The airborne spread of the coronavirus has been well documented. Famously, outbreaks have emerged from choir practices and other indoor gatherings, with infection rates so uniquely high that it’s unlikely everyone got infected by touching the same surface. Over a long period in an enclosed space, it seems, singing can spew virus into the air until it accumulates to the point of danger for people who are well over six feet away.
Once a virus is hanging in the air—and we know that the coronavirus can linger for hours—it will travel with air currents. One ominous study of a restaurant in Guangzhou, China, documented how air-conditioning appeared to spread the virus between tables at opposite sides of the room. The issue wasn’t that the virus was traveling through the air-conditioning unit, but that it was getting pushed around the room by the stream of air. The takeaway is that while airflow is good when it’s coming from open windows, it could make things worse when it’s coming from an AC unit that’s blowing air around a closed room. Coughing in a well-ventilated room is sort of like peeing in a river as opposed to a hot tub: Ideally you wouldn’t do either, but one is definitely worse.
Read: The moral history of air-conditioning
There’s currently no clear evidence of this virus spreading from room to room through air-conditioning, as you’re curious about, Peter. I’d be more concerned about the air at the rallies themselves. But the presumption is that it could, and buildings should take precautions to prevent that possibility.
“In a high-functioning building with a well-conditioned HVAC, you shouldn’t expect that there would be spread between different rooms,” Krystal Pollitt, an epidemiologist at the Yale School of Public Health, told me. But that’s only if things are up to code and working properly.
For example, commercial building codes in many places require “negative-pressure ventilation” in bathrooms—meaning air is propelled outside by an exhaust fan. This is especially important during the pandemic because of something called “toilet plume,” which I wish I’d never read about. Basically, when you flush a toilet with the lid up, the rush of water can aerosolize the contents of the bowl and send some of them up into the air, kind of like an enormous cough. During the SARS coronavirus outbreak, in 2003, a cluster of cases in Hong Kong was attributed to one person with diarrhea in a poorly ventilated apartment building.
In light of the pandemic, various professional organizations have issued new recommendations for building ventilation, but how widely they’ll be followed is unclear. The fixes aren’t actually groundbreaking: They’re mostly things that everyone was supposed to be doing all along, such as ensuring that bathrooms have exhaust fans and that air filters are changed regularly and of high-enough quality to catch the virus. That means they should be high-efficiency particulate air (HEPA) filters or MERV-rated 13 or 14, which are essentially the N-95 masks of air filters.