The Problem
Viruses, including SARS-CoV-2 (the cause of COVID-19) may be transmitted via airborne droplets from coughs, sneezes, talking, and simple exhalation. The virus can be transmitted directly by inhaling droplets, and indirectly picked up from surfaces where droplets are deposited.
Close contact direct transmission is via larger droplets (>10 microns), typically from coughing and sneezing. These tend to fall to surfaces within 1 to 2 metres from the affected person, which is why the social distancing rules are based on this distance. Larger droplets fall quickly, but smaller droplets (<5 microns) evaporate quickly and then may stay airborne for hours and may be carried long distances by indoor air currents. These smaller droplets arise from the evaporation of larger droplets and from coughing, sneezing, talking and breathing.
It is not yet known how significant these smaller droplets are for the overall transmission of SARS-CoV-2, but this transmission route is well-documented for SARS-CoV-1 and other viruses. There is a particular risk in enclosed spaces such as buildings, because exhaled air may linger, making it easier to breathe in and more likely to settle on touched surfaces.
Despite the global scale of the problem, relatively little research has been undertaken to quantify the risk of airborne transmission in enclosed spaces, and the majority of previous studies stem from the work of Wells (1955) and Riley et al. (1978), using an analytical expression known as the Wells-Riley equation (see Box 1). As lockdown is reduced, this risk is becoming increasingly important to all of us, and understanding and mitigating it could be vital in avoiding further waves of infection.
The Industry Response
Although the science is still evolving rapidly, existing evidence is sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit infection risk indoors. Therefore, industry bodies have moved quickly to alert building managers to the risk and options for management. ASHRAE in the US, REHVA in Europe and CIBSE in the UK have all published advice and guidance on HVAC systems.
Some of the key strategies recommended are:
- Ventilation: this is exchanging indoor for outdoor air, effectively removing the risk. This can be by opening windows, by HVAC systems, or a combination of both.
- Circulation: this can break up concentrations in certain areas. But it carries risks, for example in directing air with higher concentrations of droplets to other people, or preventing the settlement of droplets.
- Filtration of recirculated air: although virus particles are around 0.08-0.16 microns, they can be removed by modern HEPA filters. Care is required in cleaning the filters which may contain active viruses;
- Active eradication: heat, UV light, and other techniques are used to kill the virus in recirculated air. Combined with filtration, this can be particularly effective in enclosed spaces.
So there are a wide range of options available for mitigating the risk. Operators of enclosed spaces need to determine which if any are appropriate for them.
Understanding by Measuring – The NAQTS Approach
The key to deciding which strategy to adopt and assessing effectiveness, is to have reliable measurements. That way, building operators can decide if they have a problem and can design and evaluate solutions based on real data rather than guesswork.
Direct measurement of viral load in the air is possible but expensive and time-consuming, so is really confined to laboratory situations. At NAQTS using the NAQTS V2000 integrated indoor air quality monitor we focus on two main areas: carbon dioxide (CO2) and Particulates.
By measuring CO2 we can continuously evaluate room ventilation rates, and the proportion of CO2 that is being generated by exhaled breath (rebreathed fraction).The potential infection risk for airborne disease is present in higher concentrations in exhaled air than ambient air. By measuring CO2 we can get an accurate picture of the concentration of exhaled air in the room. For example, in a room that has 1000ppm CO2 (~600ppm higher than outdoors) the rebreathed fraction is 1.6%, whereas in a room that has 3000ppm (which can be commonly observed in crowded indoor spaces) the rebreathed fraction is 7% – more than four times greater. CIBSE recommends aiming to reduce the concentration of CO2 to the same as outdoor air (~400ppm), making sure that this risk is minimised.
