed at which air is discharged from a ventilation system – the efflux velocity – can determine stack height. By increasing the velocity of vertical discharge, fumes can be pushed higher and the stack height reduced. CFD modelling can be used to predict whether or not the concentration of released fumes will exceed the required parameters of the nearby receptors. It should be borne in mind that higher efflux velocities require more energy. 

The importance of heat recovery

As there is a requirement for labs to have a lot of air circulating in the building, it is important to recover as much of its heat as possible. However, effectively capturing and using waste heat can be difficult in practice. In some cases, the temperature difference may be insufficient to extract heat efficiently, limiting the feasibility and effectiveness of heat recovery.

Integrating heat recovery systems can be difficult, as it will introduce pressure drops. It can also create foul air that can be corrosive, which means the ductwork must be made with corrosion-resistant materials that will not be damaged by this. 

Science buildings are one of the sectors being considered by the Net Zero Carbon Buildings Standard (NZCBS). Simon Wyatt, sustainability partner at Cundall, is leading the NZCBS¹ sector group and is collaborating with market leaders to create assessment frameworks for buildings in the sector. It is still early days for the sector, and there is a lot more data that is needed before benchmarking of life sciences buildings is taken seriously.

About the author:
Rob Van Zyl is a management board partner at Cundall

References:

1. UK Net Zero Carbon Buildings Standard www.nzcbuildings.co.uk

The post Science in the city: the challenge of retrofitting labs appeared first on CIBSE Journal.

Thorough research and testing of the Diffusion Highline 235 modular fan coil range has resulted in a product that considers whole life costing through the use of TM65 and local sourcing to reduce transport miles, the judges added. Energy, acoustics, performance, and the flexibility modularisation brings to deployment and onsite repairs have also been considered.

Working closely with customers, Diffusion researched every UK fan coil on the market to assess how it could improve the design to meet the changing needs of the industry.

As a result, its Highline range has been increased to eight, the modularity of which now allows almost 300,000 configurations. This means customers can select a unit that exactly matches their performance requirements rather than having to over-specify, ensuring the lowest energy consumption.

At design stage, the emphasis was on using fewer materials, reducing the volume of materials transported, minimising carbon footprint, and lowering running costs per unit size.

Leveraging high-efficiency EC/DC motor and fan assemblies, the units achieve a specific fan power as low as 0.14W.L^-1.s^-1, significantly reducing energy consumption and operational costs. Forward-curved centrifugal fans provide the most efficient airflow and acoustic performance in all models. Further acoustic benefits are achieved through ‘0’ fire-rated foam insulation.

The unit’s heat exchangers are manufactured from solid drawn copper tubes, mechanically expanded into pre-formed collars in rippled plate aluminium fins. Multi-circuit design ensures maximum thermal performance. For optimum heat transfer into the airflow, electrical elements are 8mm-diameter, fully sheathed, stainless-steel rods, with spiral-wound fins.

Highline 235 is supplied with Diffusion’s Lifetime Eco wire-mesh filter, which can be simply vacuum cleaned in situ. It lasts the lifespan of the unit. 

ISO-grade media filters are also available. When filters need to be cleaned or replaced, they can be easily removed from either the side of the unit or from beneath it.

In spaces where noise levels significantly influence occupant satisfaction, the Highline 235 range can achieve noise levels ranging from NR25 to NR40. Discharge plenums are available in rectangular or circular spigots, and inlet and discharge attenuators are available in lengths to meet requirements.

British designed and manufactured with a short supply chain, 70% of Diffusion’s fan coil units (FCUs) are transported less than 24 miles to end users in London, keeping carbon emissions to a minimum. The modular, configurable design means building owners can reuse the FCUs by repositioning them.

The CIBSE TM65 data-collection methodology was used to collect accurate and detailed embodied carbon information about the system. Working from a component level, this methodology ensures data is comprehensive and up to date.

Diffusion uses its in-house test facility to offer volumetric, acoustic and thermal performance testing, and customers can watch their chosen products being tested and certified. They can also input their building’s design parameters into Diffusion’s software to select the ideal FCU for their required temperature and flowrate. This includes data on correct heat exchanger selection.

The judges said the range of innovations among award entries this year showed that innovation doesn’t need to be ‘epic’ to be influential and beneficial. They also illustrated the importance of product testing.  

• For more on the winners at the CIBSE Building Performance Awards, visit www.cibse.org/bpa

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Activities such as cooking and cleaning can produce high pollutant concentrations, with potential impacts on human health, including respiratory and cardiovascular diseases^2,3. However, environmental regulation still focuses on outdoor air quality, with the exception of specific occupational exposures⁴.

A joint project between the universities of York, Chester and Nottingham has studied the production of different chemicals during cooking and cleaning. The Impeccable (Impacts of cooking and cleaning on indoor air quality: towards healthy buildings for the future) project used state-of-the-art equipment, coupled with computational modelling¹¹ to understand the complex underlying chemistry that occurs during and after domestic cooking and cleaning activities.

The study recommends the most effective ventilation strategies to reduce the air pollution created by cooking and cleaning.

The climate emergency has forced us to think more carefully about energy efficiency. Buildings are becoming increasingly airtight, but we have the potential to increase our exposure to indoor air pollution if sources such as cooking and cleaning dominate our personal exposure to air pollution.

Measuring VOCs while cooking in the home lab

We need to understand the sources and reactions of pollutants indoors, particularly as an increasing weight of evidence shows that secondary pollutants are more harmful to health than primary emissions⁹. For instance, the carcinogen formaldehyde is a reaction product of limonene oxidation, the latter species being a key component of many cleaning formulations¹⁰.

Cooking generates high concentrations of particulate matter (PM), nitrogen oxides (NOX) and carbon monoxide (CO). Emission rates depend on the cooking method, and oil and food types.

Emissions of PM are higher for frying than boiling and steaming, and higher when you fry meat compared with vegetables². During cooking, PM concentrations can reach several hundred mg/m³, particularly when frying², exceeding acute health standards over several hours⁵. Cooking can also generate ultra-fine particles (< 100 nm in diameter), which are associated with adverse effects on the respiratory and cardiovascular systems⁶.

Cleaning is another regular activity indoors, with bleach cleaning having a large impact on gas-phase and surface chemistry⁷. The prevalence of asthma among domestic cleaning staff suggests that cleaning activities may cause adverse health effects⁸.

