Winter is Coming – A New British Report Has Advice for Dealing With Covid-19

Ventilation may be the biggest problem when everyone is inside.

English schoolroom with inappropriate social distancing
English schoolroom with inappropriate social distancing.

 Getty Images

The question of how the virus that causes Covid-19 is transmitted has been controversial. Building scientists and engineers have been saying since March that it is carried on aerosols and can travel long distances; the Centers for Disease Control and Prevention (CDC) continues to say only that it is "thought to spread through close contact from person-to-person." As we noted in the brief period when the CDC accepted the aerosol theory, it would have big implications: "Basically, you do not want to go to a Trump rally, or any other crowded, poorly ventilated indoor space where people don't wear masks."

Now a report from the UK's Royal Society RAMP task force, "The ventilation of buildings and other mitigating measures for COVID-19: a focus on winter 2020," looks at the issue of how the virus moves around and makes recommendations for the government, public health agencies, ventilation practitioners (designers and manufacturers) and "interested scientists."

The report assumes that Covid-19 can be spread by droplet and contact (which is what the CDC says) but also that it is airborne.

"Indoor spaces that bring individuals together over long periods, e.g. open-plan offices, school classrooms and the like, or those that lead to increased respiratory activity, e.g. gyms, choral halls, etc..., are expected to make the airborne spread of the virus an important consideration."

The challenge is to keep the reproductive number (R), the average number of infections arising from a single case, below 1. The good news is that if it is a group of people who meet regularly (like office workers or students) in properly ventilated spaces, then the risk is low. Masks, social distancing, and cleaning of surfaces can deal with droplets and contact transmission; ventilation is more complicated.

Hospital ward in Scutari with Florence Nightingale
Hospital ward in Scutari with Florence Nightingale.  Simpson-Walker / Wellcome Collection / Public Domain

The report starts by reminding us that these issues have been understood for a while:

Florence Nightingale is credited with having promoted the idea that the indoor environment plays a critical role in determining health outcomes. Her pioneering work on hospital ward design is still highly relevant with the guiding principles of high ceilings, adequate natural lighting, and sufficient ventilation proving sound design for any indoor environment. However, modern architectural approaches, which often rely on mechanical means to condition the environment, may not follow these principles. 

[More on Flo and airflow on Treehugger here.]

Concentrating their review on open offices and classrooms, the report notes that social distancing is effective outdoors, in well-ventilated spaces, and with short interaction times. "However, for longer exposure times and/or in poorly ventilated spaces, social distancing is unlikely to be sufficient as the dilution at room scale will not reduce the aerosol concentration enough to avoid an infectious dose." So the ventilation rate has to be increased, a problem in many buildings.

"A concern for the northern hemisphere winter is that buildings will become less well ventilated, with a lower supply of outside air in order to maintain warm conditions indoors. Ventilation systems that recirculate a proportion of the indoor air, primarily to temper the temperature of the outdoor air without increasing energy consumption, are common but it is crucial to regard only the flow of outdoor air as contributing to the ventilation rate."

If offices and classrooms cannot be adequately ventilated, then the design occupancy has to be reduced. Portable air filters can also be used; fans should only be used where there are areas of stagnant air. The report also points out a real problem with all those plexiglass screens that are being installed everywhere; they might mess up the airflow.

"This highlights the main problem with the use of screens indoors; the impact of the screen on the airflow patterns within a space is very difficult to predict. While the exchange of air between two areas of a room may be reduced, the presence of a screen can lead to areas of stagnant or recirculating flow where the virus could accumulate." 

In the end, the report concludes that the virus is airborne, and offers a grab bag of suggestions.

  • Upper room ultra-violet light (where light is carefully aimed so that it doesn't shine on people) may be useful.
  • Air ionization probably isn't.
  • Air filters work, but "are likely to require complementary modifications to the ventilation system to, at the very least, account for the changes in pressure drops across the system."
  • Portable filters probably work if properly placed.
  • Screens may stop droplets, but their use against airborne transmission is problematic.
  • "The primary control measure for airborne transmission indoors is ventilation and achieving adequate outdoor air supply rates to ensure an acceptable level of risk should be the priority."

In other words, the British report pretty much confirms what the building scientists have been saying since March: It's an aerosol, it's airborne, and building ventilation matters most.