I’m glad to see the possibility of airborne transmission acknowledged clearly in REHVA’s guidance document on Covid-19. However, I think there would be debate about what it says about humidity. The document says humidification and air conditioning have no practical effect, and concludes that there would have to be an unacceptably high humidity to have an effect on the virus.
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.
1 Dr Stephanie Taylor, Using the indoor environment to contain the coronavirus, Engineered Systems, 16 March 2020