The Impeccable project

The experiments undertaken range from benchtop ones in a lab to determine specific emission rates from individual products or processes, through to more realistic settings, where full meals are cooked in a more home-like environment.

Measurements are taken using on- and off-line diagnostics, to determine types and quantities of various indoor air pollutants. Selected-ion flow-tube mass spectrometry (SIFT-MS) was used to measure real-time concentrations of more than 40 volatile organic compounds (VOCs) during the cooking and cleaning experiments.

This technique uses a plasma to generate ions, which then ionise VOCs present in the sample gas¹². These charged species are then separated and quantified based on their mass-to-charge ratios. An example of the real-time data obtained during a cooking experiment is shown in Figure 1.

The modelling of indoor air chemistry has been carried out using INCHEM-Py, an open-source box model that uses a detailed chemical mechanism, combined with parameterisation for indoor-outdoor exchange, gas-particle partitioning, surface deposition, and internal photolysis¹³.

The detailed model allows us to simulate the time-evolved concentrations not only of the species emitted from indoor activities, but also of those formed after chemical reactions, such as formaldehyde, peroxyacylnitrate species (PANs) and PM.

The simulated concentrations of some secondaries formed after the typical stir-fry cooking experiment are shown in Figure 2. These products are often more harmful to human health than the original emissions, so understanding how they are produced can help inform scientifically rigorous mitigation measures (such as extraction of cooking fumes or increased ventilation).

Relevance for building design and operation

Kitchen ventilation is regulated by Approved Document F of the Building Regulations in England. In new houses, there is a requirement for a cooker hood that extracts to the outside or a wall-mounted fan.

The advantage of the cooker hood is that it captures cooking contaminants before they mix in the kitchen, whereas the wall fan allows the contaminants to mix before they are extracted. The required flowrate through the hood is half that of the fan, which implies that it should capture 50% of all emitted contaminants. This is known as a capture efficiency. There is no requirement to test hoods to make sure they meet this requirement, although there is now an ASTM standard¹⁴ that tests some types of hood in a laboratory environment. Future versions of Approved Document F should require cooker hoods to conform to the ASTM standard or allocate a punitive capture efficiency to those that are untested.

Ventilating for 10 minutes after cooking has finished has a significant effect on exposure and should be recommended by public health campaigns^15,16. The higher airflow rate through the wall fan means that it reduces the concentration of contaminants in the air faster than the hood. So having multiple flowrate settings on a hood is advantageous. The best place to locate a hood is against a wall, between cabinets, directly over the gas burners/hob, and as close to the burners as the manufacturer allows. Noise is often a barrier to their use, so using short, wide, straight, rigid, noise-absorbing ducts that reduce air velocity to 2-3 m/s is best practice.

When cooking, using the back burners increases the pollutant capture efficiency¹⁶. Consider using the hood, at least on low speed, for general kitchen ventilation when using other appliances, such as toasters.

Using induction rather than gas burners, and electric rather than gas ovens, reduces the emission of nitrogen dioxide (NO₂), but the chronic harm from exposure to NO₂ is significantly less than that from fine PM. A good cooker hood will capture NO₂ from a burner, so using the hood is more important than changing from a gas to an induction stove.

In very airtight homes (<1 m³/h/m²) make-up air into the kitchen or home is needed and this can be provided using mechanical ventilation with heat recovery (MVHR). Reclaiming thermal energy from the exhaust stream can be problematic in MVHR systems because grease can clog the heat exchanger. This problem has yet to be solved and may require more frequent cleaning of filters.

Some homes have recirculating fans, which pass air through a particle filter and resupply it into the kitchen, rather than exhaust it outside. In this instance, it is imperative to use induction hobs. Electric hobs have their own complications because they can emit PM when they get dirty. For homes that have no fans in their kitchens, windows and doors should be opened when cooking and cleaning to provide ventilation in the short term. In the medium term, a mechanical system should be installed and trickle ventilators kept open at all times.

We would like to thank EPSRC for funding for the Impeccable project (EP/T014474/1).

About the authors
Professor Nicola Carslaw is professor of indoor air chemistry in the Department of Environment and Geography at the University of York

Dr Helen Davies is research associate in chemistry in the Department of Environment and Geography at the University of York

Dr Benjamin Jones associate professor in the Department of Architecture and Built Environment at the University of Nottingham.

References

1. Nazaroff, W. W.; Goldstein, A. H., (2015), Indoor Air, 25, (4), 357-61
2. Abdullahi, K.L. et al. (2013), Atmos. Environ. 71, 260-294
3. Wolkoff, P. (2013) Int. J. Hygiene Env. Health, 216, 371-394
4. Weschler, C.J., and N. Carslaw, Environ. Sci. Technol., 2018, 52, 2419-2428
5. Logue, J.M., et al. (2011), Indoor Air, 21, 92-109
6. Rim, D. et al. (2013), Environ. Sci. Technol., 47, 1922-1929
7. Wong, J. P. S et al. (2017) Indoor Air, 27, 1082-1090
8. Medina-Ramón, M. et al. (2006) Occupational and Environmental Medicine, 62, 598-606
9. Buchanan, I.S.H. et al. (2008), Indoor Air 18, 144-155
10. Wang, C., M., et al. Environ. Sci. Proc. Impacts, 2017, DOI: 10.1039/C6EM00569A
11. Impeccable website: impeccable.york.ac.uk
12. Smith, D., Španel, P., (2005), Mass Spectrometry Reviews, 24, 661-700
13. Shaw et al. (2021). Journal of Open Source Software, 6(63), 3224, doi.org/10.21105/joss.03224
14. ASTM, Measuring Capture Efficiency of Domestic Range Hoods. Report. 2018.
15. Holgate S et al. The inside story: Health effects of indoor air quality on children and young people. Royal College of Paediatrics and Child Health; 2020.
16. O’Leary C, et al. Setting the standard: The acceptability of kitchen ventilation for the English housing stock. Building and Environment. 2019:106417.

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Led by Loughborough University, the ‘Airbods’ project is a collaboration of academic institutions and engineering research firm Wirth Research, and made a vital contribution to the government’s Events Research Programme (ERP), which evaluated the transmission risk at big events.

Standing for ‘Airborne Infection Reduction through Building Operation and Design for SARS-CoV-2’, the Airbods project’s high-resolution CO₂ monitoring and modelling informed the government’s decision in July 2021 to resume the normal operation of large events in culture, music and sports industries while improving venue safety.

