In this episode of the podcast Social Distance, executive producer Katherine Wells and staff writer James Hamblin discuss the viral-transmission risks of indoor air and the best way to mitigate them.

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What follows is an edited and condensed transcript of their conversation.

Katherine Wells: This question comes from one of our colleagues, Peter Nicholas, who’s our White House reporter. Dear Dr. Hamblin, as an Atlantic reporter covering the White House, I’ll soon be traveling to cover things like Trump rallies. Am I at risk of contracting the virus if I stay at a hotel? Could the virus be passed through the hotel’s HVAC system?

James Hamblin: I thought this would be a really simple question. Before anyone freaks out, the answer is: In the United States, in most developed countries, we have building codes that require that ventilation systems are well maintained and filters are changed regularly, and this should not be a serious concern. But it is also not impossible that [the virus could transfer] this way.

Wells: Please tell us more.

Hamblin: The focus on how this virus spreads has shifted from surfaces to airborne, aerosolized transmission in terms of how important we think things are.

Wells: Which is why we’re very much focused on being outside when gathering right now as reopenings are happening, because the air is moving around a lot more?

Hamblin: Right. If I could use a weird metaphor: When you’re outside, if you cough, it’s sort of like peeing into a river, as opposed to if you’re in a room that’s not well ventilated, it’s like peeing in a hot tub. Ideally, you would do neither, but one of them is definitely worse. The virus aerosolizes, obviously, when someone is coughing and yelling and singing, but also it can come out in things called toilet plumes.

Wells: What is a toilet plume?

Hamblin: So when you flush the toilet, not everything goes down. Some small portion of what was in that bowl gets misted into the air. This is all hypothetical right now, but we know that people shed virus in their feces, sometimes for long periods after they’re no longer even sick.

Wells: Wait, so people who are fully recovered can continue to shed contagious virus through their feces long after they’re recovered?

Hamblin: There is one key word in there you said, which is contagious. We don’t know exactly the dose of virus that you’d need to be exposed to in order to get sick. It’s very unlikely that there would be enough virus from inhaling someone’s toilet plume one time for anyone to get sick.

Wells: Does this mean that shared public restrooms, even if no one is in them while you’re using them, are dangerous?

Hamblin: Well, the answer that I came down with is they’re dangerous if they’re not properly ventilated. It’s exactly for reasons like this that commercial buildings are supposed to have exhaust fans that are constantly running that are pulling air out of the bathroom and blowing it out outside.

Wells: Exhaust fans, ventilation, the filtration system on the building—those are the things that clear the virus?

Hamblin: Yeah. In restrooms, specifically, because we know about toilet flushing, because we know it’s just full of infectious particles of various sorts, so you’re supposed to have a ventilation fan running in there constantly. Even if there is a toilet plume, it’s not hanging out there very long. We should assume that that’s safe.

Where it could be an issue is if that wasn’t working and, obviously, toilets aren’t the only way we aerosolize this virus. And when you have air-conditioning going, you’re basically recirculating a lot of air. So when you recirculate that air, it goes through a filter. There are some filters that are good enough to catch this virus and remove it while it’s being recirculated. In that building you’re much safer than if you’re in a building that is running air-conditioning and hasn’t upgraded its filters appropriately, doesn’t have the best kinds of filters, or doesn’t have a system that can handle those high-quality filters.

Wells: Well, how would a person know if the building they were entering had the kind of filtration that you need or not?

Hamblin: You know, indoor-air quality has been something that a lot of people have been ringing alarms about for a long time. Take, for example, occupational settings like nail salons, where ventilation is really important, and notoriously there are lots of exposure problems that happen in places like that. Building codes are enforced to various degrees in various places. Usually it’s run sort of like getting audited by the IRS. No one’s coming every week and checking to make sure your ventilation system is right.

Wells: Right. It’s just like a spot-check thing.

Hamblin: Yeah. If you really want to save money, you know, you can stretch that filter another month across your various properties. And so there is not a system in place that ensures that this is definitely happening. But it’s a moment where places are already starting to realize that workers and consumers are going to be conscious of this and probably hold them accountable. There are also third-party certification systems.

Wells: Oh, does LEED certification cover this?

Hamblin: Right. We’ve, in the very American way, privatized some of these certification processes.

Wells: What about if I wanted to get my haircut at my neighborhood salon and it’s on the bottom floor of a small building and they have an air conditioner on. Is that the kind of place where you just can’t assume that it’s okay?

Hamblin: It's going to depend where you are from country to country. The places that this has been an issue, with the SARS coronavirus back in 2003, was in Hong Kong, where it was spread within an apartment building to people on different floors. I think when you can sense stagnant air is the best rule-of-thumb advice. When there are places you can tell are mildewy, you can tell they feel stagnant, or there is a smell of chemicals that are lingering in a space—those would be signs that these are not well-ventilated spaces.

To come back to Peter’s question: Here in the U.S., you’re probably going to be staying in some place that isn’t going to smell like mildew and seem like there’s never been airflow through there. So this is much more of an issue for people who are probably working in cramped conditions or places where they don’t have air-quality standards.

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