Particulates come into play when filtration is being used. The size of the particle affects how long aerosols will stay airborne, with the smallest particles (called ultrafine particles) staying airborne for up to eight hours, whereas larger particles will fall out of the air more quickly and land on surfaces after a few minutes. Ultrafine particles are not part of standard air quality measurements as they are unregulated. NAQTS incorporate technology to measure ultrafine particles because of their potential significant health effects. By comparing the amount of particulate matter in the air outside with that inside, we can determine if the air filter is working. By looking at the pattern over a 24-hour period and comparing it with occupancy, we can understand the effectiveness of ventilation and filtration systems in clearing and filtering particulates generated by the occupants – including the virus.
NAQTS can run these tests under different ventilation conditions and at different locations within the building, typically gathering data over a few days to ensure that it is representative. This then allows us to understand the risks and the effectiveness of mitigation strategies being used. NAQTS is currently involved in assessing ventilation and filtration systems in a range of indoor environments in the UK.
The NAQTS Approach – UK School Classrooms
As part of a nationwide project with Johnson Matthey Plc using the NAQTS V2000 in 20 primary schools to evaluate indoor air quality, NAQTS continuously measured CO2, particulates, and other air pollutants and environmental parameters.[1] The project ran from January to May 2020, and gave NAQTS a unique perspective on classroom indoor air quality, particularly in the context of airborne virus transmission as it straddled both the pre-lockdown and lockdown periods.
Through analysing the measured CO2 concentrations, we calculated air changes per hour (ACH) in different school buildings, and the results showed a wide range, between around 0.1 to 1.8 ACH. Figure 1 shows what a typical school day can look like, with CO2 concentrations regularly exceeding 3000ppm in periods of the school day when the classroom is occupied. During the evening the CO2 level decays exponentially down to ambient levels of around 400ppm. NAQTS uses this decay rate to determine the room ACH..
Through continuous collection of this CO2 data, NAQTS was able to plug these results into the Wells-Riley equation, with each dot representing a school. As can be seen in Figure 2 the probability of infection is highest when the room ventilation rates are low. Improved ventilation causes a significant reduction in the chance of infection for all but measles.
The results suggest that for viruses such as SARS-CoV-1 and TB, the probability of airborne transmission can be reduced by 80% or more by increasing ventilation within normal achievable rates. If filtration systems are also used, this infection probability can be further reduced by the removal of particulates containing viruses.
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It is stressed that these results are preliminary and based on very limited data; we do not yet know the airborne transmissivity of SARS-Cov-2; and that airborne transmission is only one component of risk.These examples were taken in school classrooms, but this applies in any indoor environment. Through the use of the NAQTS V2000 and its measurements of particulates in conjunction with the measurement of CO2, NAQTS can provide a comprehensive and detailed understanding of the building operation with regards to air quality, and provide engineering-based advice to protect building occupants from poor indoor air quality, and support the back to work initiatives around COVID-19, ensuring that building occupants are adequately protected.
The first step in improving our air quality is to understand it.
References / Further Reading
- Morawska, L., Tang, J. W., Bahnfleth, W., Bluyssen, P. M., Boerstra, A., Buonanno, G., … Yao, M. (2020). How can airborne transmission of COVID-19 indoors be minimised? Environment International. https://doi.org/10.1016/j.envint.2020.105832
- Stephens, B. (2013). HVAC filtration and the Wells-Riley approach to assessing risks of infectious airborne diseases. NAFA Foundation Report, (March), 44.
- CIBSE website https://www.cibse.org/coronavirus-covid-19/coronavirus-covid-19-and-hvac-systems
- ASHRAE position document https://www.ashrae.org/file%20library/about/position%20documents/pd_infectiousaerosols_2020.pdf
- REHVA COVID 19 guidance document https://www.rehva.eu/fileadmin/user_upload/REHVA_COVID-19_guidance_document_ver2_20200403_1.pdf
- Buonanno, G., Stabile, L., & Morawska, L. (2020). Estimation of airborne viral emission: Quanta emission rate of SARS-CoV-2 for infection risk assessment. Environment International. https://doi.org/10.1016/j.envint.2020.105794
[1] Johnson Matthey Plc owns the data presented here, which was measured as part of a programme they coordinated. This is part of an ongoing programme and results are still being processed.