Praising the outcome of the Airbods project, the awards judges said: ‘The research has resulted in usable guidance and tools to help designers identify and mitigate infection control risk. It has enabled policy-makers and stakeholders to evaluate the changes in risk that modifications to ventilation rates, occupancy density, and exposure times have at a population scale, so that interventions can be better focused.’

The Airbods project began in March 2021, when the government’s Engineering and Physical Sciences Research Council (EPSRC) issued a call for rapid responses for funding. Most of the participants in the Airbods project team had worked with each other before on ventilation and indoor air quality projects and publications, while a small number of the group had been meeting every week during the pandemic to discuss and share ideas about how they might respond to what had become a global pandemic with far-reaching economic and social impacts.

The team included specialists from academic institutions including University College London (UCL), University of Nottingham, University of Sheffield, University of Cambridge and London South Bank University.

The Airbods project informed the government’s decision to resume the normal operation of large events in culture, music and sport

The Airbods team was asked to contribute to the ERP by carrying out the environmental study of a wide range of venues and events between April and July 2021. This part of the programme, led by the UCL team helped to build the evidence based on air quality and improved understanding of potential risks of airborne transmission, and their mitigations.

Malcolm Cook, Professor of Building Performance Analysis at Loughborough University, led the Airbods programme. ‘It has been a privilege to lead such a committed and experienced team,’ says Cook. ‘The collaboration across the project partners was highly focused from the start: it was exciting but very intense. The whole team worked exceptionally hard and the experience couldn’t have been better in terms of teamwork and commitment at a time when many people had personal challenges of their own. Our team of eager, early career researchers certainly got more than they bargained for when they willingly accepted the challenge to prepare hundreds of sensors and loggers for installation into some of the largest, most complex entertainment and sports venues in the UK!’

Working rapidly to develop evidence for the ERP, the University of Nottingham team developed analytical models to estimate the risk of SARS-CoV-2 long-range aerosol transmission indoors at both individual and population scales. A ‘relative exposure index’ estimated the relative inhaled doses in a comparator and reference scenario in the presence of an infected person.

The index was used as evidence by the UK Scientific Advisory Group for Emergencies (SAGE) Energy Modelling Group, and forms the basis of the CIBSE COVID-19 Relative Exposure Calculator released with CIBSE’s COVID-19: Air cleaning technologies documents. This tool enables building operators and consultants to assess the effectiveness of ventilation and air-cleaning interventions at reducing risks in their own particular scenarios.

An extension of Airbods work considered the population-scale effects of ventilation in indoor scenarios, enabling policy-makers to evaluate the changes in risk that modifications to ventilation rates, occupancy density, and exposure times have at a population scale. This allows potential interventions to be better focused.

Cook, who has been working in ventilation modelling and measurement for around 30 years, says that, prior to Covid-19, it had been difficult to engage people’s interest in the importance of indoor air quality. ‘Its invisibility means it is often overlooked,’ he said.

‘However, when the pandemic hit, everybody wanted to talk to us about how to improve their pubs, restaurants and other entertainment venues. Our computer simulation gave us the means of visualising what was going on and, for instance, creating graphics, animations and computer models that could show people the impact of people breathing out moisture particles with virus on them in different scenarios.’

The Airbods project’s initial phase was fast-moving, Cook says, which was itself a challenge. ‘Within two weeks, the team had completed risk assessments and secured ethical clearance as well as purchased and calibrated hundreds of sensors, and rolled them out into the field, while ensuring the computer interfaces were working as they should.’

Between April and July 2021 pilot events were monitored in indoor and outdoor settings, with variations of seated, standing, structured and unstructured audience styles, and a range of participant numbers.

These included the World Snooker Championships at the Crucible in Sheffield,  The FA cup and the EURO2020 football events at Wembley Stadium, London, and the BRIT Awards at the O2 in London.

The team used wireless technology to encrypt, transmit, and cloud-store the data, measured by sensors sampling environmental variables at two-minute intervals. This enabled real-time data analysis, while the team used the live information to respond rapidly to any data disruption and to correct errors and malfunctions. The team also comprised specialist microbiologists who conducted simultaneous microbiological sampling to provide information on the types of areas and surfaces in which it is most likely to encounter bacterial contamination.

Every week, the Airbods team would meet government bodies, the DCMS and BEIS to share results and discuss upcoming tests, while feeding their findings back into the next round of tests. ‘It was very fast moving, compared to typical academic research, but very exciting. We produced outputs in a very short period of time, reporting on each event as it happened. The final government report brought it all together, with our work undertaken alongside other organisations monitoring other aspects of the events, such as crowd dynamics and so on.’ Around 20 people from the Airbods team were involved in the research, with around 40 other participants.

Measuring CO₂ as a surrogate for the virus revealed that many buildings aren’t ventilated well enough, Cook says. ‘One of the key findings was that if buildings were ventilated as they should be, then the risk of transmission was relatively low. The problem we found was that many buildings weren’t ventilated as intended and may not have taken into account occupancy patterns throughout the building.

CIBSE guidance is for airflow of around 12 L·s^–1 per person – if that level of ventilation was present, then we may not have had the level of transmission that we suffered. The problems were because we had poorly ventilated, poorly operated, and poorly maintained spaces.’

Cook explains that building owners and operators didn’t always know how the ventilation system was intended to operate most effectively and, related to that, some systems were poorly maintained and weren’t delivering what the design intended. He says the team was surprised to find that transmission potential was found even at outdoor events. ‘At Wembley Stadium, for instance, we were finding high CO₂ levels on the terraces as well as around the bar areas and toilet queues, indicating a risk.’

A series of pilot events were monitored, including Royal Ascot and major football events at Wembley Stadium, left

The main recommendations for reopening were, therefore, to limit capacity, maintain testing and physical spacing for the bar area indoors, and to reduce all ventilation recirculation of air to zero, using only fresh air ventilation. Cook acknowledges that post-pandemic, ‘Covid-fatigue’ has started to see general interest in the subject starting to wane, particularly with government moving on to other issues. However, he says: ‘I don’t think it will ever disappear from the minds of our engineering fraternity. Architects and building designers will remain interested in how they can make their buildings healthier for a long time.

‘Healthy buildings and wellbeing are still very much on the agenda, and it’s up to organisations such as CIBSE and its members to keep it there.’

The Airbods team is still active, and, as well as working on a number of papers on the research project for publication in academic journals and contributing to new guidance, it will be running events in conjunction with CIBSE.

