Covid-19 and the role of Indoor Air Quality
As buildings are reoccupied new challenges have appeared, not least limiting the risk of spreading the virus
A lot has changed since the UK went into a nationwide Coronavirus lockdown in an attempt to reduce the risk of community transmission. Since then, and despite the lockdown, 56,800 deaths were registered in Britain where Covid-19 was mentioned on the death certificate, according to the Office for National Statistics (ONS). Obviously we will need to continue to protect ourselves against this deadly virus. As the UK emerges from lockdown, it is clear that we will have to reconsider hygiene concepts in closed rooms.
It is thought that the Coronavirus spreads via larger respiratory droplets as well as via direct contact with contaminated surfaces (so-called fomites) and infected people. But, and this is important, a growing body of evidence suggests that the virus spreads indoors via small airborne droplets, so-called aerosols. Because they are so small, aerosols can linger in the air for long periods of time - particularly in badly ventilated indoor environments -, enabling airborne transmission. This is not a new phenomenon. Other contagious viral diseases, such as smallpox or measles, are known to be transmissible via the airborne route.
If planned poorly or not serviced properly, air conditioning units can quickly turn into transmitters of disease-causing germs. Conventional air conditioning systems only control temperature and air humidity. An effective ventilation system, perhaps augmented by air disinfection and particle filtration, can help to reduce the overall indoor infection risk from airborne particles by diluting and controlling airborne pathogenic material, thereby reducing the time during which people are exposed to any airborne viral aerosols, and reducing the chance for aerosols to settle on surfaces.
Experts advice following the precautionary principle and taking measures to mitigate airborne transmission risk, including the provision of sufficient and effective ventilation, and supplementing general ventilation with airborne infection controls, such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights.
There are a number of steps that can be taken to improve indoor air quality, beginning with choosing a ventilation system that has HEPA filters (which filter out 99.99% of all micro-organisms, such as bacteria and viruses), UV lamps (which reliably kill viruses, germs and bacteria that have been accumulated on the particulate air filter), antimicrobial interior coating (inner plates coated with silver ions, which destroy viruses, germs and mould), and thermal disinfection (a new technology that dries out the biofilm resulting from the cooling process, reliably killing micro-organisms).
For commercial and public buildings, the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) advises against the use of air recirculation units unless they are equipped with HEPA filters and UV lamps. In demand-controlled ventilation systems, REHVA recommends changing the CO2 setpoint to 400 ppm in order to maintain the operation at nominal speed. And far from switching the ventilation off in buildings that have been vacated due to the pandemic (e.g. offices or schools), it recommends operating ventilation continuously at reduced speed during normal operation hours.
Where central recirculation can’t be avoided because of limited cooling capacity, the outdoor air fraction should be increased as much as possible and additional measures taken for return air filtering. Air handling units and recirculation sections that are equipped with return air filters would require HEPA filters help to remove airborne particles and viruses from the return air. Where these can’t be installed into an existing system, REHVA recommends that low-efficiency return air filters be replaced with ePM1 80% (former F8) filters, which have a reasonable capture efficiency for virus-laden particles (capture efficiency 65-90% for PM1).
Well-maintained air recirculating units securely filter large droplets as well as aerosols containing Coronavirus (and of course other viruses) and may have a complementary role in decreasing transmission rates in indoor spaces by increasing the rate of air change. Efficient ventilation not only reduces infection rates, but also improves occupant wellbeing, and ultimately productivity.
Malcolm Youll is Managing Director of Weiss Technik UK Ltd
Headline image by Sergio Santos and is used under the creative commons license.