covid-19: what we know about heating, cooling and ventilation
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new studies are shining a light on hvac systems and the coronavirus.
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this story was originally published on august 14, 2020 and has been updated on october 20.
- what “airborne transmission” really means
- how heating in the colder months will affect covid-19’s transmission
- ventilation is important, but so is airflow and pathway
avoiding crowds, physical distancing, hand washing, and wearing face masks have been the main messages for the last few months to keep covid-19 case numbers down. but as cooler weather sends us indoors and schools have reopened, there’s a new concern: how to keep indoor air clean.
in a new study published in journal of fluid mechanics, researchers from the university of cambridge say wind-driven ventilation is a natural form of ventilation but because office buildings are often designed to be airtight to create comfortable temperatures, it could increase risk of covid-19 transmission. maximizing ventilation while keeping comfortable temperatures without using excessive energy consumption is a “difficult balance,” say study authors.
in india, a research institute has suggested that intensive care units (icus) treating covid-19 patients get rid of air-conditioning units to prevent recirculating air that could spread covid-19. the study suggests icus be fitted instead with exhausts that create a one-way flow for infected air to be treated with either very hot water or a disinfecting soap or sodium-based solution before being directed outside.
another paper, published in the journal lancet respiratory medicine which looked at infectious aerosols and how to protect healthcare workers, says there needs to be more attention paid to the fact that smaller particles of sars-cov-2, the virus that causes covid-19, can remain suspended in the air, unless there is an air current or dilution ventilation to remove them.
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and the question is quickly becoming can we do this affordably and effectively?
the author, dr. kevin fennelly, a maryland-based pulmonologist, writes that powered air-purifying respirators will help infection control, since air disinfection with uv germicidal irradiation may be useful in settings where people congregate.
currently, public health infection control policies, like hand-washing and physical distancing, are based on the premise that infections are spread by large droplets, anything larger than five micrometres in diameter — in other words, coughing and sneezing. however, many pathogens, like those in influenza, for example, can be spread through smaller aerosols.
in early july, hundreds of scientists, engineers, epidemiologists, virologists, aerosol physicists, doctors, and more signed a letter asking the world health organization to acknowledge microdroplets as a mode of covid-19 transmission. on july 9, the who responded saying airborne transmission could be a method but requires more study.
covid-19 is not “airborne” the way measles is
there has been a lot of misunderstanding around the term “airborne transmission” and its meaning, says dr. nelson lee, an infectious diseases physician and professor at the university of alberta, one of the 239 scientists who signed the letter. typically, when we think of airborne transmission, we think about diseases with a long-range airborne transmission such as measles and tuberculosis. measles is one of the most contagious, with virus droplets able to linger in the air for up to two hours after an infected person has left the room. people with active tuberculosis are advised to stay in separate, ventilated rooms separate from their friends and family to avoid transmission (those with tb medication are not contagious).
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but airborne transmission actually depends on the size of the particles. by definition, an aerosol is any particle smaller than five microns. in this letter, researchers are referring to situations — especially in indoor settings — where microdroplets can spread beyond two metres. typically, droplets larger than 10 micrometers — which happens when an infected person coughs or sneezes — are large enough to fall to the ground within two metres due to gravity. this happens within seconds, and if you’re outside the two-metre zone, you’re safe. however, there’s increasing data showing that droplets can travel further than two metres, depending on the environment. this is the rationale behind widespread mandatory masks in indoors, to limit the amount of droplets each person transmits, says lee.
“it is important to look at all aspects of indoor settings in terms of airflow and air exchange to improve the ventilation,” he says. “you want these virus microdroplets to be diluted or removed as much as possible.”
another signatory, jeffrey siegel, is a civil engineering professor at the university of toronto who has spent his career studying ventilation, urges that even with improved ventilation, we cannot forget the preventative measures we started out with at the beginning of the pandemic.