The team is preparing a bid for more funding, broadening its scope to cover topics including wellbeing, intelligent buildings and energy consumption – looking at how safe environments can be provided in an energy efficient way. ‘The project will enable us to undertake more experiments and computer modelling to gain a deeper understanding of how air flows and interacts with people in buildings.’

Airbods’ contribution during Covid-19 is not only of academic interest – the intention is to help inform the future. ‘All the modelling and monitoring we’ve done have been geared to understand how we can do better in the next pandemic,’ Cook says.

‘Also, crucially, it’s about making buildings healthier – how can we better design, use and operate buildings to minimise absences due to viral infections.’

More findings and guidance on infection resilience: https://airbods.org.uk 

The post How the Airbods study hastened the return of large-scale events appeared first on CIBSE Journal.

The guide gives a brief introduction to removal mechanisms for SARS-CoV-2, a summary of the existing guidance related to the different technologies employed for air cleaning, and several tools and worked examples to help specifiers determine the potential impact of an air cleaner introduced into a space.

Chief among these is the Relative Exposure Index Calculator. This article gives a brief introduction to the topics covered in the guide.

Government statutory guidance on Covid-19 safety should be followed at all times. Air cleaning devices should not be used as a substitute for adequate ventilation.

Devices that clean the air have existed for many years. They range from the well known, such as mechanical filtration, to more novel and less studied methods, which employ a variety of catalysts, ionisers, electrostatic or other techniques to physically or chemically alter particles passing through them. 

Historically, many of these devices were marketed for the removal of chemical pollutants, such as volatile organic compounds (VOCs) or particulates, with less of a focus on pathogens. With the emergence of Covid-19, attention has turned to how these devices can be applied to reduce infection risk, particularly from airborne particles carrying SARS-CoV-2. 

As SARS-CoV-2 is a relatively new virus, there is not a large body of research on the specific effectiveness of any technology for application against it. However, organisations including the World Health Organization (WHO) and the Scientific Advisory Group for Emergencies (SAGE) have reviewed the literature that does exist and made recommendations based on the best available evidence. The new guide summarises these recommendations. 

There is also a lack of guidance on the specification and selection of air cleaning devices, especially in commercial settings such as offices. The suitability of air cleaning devices is determined by several factors, including the existing provision of outside air, the level of occupancy, the location of the device, its removal efficacy, and how much air passes through the device over time. The guide highlights the calculations a specifier should undertake to assess whether an air cleaner is suitable for a space.

On top of these physical factors, the internal chemical environment is extremely complex. Many of the novel technologies rely on chemical reactions as a cleaning mechanism, with some capable of generating pollutants that are harmful in themselves, including ozone and other VOCs. 

Having a full understanding of the risks, however small, and the long-term maintenance requirements of any technology placed in a building is key to ensuring long-term occupant safety. Flowcharts containing questions suppliers should be able to answer are included, to help non-specialists navigate this complexity. 

It is worth noting that the guide does not address the use of ultra-violet germicidal irradiation (UVGI). The authors acknowledge that UVGI can be extremely effective against pathogens and is well proven, particularly in healthcare settings. The safe application of UV technologies, however, is an equally complex and specialist science, and it was decided that it would be best served by a guide of its own. References are provided to existing guidance on UVGI.

The guide was created following a thorough review of the existing science and guidance on air cleaners, drawing on expertise from the world of air quality, chemistry, biology, ventilation, and mechanical engineering. While it doesn’t examine the detail of specific technologies, the authors hope it is referenced thoroughly enough to serve as an entry point to allow those seeking deeper knowledge to find it.

Safety first

The Covid-19 pandemic created an urgent need for safer indoor spaces. Despite this, the drive towards healthy air must be undertaken in a way that not only deals effectively with airborne viruses, but also does not lead to unintended health impacts. 

The market for air cleaning devices includes an extremely wide range of products, technologies and prices, with consumer devices available from less than £100. As an industry, it is lightly regulated, particularly in terms of the chemistry of the devices, where thorough testing is expensive. As a result of this, it is difficult for the non-specialist to determine what health impacts any device could have. 

In general, the recommendation is to err on the side of caution. SAGE Environmental Engineering Group and others, including ASHRAE and the Environmental Protection Agency, caution against using devices that produce ozone, ions or other chemicals without independent evidence for their safety and efficacy, as the by-products created by these technologies may act as respiratory irritants. 

The guide includes a flowchart of questions for the specifier to ask potential suppliers. While trying not to rigidly favour any particular technology, more established technologies such as mechanical filtration, which are relatively inert from a chemical perspective, are simpler to specify.

From a ventilation perspective, the guide describes the methods that specifiers can use to translate a manufacturer’s claims into a robust assessment of performance:

Equivalent ventilation rate (eqACH)

In simple terms, this metric is used to measure the amount of air treated by a device in terms of the equivalent air changes per hour of clean air. Ventilation should always be the first choice, but in spaces where this is not possible, this metric can be used to assess the ability of a device to clean the air.

The clean air delivery rate (CADR)

The CADR is a commonly used metric that can be useful for comparing devices and for comparing the impact of dilution through ventilation. In the absence of this test-derived data, the guide provides a method to estimate the CADR from a product data sheet.

The relative exposure index (REI)

Developed by Benjamin Jones et al,² the REI is used to highlight types of indoor space, respiratory activity, ventilation provision and other factors that increase likelihood of far-field exposure to SARS-CoV-2. 

The guide is accompanied by a simple yet powerful spreadsheet tool, which enables specifiers to assess the impact of different interventions on a space, including ventilation, occupancy rates and the introduction of an air cleaning device. An example is shown in Figures 3a and 4a.

An air cleaner is for life, not just for Covid

Responsibility for maintenance and operation of any devices should be identified at the outset. The likely users of the devices should also be educated on the correct operation of these. Studies have shown that the effectiveness of devices in practice is linked to issues including thermal comfort and noise, so ensuring building occupiers know what they are for – and how they work – is key.

Devices based on mechanical filtration should follow standard industry practice on filter replacement and safe disposal. The long-term performance of novel air cleaning devices is not well studied. Again, it is recommended that specifiers err on the side of caution and ask for clear, concise information on how to maintain any device, including replacement intervals of any operating parts.

Healthy people and a healthy planet

The guide has largely taken a relatively short-term look at the application of air cleaners. This was on the basis that the current pandemic will, hopefully, be brought under control in a reasonable period and that their use will be temporary. 

