The recommendations given in this blog are taken from the “REHVA COVID-19 guidance document, April 2, 2020”1. That document may be subject to revision if new scientific evidence is reported and readers are urged to visit the REHVA website for the latest advice. It is recommended that readers also act in conjunction with advice from the W.H.O. on “Getting your workplace ready for COVID-19”2. It should be noted that in the absence of evidence regarding SARS-CoV-2 (COVID-19) the advice listed here is mostly based on evidence relating to the previous SARS-CoV-1 epidemic.
This advice is intended for anyone working in proximity to mechanical air exhaust systems and may be of interest to those providing FM services to commercial buildings. It relates to public and commercial buildings where there may be occasional occupancy by infected persons. Hospital and health care settings are explicitly excluded.
Although there are thought to be three transmission routes of the infectious agent, the standard assumption is that transmission via large droplets (droplets or particles emitted when sneezing, coughing, or talking), and via surface contact (surface to hand, hand to face) dominate3,4. A third route has been recognised by the WHO, via fᴂcal matter via droplets caused by flushing WC’s5.
Droplets are formed from coughing and sneezing and fall onto surfaces, such as desks, tables, or handrails, no further than 1-2m. People then catch the disease by touching the surface and then touching their eyes, nose, or mouth. If people are standing within 1-2 metres of an infected person, they may catch it directly by breathing in droplets sneezed, coughed, or exhaled by them. This is the reason for the 2m separation rule in social distancing. Because of the short travel distance, this is unlikely to be affected by air handling systems.
Also generated by coughing, sneezing, or talking, these may stay airborne for hours and can be transported long distances. SARS-CoV-2 remains active for up to 3 hours in indoor air and 2-3 days on room surfaces under common indoor conditions6. Therefore small virus particles can travel long distances carried by the airflows in rooms, or the extract air ducts of ventilation systems.
Although there is not yet direct evidence relating to SARS-CoV-2, there is clear evidence relating to SARS-CoV-17,8. There is also no reported data or studies to rule out the possibility of the airborne-particle route, however, Coronavirus SARS-CoV- 2 has been isolated from swabs taken from exhaust vents in rooms occupied by infected patients.
In conclusion, there is clear scientific evidence that mechanical air extract systems can transport virus particulates through the air system and be extracted to air. Hence a virus can be present some distance away from an infected person through such a system, at the air extract point.
REHVA proposes adopting an ALARA principle (As Low As Reasonably Achievable) and to take a set of practical measures to help to control the airborne route.
In short, supply as much outside air as is reasonably possible.
Coronaviruses are resistant to environmental changes and only extremes, such as RH >80% and temperatures ≥30ºC, have any effect.
There is a risk that heat recovery devices could carry over the virus attached to particles from the exhaust side to the supply side, via air-leaks.
Regenerative thermal wheels may be susceptible to significant leakage due to poor design or lack of maintenance. The most common fault is that fans are mounted such that a higher pressure is created on the exhaust side, causing leakage into the supply air.
If a leak in the heat recovery system is suspected, pressure adjustment or bypassing is recommended.
Virus particles can re-enter a building when air-handling units are equipped with recirculators. These should be avoided by closing the recirculation dampers, either via BeMS or manually. Whilst this may cause problems with heating or cooling capacity, this must be accepted.
Dampers should be closed even if filters are fitted as standard efficiency filters (F4/F5 or ISO coarse / ePM10 class) will not filter out virus particles.
Some fan coil and induction systems operate with room-level circulation. If possible, it is recommended that these units are turned off as the fan coil units have a coarse filter that will not stop the virus. If they cannot be switched off, it is recommended that they run continuously to prevent the virus from forming a sediment in filters which becomes re-suspended when the fan is turned on.
Viruses attached to small particles will not deposit easily in ventilation ducts and will normally be carried out by the airflow.9
Instances of outdoor virus contamination are rare, and even if air exhausts are close to air intakes, at 80-160 nm, the virus is much smaller than the capture area of normal outdoor air filters (F7 or F84 or ISO ePM2.5 or ePM1)10.
To be effective air cleansers need at least HEPA level efficiency. However, because airflow through air cleansers is limited, the area of floor covered effectively is typically <10 m2.
Devices that use electrostatic filtration systems (not the same as portable room ionisers) can be effective, as can specialist UV cleaning equipment.
SSE Energy Solutions have adapted its COVID-19 RAMS to acknowledge the potential increase of risk and will now review the ductwork and AHU schematics to understand where exhaust air is extracted from and to. Any form of control measure required to keep operatives safe would be implemented on a case by case basis.
Occupant wellbeing is an emerging and future area of interest in the Building Control sector; air quality especially. The UK market has been slow to respond beyond the current functional procurement of ‘standard’ building control solutions whilst air quality solutions, for example, have been adopted more readily elsewhere in the world. This could change in the UK following the COVID-19 pandemic.
We are currently reviewing a number of solutions known to kill flu and other viruses11 that could be installed within air ventilation systems to eradicate the potential to circulate viral infections in buildings managed by control systems.
11 These have not yet been tested against SARS-CoV-2