“ventilation is a secondary measure, not a primary measure,” he says. “ventilation will not help people if they aren’t wearing masks, if people aren’t physical distancing, and if we aren’t hand washing. however, it is the most important secondary measure we should all be thinking about.”
siegel says as winter arrives and we turn up heating in buildings, this turns the air relatively drier. not only does it cause our hair to dry out and the occasional electric shock, but dry air also becomes a challenge for disease transmission.
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“the drier the air, the more the droplets that carry this virus shrink in size. and the more they shrink in size, the further they’ll transport indoors,” he says. “the idea of two-metre physical distancing is kind of fiction as it is, but it’s really fiction when the air is dry. it’s very easy for these droplets to move — tens of metres is not unreasonable.”
he looks to the u.s., pointing out that some school districts have given teachers portable hepa — high efficiency particulate air — filters, that they can run near their desk to trap covid particles.
“it’s a secondary measure but it provides a little bit more protection for them against any emissions that happen in a classroom, he says. “the cost is quite small compared to some of the things we’re dealing with.”
however, ventilation is not enough. airflow is also critical.
“if a window can be opened, fresh air will increase the exchange in that environment which will help,” says lee. “but whether opening a window will be enough is very setting-specific. there are some [areas] that will not be adequately ventilated.”
one option that may cost less is to provide outdoor teaching spaces.
“we should absolutely be doing as much teaching outside as we can for as long as we can. the cost of doing that is so cheap compared to other options and the ventilation [problem] goes away completely,” says siegel. “outdoor air has very good ventilation. i barely worry about my exposures outside at all. i put on my armour like everyone else does when i go in indoor environments, but outside, i don’t worry.”
“[this is] the way i look at it,” he says. “let’s improve the air quality in our indoor environments and that will probably have covid-19 risk-reduction benefits. even if it doesn’t, it will have a whole host of other health benefits from reducing indoor air quality. so, the absolute worst case scenario is that we’ve improved indoor air quality and all the benefits that come from that. and the best case is that we actually do something for covid-19.”
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ventilation is part of a larger picture
humber river hospital, a hospital located in north york, was hit particularly hard by covid-19, consistently ranking either first or second in the number of cases of covid-19 in ontario. humber serves northwest toronto, the epicentre for covid-19 in toronto, where there were more cases per capita than anywhere else in toronto. only did this week did they discharge their last covid-19 patient (to date).
“we had a lot of covid in the hospital and didn’t have transmission in the hospital, which to me is remarkable,” says dr. michael gardam, an infectious diseases physician and humber river hospital’s chief of staff. “i chalk that up to a combination of factors.”
besides humber’s design with fresh air ventilation, it is also a new hospital with larger hallways, larger waiting rooms, and many more single rooms. given that the virus transmits through close contact, the lack of crowds has made a difference.
“i know we are very unusual with having all that fresh air ventilation,” he says. “transmission at our hospital was really negligible in terms of healthcare workers and other patients. we were very much able to control this. the big question is why?”
in his experience, when hospitals move from older buildings to newer buildings, infectious disease problems disappear. at one facility gardam worked at in the niagara region, after moving to a newer facility, the spread of hospital superbugs “basically vanished.”
“a lot of it is to do with the fact that they had bigger hallways, more space to store their stuff, the place wasn’t as cluttered and they had multiple single rooms for patients.” he says. “the difference was dramatic.”
gardam also says the hospital’s administration has been good at keeping the supply chain for ppe steady.
clearly, many factors have potentially played a role in humber’s success at keeping virus transmission low — but like a lot of things about covid-19, we still know so little that it’s difficult to isolate what most contributed to the achievement. this is similar to the justification behind wearing masks, hand washing, physically distancing, and closing down venues that create crowds, he says.
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“what’s the proportion of transmission occurring from surfaces? are we are we pushing hand-washing too much, are we pushing all cleaning protocols too much?” he says. “the answer is we don’t know. in terms of spread, it’s clearly not hurting. but i can’t tell you if it’s contributing 10 per cent or 30 percent. i have no idea. that’s our challenge.”
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