While it is not covered explicitly, the long-term energy implications of any air cleaning device should still be at the forefront of everyone’s minds. All air cleaning devices either use energy directly or increase the consumption of existing systems – for example, increasing the grade of filters in a ventilation system. 

As such, the specification of air cleaners, especially those intended to be permanent, must look at the potential longer-term energy consumption of the devices. This should ideally form part of a wider assessment of methods to reduce infection risk and improve air quality, including increasing ventilation where possible.

Any measures should be combined with approaches that mitigate energy consumption, such as demand control. 

About the author
Edwin Wealend is the head of research and innovation at Cundall and co-author of the guide alongside Chris Iddon and Dzhordzhio Nadzhiev

References:

• Jones, B et al, 2021. Indoor particle behaviour diagram – accessed 5 May 2021.
• Jones, B et al, 2021. Modelling uncertainty in the relative risk of exposure to the SARS-CoV-2 virus by airborne aerosol transmission in well-mixed indoor air. Building and Environment, Volume 191

Covid-19: Air cleaning technologies is available to download now from the ‘Emerging from lockdown’ section of the CIBSE website 

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When the WHO finally admitted that airborne transfer was a possibility (almost nine months after 239 scientists wrote an open letter urging it to accept the airborne route)³ it continued to insist that aerosol transmission was rare, and fomites were key to the spread of infection.⁴ 

Professor Cath Noakes changed her Twitter handle from #hands #face #space to #ventilate to encourage people to start taking ventilation seriously. Still, unanswered questions remained: is it about air changes per hour? Does how we ventilate matter at all?

We see air changes per hour (ACH) requirements – 10L.s^-1 per person, or 15L.s^-1, or as much as 210L.s^-1 per infected person, for example – thrown around with authority, yet we don’t see much of a discussion about how breath spreads indoors and whether some ventilation methodologies manage this better than others. Interestingly, this debate took place years ago following SARS-CoV-1, with intriguing outcomes. 

The fact many super-spreader events happened in settings with good or adequate ventilation should have raised alarm bells immediately. The Skagit Valley Chorale practice that resulted in 53 infections (out of 61 attending) and three deaths, had a modern, forced-air heating and ventilation system installed that was running at 0.7 ACH during the event, designed for occupancy of up to 180.⁵ 

The call centre in Seoul, where 94 out of 216 employees were infected, had a functioning, positive pressure HVAC system in place, delivering a recommended airflow rate.⁶ This inconsistency should have been spotted earlier, but this was happening while the WHO was peddling its unhelpful fomite theory. 

How do we ventilate?

There are two main ventilation methodologies (when it comes to airflow dynamics): mixing ventilation and displacement ventilation. Each has had dominance at some point in the past. Despite displacement ventilation previously having a dominant position, it has lost some popularity. This may be because developers have worked out they can save 2% on the build cost if they pack building services into the ceiling void. 

You might be old enough to remember sockets, as well as air vents punctuating the carpet under your desk – that was when displacement ventilation ruled. 

The objective of mixing ventilation (always mechanical) is to ensure that everyone gets the same quality air, wherever they are in the room. It’s not all fresh though, it is mixed with the room air so everyone in the room breathes elements of other people’s exhaled air. 

Displacement ventilation, however, (frequently natural) adheres to the rules of physics, using buoyancy (warmer air rising) to facilitate the removal of stale air. Displacement ventilation needs considered design and architect and engineer to work together, which can be hard. 

Mixing ventilation is much easier to implement and manage, needs ducts and fans; and is less demanding on the actual space, opening sizes and the layout. 

Most importantly, the way we ventilate seems to matter a great deal when it comes to pathogen spread indoors – not least when considering the impact of coronavirus. 

History

None of this, especially with the infection spread in mind, is new. In 2010 Hua Qian et al published a paper titled Natural ventilation for reducing airborne infection in hospitals.⁷ The researchers conducted real-life tests of various methods used in a hospital ward, complete with thermal manikins and tracer gas (sulphur hexafluoride) to measure air exchange rate. 

The authors observed that downward ventilation systems could not produce a unidirectional airflow pattern, since thermal plumes of manikins induced mixing and disturbed pollutant removal, while a higher location for exhausts resulted in more effective pollutant removal from the ward. They concluded that natural buoyancy-driven displacement ventilation was much more effective at removing pollutants than a top-down mixing one.

In 2011, two further papers were published, collating performance data for mechanical, natural and hybrid ventilation systems and attempting to quantify the difference between displacement and mixing ventilation with regards to effectiveness at removing pathogens. Amir Aliabadi et al concluded that a vertical, upward-type displacement ventilation that introduces fresh, cool air near the bottom of the room is far superior to top-down mixing ventilation, as the buoyant force takes the warm and polluted air (possibly containing airborne pathogens) close to the ceiling and subsequently the exhaust for removal.⁸ 

Yonggao Yin et al managed to conduct comparative tests (again, in a hospital ward) and arrive at a numerical evaluation: 4ACHD > 6ACHM (displacement ventilation with 4ACH removed tracer gas and fine aerosols much more effectively than the mixing type ventilation with 6ACH).⁹ If any exhaust was located at low level, (ventilation flow against buoyancy) the pollutant concentration at breathing zone would be even worse than when using a mixing type of ventilation. The paper concluded that for the best result for pathogen removal, all exhausts must be located at high levels, preferably closer to the pollutant source with fresh air delivered low. 

Back to the present

Several studies published in 2020 and 2021 arrived at similar conclusions. One focused specifically on the mixing ventilation in hospitals and established that even at 12ACH (equivalent to 120L.s^-1 per person) the top-down mixing ventilation failed to remove virus pathogens from two-person wards.¹⁰ 

A team at the University of Cambridge found that mixing ventilation systems disperse airborne contaminants evenly throughout the space. These contaminants may include droplets and aerosols, potentially containing viruses. 

Figure 3: Illustration of ventilation flows with the various flow elements such as the body plume, exhaled breath, inlet flows, stratification and arrows indicating entrainment and mixing

The conclusion was that displacement ventilation, which encourages vertical stratification and is designed to remove the polluted warm air near the ceiling, is the most effective at reducing the exposure risk.¹¹ 

A University of Oregon study concluded that recirculating or mixing airflow has the potential for high spread of coronavirus-infected droplets within densely occupied spaces, even with just one person exhaling the virus droplets. Apart from the recirculation, transmission appears to be facilitated by the type and velocity of turbulent airflow designed to reach deep into the occupied space.¹² 

Conclusion

We need to take a closer look at how we ventilate buildings, especially with high occupancy. Thinking just in terms of ACH is way too simplistic. We spent the past few decades sealing office windows and fitting positive pressure, mixing ventilation into ceiling voids – I believe we were going in the wrong direction. 

