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Joseph G. Allen is an associate professor, director of the Healthy Buildings program at Harvard University’s T.H. Chan School of Public Health and co-author of “Healthy Buildings: How Indoor Spaces Drive Performance and Productivity.”

It’s not something people like to talk about, but there’s a potential important source of covid-19 spread that deserves some attention: toilets.

Studies are showing that the novel coronavirus can be detected in stool samples and anal swab samples for weeks. In fact, scientists are testing wastewater as an early tracking system for outbreaks. And a recent case on an airplane identified the airplane bathroom as the potential source.

Here’s what we know: When you flush a toilet, the churning and bubbling of water aerosolizes fecal matter. That creates particles that will float in the air, which we will now politely call “bioaerosols” for the rest of this article.

Surprisingly little has been published on toilets and disease risk (well, maybe not surprisingly). But there are a few brave scientists out there. Take one 2018 study of flushing toilets in a hospital. Researchers found high concentrations of bioaerosols when a toilet with no lid was flushed.

How many airborne particles, exactly, are we talking about? Upwards of 1 million additional particles per cubic meter of air. Not all of the aerosols generated will carry the virus, of course. But, let’s be clear: When you flush the toilet, you’re breathing in toilet water, and whatever is in that toilet water — including viruses and bacteria.

The study also found that bioaerosols spread around the room and lingered in the air. In fact, basic aerosol physics tell us these bioaerosols will stay aloft until one of four things happen: they settle out onto surfaces, they are removed through dilution, they are removed through filtration or they are removed by your lungs.

When large bioaerosols settle onto surfaces in the bathroom, they create the potential for fomite transmission. (A fomite is any inanimate object, such as a countertop or door handle, that, once contaminated, can act as a source of transference to another person.) Most aerosols, however, will be small and will linger in the air.

This is why bathroom exhaust ventilation is so critical. The design standard for bathroom exhaust requires approximately one cubic foot of air removed per minute for every square foot of bathroom. So, a 7-by-10-foot bathroom requires about 70 cubic feet of exhaust per minute. This equates to roughly eight air changes per hour, meaning the full volume of air in the bathroom is replaced every 7.5 minutes.

The toilet issue could extend beyond your own bathroom. During the first SARS epidemic in 2003, there was evidence of an outbreak in an apartment building linked to sewage. The cause was empty U-traps in drains. Normally, these have a little bit of water in them that stops vapors. Investigators found that some apartments had drains with empty U-traps, which allowed bioaerosols from a sick patient to waft through the building’s pipes and into the air of other apartments.

The Chinese government recently reported something similar for the novel coronavirus. Investigators detected viral particles in a long vacant apartment on the 16th floor of an apartment building, one floor above where a sick patient lived. This set off an investigation, leading to the discovery of aerosols generated from flushing in the 15th-floor apartment reaching the 25th and 27th floors of the building, where two people on each of those floors had previously been recorded to have had covid-19.

So, what to do with all of this information to protect yourself? First, bathroom ventilation is your best defense. If you have an exhaust fan, turn it on when you enter the bathroom and leave it on when you’re done. If that fan is not on, or you don’t have one, crack open any windows (even just a few inches will help).

Second, make sure your U-traps aren’t dried out. An easy to way to check this is the simple sniff test. Do you smell sewer gas smell coming out of your shower drain or your sink drain? If so, that’s telling you your U-trap might be empty.

Some other common sense — though still worthy — tips: Flush the toilet when you’re done (this is mostly for parents with children). Flushing removes nearly all of the fecal matter, but, be mindful, some will remain on the walls of the toilet.

Close the lid. Research has shown that lidless toilets increase the risk of bacteria and larger particles escaping into the air. These settle out onto adjacent surfaces, creating a fomite hazard.

Clean and disinfect bathroom surfaces, and wash your hands vigorously after using the bathroom.

Last, and perhaps most importantly, trust your nose. Don’t go in if it smells bad. That’s your first hint that the air hasn’t been exhausted properly. Wait to let the room air out a bit.

The fecal-oral route is a known transmission pathway — I was worried enough to write about it in early February. While we don’t have a proven case of such transmission for covid-19, there’s no need to wait for the science to clear the air on this. You can take matters into your own hands and clear the air yourself.

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