The question is how can we adapt existing buildings to a Covid-conscious future? Not every building can be naturally ventilated – although, surprisingly, many can – and not every HVAC or every space can be easily converted to displacement ventilation (supplying air slowly at low level and extracting at high level). 

Those hard-to-adapt spaces may need to transition to lower occupancy levels or to more individual setups (less open plan) accommodating more flexible working arrangements. This means that working from home might not always lead to a reduction in office space requirement – and lower estate costs. 

About the author
Tom Lipinski is founder and technical director at Ventive

References:

1. Exaggerated risk of transmission of COVID-19 by fomites, The Lancet 
2. How the world missed Covid-19’s silent spread, The New York Times
   
3. 239 experts with one big claim: the coronavirus is airborne, The New York Times
4. Roadmap to improve and ensure good indoor ventilation in the context of COVID-19, WHO, 
5. SL Miller, WW Nazaroff, JL Jimenez, A Boerstra, G Buonanno, S J Dancer, J Kurnitski, LC Marr, L Morawska, C Noakes, 2020. Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading event, International Journal of Indoor Environment and Health, 314-323.
6. Birnir, Björn, 2021. Ventilation and the SARS-CoV-2 Coronavirus Analysis of outbreaks in a restaurant and on a bus in China, and at a Call Center in South Korea
7. Hua Qian, Yuguo Li, WH Seto, Patricia Ching, WH Ching, HQ Sun, 2010. Natural ventilation for reducing airborne infection in hospitals, Building and Environment Volume 45, Issue 3 559-565.
8. Aliabadi, Amir, A & Rogak, Steven & Bartlett, Karen & Green, Sheldon, 2011. Preventing Airborne Disease Transmission: Review of Methods for Ventilation Design in Health Care Facilities. Advances in preventive medicine 124064. 10.4061/2011/124064.
9. Y Yin, W Xu, JK Gupta, A Guity, P Marmion, A Manning, B Gulick, X Zhang & Q Chen, 2011. Experimental Study on Displacement and Mixing Ventilation Systems for a Patient Ward. HVAC&R Research
   1175-1191.
10. Juan Ren, Yue Wang, Qibo Liu, Yu Liu, 2021. Numerical Study of Three Ventilation Strategies in a prefabricated COVID-19 inpatient ward. Building and Environment 188 (2021) 107467.
11. RK Bhagat, MSD Wykes, SB Dalziel, PF Linden, 2020. Effects of ventilation on the indoor spread of COVID-19. Journal of Fluid Mechanics Volume 903.
12. Leslie Dietz, Patrick F Horve, David A Coil, Mark Fretz, Jonathan A Eisen, Kevin Van Den Wymelenberg, 2020. 2019 Novel Coronavirus (Covid-19) Pandemic: Built Environment Considerations To Reduce Transmission. Applied and Environmental Science.
13. RK Bhagat, MSD Wykes, SB Dalziel, PF Linden, 2020. Effects of ventilation on the indoor spread of COVID-19. Journal of Fluid Mechanics Volume 903.

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‘It was seen as a curiosity,’ says Noakes, professor of environmental engineering for buildings at the University of Leeds. ‘They didn’t see it being important, as they didn’t see transmission happening. It suddenly matters now.’

Noakes’ knowledge of ventilation and its impact on air quality has put her at the forefront of the UK’s struggle to minimise the spread of Covid-19. Her contribution was recognised last month when she was made an OBE for ‘services to the Covid-19 response’.

She sits on the Scientific Advisory Group for Emergencies (SAGE), advising the government on its response to the pandemic. She also convened and chairs the Environment and Modelling Group (EMG), a cross-disciplinary sub-group of engineers, architects, clinicians, modellers, microbiologists, behavioural scientists and public health specialists. It provides advice to the government on the physics of Covid-19 spread and the risks of exposure in buildings, and puts forward mitigation strategies to keep people safe.

Sir Patrick Vallance, the UK government’s chief scientific adviser, said that, under Noakes’ leadership, the ‘EMG work has had widespread and significant impact, not just on the government’s response to Covid-19 – where it has informed policy across a range of departments – but also on public advice that is supporting the safe reopening of businesses and public services’.

Noakes also supported CIBSE in producing guidance on ventilation for Covid-19 and she has contributed to the World Health Organization (WHO) guidelines on ventilation for control of the virus. She was one of 36 experts in airborne infection who called on the WHO to accept that Covid-19 could be transmitted through aerosol routes.

Her work has also been recognised by The President’s Special Award for Pandemic Service by the Royal Academy of Engineering.

At Leeds University Noakes has helped devise a strategy for keeping staff and students safe during the pandemic. This involves keeping well ventilated and keeping interactions to as small a social circle as possible. ‘If you do interact, interact outside,’ she advises.

Active on Twitter, Noakes (@cathnoakes) takes time to answer questions from the public, and offer the latest advice and research on Covid-19 – as well as humorous snapshots of everyday life in Yorkshire.

Noakes says she never over-simplifies the science, even with politicians who want yes or no answers. ‘We wrestled with this for quite some time, but we can’t make it less complex,’ she says. ‘It is what it is. It’s physics.

‘One of the real challenges we are finding with ventilation is that people want a simple rule – but there isn’t one. It’s a specialist area and we need to recognise it in that way.’

Even now, Noakes says it’s difficult to say categorically that airborne transmission of Covid-19 is definitive because of the complex multiple routes of transmission, but an increasing body of evidence points to the airborne route being a major transmission factor.

‘When it’s poorly ventilated, and where people are generating large amounts of aerosol through activities such as singing and loud speech, that is when transmission is most likely,’ says Noakes.

She highlights the infamous choir practice in Washington State, where a superspreader infected 52 people over 2.5 hours; and a case in South Korea in August, when 27 people were infected in an air conditioned Starbucks with recirculating indoor air.

With Covid-19 infections rising sharply across the UK, Noakes is now focusing on keeping building occupants safe over the coming winter.

‘Winter adds risk factors together,’ she says. ‘People want to socialise indoors, and we know that the virus survives in colder and dryer conditions. We also know it’s susceptible to the UV in sunlight and, in winter, we don’t have sufficient UV to kill the virus so effectively. We are also generally more susceptible to viruses in the winter.’

Using ventilation in the winter is more difficult because drawing in too much outdoor air – either through natural or mechanical ventilation – will cause thermal discomfort. Rather than having windows open all the time, Noakes suggests having them open slightly to maintain baseload, before opening them more fully to air a space.

‘If you combine the airing period with when people leave a room, you can effectively flush the room and reset it to zero for the next occupancy period,’ she says. ‘That’s one way that we could try to manage comfort and energy, and the need for ventilation.’

Noakes says maintaining humidity above 40% and temperature above 18-20°C is important. The likely cause of large clusters of infections among food-factory workers was the recirculation of chilled air in the processing plant, she says.

Ultraviolet germicidal irradiation (UVGI) and Hepa filters may be solutions in poorly ventilated spaces where there is limited opportunity to bring in outdoor air. However, Noakes says people have to consider the unintended consequences.

Some air purifier technologies can produce ozone. For example, when oxygen particles are broken apart by high-energy UV-C light, the atoms combine again with other oxygen atoms to form ozone.

‘Ozone is a respiratory irritant,’ says Noakes. ‘You need to think “are people going to be exposed to air chemistry that may be detrimental to their health?”’

She also warns that chemicals from UVGI may degrade materials, while decontaminates from foggers [equipment producing fine spray] may be harmful to occupants if not handled properly. ‘If people are decontaminating space using foggers, they need to think about the chemical being used. Is there off-gassing involved, and how long before you can go safely back into the space?’

Noakes says suppliers often don’t say what the ‘fog’ contains, and she believes there is an important role for engineering professionals to advise those responsible for making buildings Covid-19 secure, such as school principals.

‘The headteacher makes sure the boiler is running, but it’s not their role to maintain and set the boiler,’ she says. ‘It’s the same with ventilation. It’s their responsibility to make sure it’s running smoothly, but it’s up to someone else to set it up and make sure it’s working correctly.’

CO₂ monitors can be used as an indicator of poor air quality, but Noakes warns that they are not necessarily an indicator of good air quality. ‘CO₂ has some benefits, particularly for multiple occupant spaces. If it’s more than 1,500ppm, then you need to do something. If you have higher aerosol generation in a space then the threshold should be closer to 800,’ she says

‘But you can’t say that a low CO₂ means a space is safe. In a large area, for example, CO₂ will build up very slowly. It may appear to have a low CO₂ value, but there may be areas where it’s higher.’

Noakes believes the Covid-19 pandemic will force people to take ventilation seriously and help safeguard buildings against the risk of infections.

‘There are a lot of people working on energy and air quality, but there are very few people working on infection transmission,’ she says. ‘With the right heat-recovery systems, which ensure good ventilation that is energy efficient, there’s no disadvantage. The long-term payback is improved health and productivity, and lower energy use.’

Noakes accepts that, in the short term, many will have to live with existing systems, but, with investment, we will all benefit. ‘It’s taken a pandemic to get people looking at ventilation,’ she says, ‘but if we do invest as a nation, there’s a potential big win.’

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That conclusion is similar to what you will find in the section on temperature and humidity in the ASHRAE Position Document on Airborne Infectious Diseases, which declines to make any ‘broad’ recommendations about humidity.

However, it does cite a number of studies that have found viral infection rates to be higher at lower humidity and suggests that practitioners may choose to take that into consideration on a case-by-case basis.

On the other hand, experts such as Dr Stephanie Taylor advocate the 40-60% RH range,¹ partly because of the other consideration that dry mucosa may be more vulnerable to infection and that, at low humidity levels, respiratory droplets evaporate more quickly to particle sizes capable of remaining airborne for extended periods.

There are many factors to consider when evaluating the advice on recirculation. Certainly, closing it off will reduce transfer of airborne pathogen containing aerosols from one space to another. It will also, assuming supply airflow rates don’t change, greatly increase the amount of outdoor air being brought in to reduce exposure by dilution.

Forcing the system into 100% outside-air mode without any recirculation may result in the need to condition a large quantity of cold, very dry air in the winter or hot, very moist air in the summer. This may have consequences for comfort, microbial growth, and occupant susceptibility.

The guidance dismisses filtration on the grounds that the filters typically found in such systems are not of sufficiently high efficiency to have a significant positive impact. That may be true, but I believe most systems can handle higher efficiency filters that may have a significant impact. In addition, ultraviolet germicidal irradiation (UVGI) can be installed in a recirculating system that has a negligible effect on pressure drop and can be designed for high, single-pass efficiencies.

ASHRAE’s Standard 170-2017 Ventilation of Health Care Facilities allows recirculation to most space types and specifies filtration requirements that are mostly below HEPA level. HEPA filters are mandatory only for protective environment rooms.

For example, in ASHRAE Standard 170, the minimum filter efficiency requirement for an operating room is MERV 14 with a lower-efficiency MERV 7 prefilter. A MERV 14 filter must have an efficiency of more than 75% for 0.3-1.0 micron particles, a range into which many virus-bearing airborne droplet residues will fall. It is not rated for smaller, virus-sized particles, but will still collect a high percentage in that range.

[The UK government guidance on filters in healthcare facilities is stated in Guidance for infection prevention and control in healthcare settings ].

A MERV 16 filter must be greater than 95% efficient in this range. UVGI that is 80-90% efficient on single pass under worst-case conditions, plus a filter that is of comparable efficiency for small particles, could be a good combination that removes or inactivates a large fraction of airborne pathogens even when there is recirculation.

I consider going to full outside air a reasonable emergency measure, but one that may be unacceptable based on outdoor conditions at the time, unless the system is designed for it.

If a recirculating system is selected for a building, I would recommend a relatively high-efficiency filtration and UVGI (or other air cleaners shown to work). A system could put in 100% outside air when occupied and then recirculate when unoccupied, to allow filtration and UVGI to remove/inactivate pathogens at lower energy cost.

I disagree that UV is only suitable for healthcare. UVGI is used in all types of buildings – residential, commercial, and healthcare – although healthcare is the niche in which it has the most obvious value. A major US manufacture I contacted estimates that the current market for UVGI is only about 10% healthcare by number of systems. Upper-room UVGI can be combined with 100% outside air, which may be the best of both worlds – no recirculation and enhanced, low-energy microbial control.

Studies of the effectiveness of upper-room systems have suggested they may be equivalent to as much as 10 air changes per hour of outside air in their ability to inactivate airborne pathogens in a space. Systems that irradiate surfaces when unoccupied can reduce the likelihood of fomite transfer, which is important given the potential infectivity of virus deposited on surfaces for several days.

Reference:

1 Dr Stephanie Taylor, Using the indoor environment to contain the coronavirus, Engineered Systems, 16 March 2020

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The Clean Air Act of 1952 put paid to inner London’s power stations like Bankside, which has subsequently been reborn as the Tate Modern art gallery.

But the past 10 years have seen this process go into reverse, says Kathryn Woolley, associate consultant, air quality at SLR Consulting: ‘They got all the power stations out of London and, over the past 10 years, we’ve brought lots of little ones back in. We’ve dotted the city with all of these power sources without much understanding.’

She is referring to the proliferation of mini-combined heat and power (CHP) plants that inner London has seen installed over the past decade. 

The driver for this spate of mini-power plants has been the backing from the London Plan –the capital’s spatial development blueprint – for CHP plants.

CHPs were seen as less wasteful of energy, avoiding the losses that inevitably occur when electricity is transmitted around the grid – and less wasted energy translates into lower carbon emissions.

The other main driver was the desire to lessen the strain on the capital’s grid, which is ageing and difficult to upgrade, given the constraints involved in working in a historic and cramped environment like inner London.

But this enthusiasm for CHP hasn’t aged well, as public concerns about air quality have rocketed up the agenda. Every week seems to see a fresh report spelling out the dire consequences of air pollution for physical and mental health.

And, while motor vehilce transport has attracted much of the public ire over poor air quality, the built environment contributes to the problem too. That includes boilers and CHP plants belching out nitrogen oxides (NO_x).

Edwin Wealend, associate at Cundall, and chair of the CIBSE Air Quality Group, says: ‘The London Plan strategy has driven the installation of CHP plants without paying too much attention to the NO_x emissions. If you are buying a home next to one of those, or in a new development with an energy centre, you could have a very significant amount of NO_x compared with the traffic.

‘NO_x can be very localised: if you look at NO_x monitoring maps for London, the highest concentrations are all along roads, but there are pockets by boiler plants, particularly where there are CHPs.’

Add to that the spread of diesel generation backup plants and pinpointing air pollution is increasingly hard, says Woolley. ‘At the minute, it’s easy to see where the source is on the roads, but you could end up with really bad pockets around power generation.’

CHPs are better than the grid in terms of overall NO_x emissions, but not at a more neighbourhood level, says Bill Sinclair, technical director of plant manufacturer Adveco: ‘Every time you fit a CHP, the NO_x emissions on a countrywide scale goes down. However, if you put that CHP in the centre of London, the local emissions go up because you were not producing electricity in the centre of London.’

The sea change in attitudes towards CHP is visible in the draft London Plan. The previous version, published under the watch of former mayor Boris Johnson, had a single policy on air quality. But Woolley has counted no fewer than 15 separate policies relating to air pollution in his mayoral successor Sadiq Khan’s update.

‘Low emission’ CHP should only be deployed in large developments where doing so enables the delivery of an area-wide heat network, the key policy states. ‘Where there remains a strategic case for low-emission CHP systems to support area-wide heat networks, these will continue to be considered on a case-by-case basis.’

And the document goes on to say that it is ‘not expected’ that gas engine CHP will fit the low-emission category with the currently available technology.

Stuart Clark, director at the consultancy Energist, says: ‘[The Greater London Authority (GLA)] still wants people to be using district heat networks, but there is a very strong drive to use electrical technologies, such as heat pumps with zero onsite emissions.’

The other factor pushing down demand for CHP is the new draft SAP 10 carbon factors, which help to determine how buildings will perform in terms of carbon emissions. 

These give a much lower carbon rating to electricity transmitted through the grid, reflecting the increased proportion that now comes from renewable power sources compared with 2014, when this measure was last calculated.

The SAP 10 methodology was published last summer but will not be applied nationwide until the next iteration of Part L of the Building Regulations is rolled out later this year. However, the London Plan states the SAP 10 carbon factors can be used as a guide when determining planning applications.

The GLA’s accompanying guidance on energy assessment spells out that onsite electricity generation technologies, which include solar PV arrays as well as
gas-engine CHP plants, will no longer achieve the level of carbon savings they have until now.

If gas-engine CHP is proposed, applicants will be expected to supply ‘sufficient information’ to justify its use and minimise the carbon and air quality impact.

This new guidance has increased the attraction of using electric technologies, such as heat pumps, says Clark: ‘Historically, electricity was a challenging fuel to use in new-build developments but is now a lot more carbon-light within London boroughs, which means heat pumps are preferable options from a carbon point of view compared against CHP.’

‘By introducing CHP, you are reducing performance of the system,’ he says, adding that developers of energy centres will have to find offsets elsewhere for the carbon emitted by gas-fired plant if they are determined to use it.

Sinclair acknowledges that the new carbon factors have changed the equation for CHP plants. ‘The less carbon-emitting technology was also the less expensive technology, so it made sense to use gas from a carbon and cost point of view.

‘With the reduction of the carbon factor under SAP 10, they are now diametrically opposite: the more expensive technology is the less carbon-emitting technology.’

On a practical level, Woolley worries about the lack of detail to flesh out the London Plan’s policies. She says: ‘The industry doesn’t know what it is aiming for. The policy is there, but the supporting planning guidance isn’t. They may have an idea, but they are not telling anyone yet.’ 

For example, while the plan says new boilers should have ‘ultra low’ NO_x emissions, it doesn’t specify how this should be defined.

Abatement measures, such as catalytic convertors and timing controls, may enable most models of gas-CHP engine to meet the more stringent NO_x emission requirements.

‘The modulated systems will flex up and down, depending on the demand of the building, which reduces the size of the thermal store, which should be more energy efficient and have positive benefits on ultra-low NO_x emissions,’ says Clark.

And Sinclair argues that gas-fired CHP engines can continue to play a role as part of a hybrid heating solution.

Heat pumps can pre-heat water in the systems cost effectively to about 45°C. But increasing the temperatures to the levels required to ensure the water is sterilised and any bacteria, such as legionella, are killed can be most cost effectively achieved by using gas heating. He says: ‘This gives you the best trade-off of running costs and savings.’

And increased reliance on electricity will revive concerns about London’s grid capacity, says Wealend: ‘We are placing increased demand on the grid at peak times.’

Nevertheless the thrust of the London policy is clear: once again, the days of power plants in the capital are numbered.

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