Editor’s Note (10/7/20): This article is being republished because it contains information relevant to the vice presidential debate that will be held tonight in Salt Lake City. Vice President Mike Pence and Senator Kamala Harris of California will debate each other from behind plexiglass screens. But if someone was infected, these barriers would not prevent the potential spread of the virus in tiny droplets called aerosols.

President Donald Trump tested positive for the novel coronavirus last week, just days after attending the first presidential debate with former vice president Joe Biden at Case Western Reserve University and the Cleveland Clinic’s Health Education Campus. While it is unclear if the president was infected at the time, people with the virus can be contagious for two to three days before developing symptoms. According to some news reports, Trump may have had mild symptoms as early as Thursday. His wife Melania Trump attended the debate and has also tested positive for the virus.

SARS-CoV-2, the virus that causes COVID-19, is now known to spread through the air—and the risk is greatest indoors, particularly in crowded or poorly ventilated spaces. Not much is known about the ventilation at the debate hall in Cleveland, and Donald Trump and Biden appeared to be roughly 12 feet apart onstage. “Based on what we know about the virus and the safety measures we had in place, we believe there is low risk of exposure to our guests,” the Cleveland Clinic said in a statement. Neither of the candidates, nor moderator Chris Wallace of Fox News, wore a mask during the debate itself, however. Further, the candidates were supposed to be tested before the event, but Wallace has said Trump and some members of his family arrived too late, so an “honor system” was used. Many of Trump’s family members took their masks off once seated at the debate.

It is thus possible that the president was contagious during the debate and could have infected Biden, Wallace or others. Biden’s campaign said the former vice president tested negative on Sunday. But SARS-CoV-2 can take up to two weeks to incubate, so he is not yet in the clear. On Saturday, former New Jersey governor Chris Christie—who has said he helped Trump prepare for the debate in a room with several people who were not wearing masks—revealed he had also tested positive for the virus.

Scientific American spoke with Linsey Marr, a professor of civil and environmental engineering at Virginia Tech, who is an expert on airborne virus transmission, about the conditions in which Trump or his family could have infected Biden or anyone else at the debate.

[An edited transcript of the interview follows.]

Is there a potential risk that Trump could have transmitted the novel coronavirus to Biden or others at the debate last week?

Sure. I think the biggest potential risks, assuming Trump was infected at the time, are to the people nearest him. That would be Biden and Wallace. They were, I think, more than six feet away. So they were out of range, generally, of the large droplets that fly like cannonballs or spitballs through the air. But when someone releases those, they also release hundreds to thousands of times as many microscopic droplets that we call aerosols. Those behave like cigarette smoke: they can stay floating in the air for a long period of time and travel much farther than six feet. The fact that Trump was talking for much of the 90 minutes means that he was potentially releasing a lot of virus in aerosols into the air, and these could then spread like cigarette smoke throughout the room. If the room is well ventilated, the virus aerosols would be removed. But if the ventilation is poor, they could build up in the air, perhaps to a high enough concentration that someone else would breathe in a lot of them.

Do we know anything about the ventilation conditions in the hall where the debate was held?

We do not know. I know people have been trying to request that information from the Cleveland Clinic and Case Western.

Even though it’s a large building, is there still a potential risk from aerosols accumulating because you’re not getting outside air mixing in as much?

Right. If there wasn’t sufficient ventilation, the aerosols could build up. It was a large space, so, you know, that’s a plus. That means there’s more opportunity for dilution. But it was a 90-minute event—probably a couple of hours by the time you get everyone in there and everyone leaves. And certainly, the longer the exposure time, the greater the chance to breathe in more virus.

The president was not just talking but often shouting or forcefully exclaiming during the event. Would that have increased the risk?

Yes, there have been studies showing that the louder you talk, the more aerosols you release into the air.

That result sort of comports with what we’ve seen in some of these “superspreading” events in nightclubs or noisy environments, right?

Yeah, or singing—noisy environments where people are talking loudly. There was an outbreak in a call center in South Korea. There, all the employees were talking. The most hazardous [situations] involve multiple people talking simultaneously. For example, if you’re at a bar or an event where there’s group singing, like the choir practice [in Skagit County, Washington State, where dozens of people were infected in March] or the call center. [At the debate] there was really only one person speaking, so that reduces the risk. But in this case, there’s a possibility he was infected at the time.

Do most modern buildings have some air filtration?

They do have some kind of filtration. The quality can vary widely—anything from pretty ineffective to a filter that can remove 80 percent of particles in the air.

Could Trump’s family members—many of whom did not wear masks while seated—have infected anyone else?

You can release virus just by breathing. That was shown in a study that was published a couple of weeks ago—up to a million virus RNA copies per hour. So certainly, I would say that people sitting near them could be at greater risk also.

Could wearing a mask have protected audience members against infection if there were aerosols in the room?

Yeah, even a fairly simple cloth mask should reduce your exposure by at least half. We don’t know exactly what the relationship is between the amount you inhale and your chances of becoming infected. But if the mask reduces your exposure by half—the amount that you breathe in—your chances of getting infected are probably reduced by roughly half, too.

Are there any other factors that might play into the likelihood that Biden could have been exposed?

I think the direction that Trump was facing when he was talking would matter. If he were facing directly toward Biden when he was talking, that would increase Biden’s exposure. Imagine it’s this plume of cigarette smoke, or puffs coming out of [Trump’s] mouth, and they’re going in a certain direction. Eventually, they do slow down—usually within six feet or so. But they can still go a little bit beyond that.

Have there been any instances of coronavirus aerosols appearing to travel farther than six feet indoors?

Absolutely. There was an outbreak at a restaurant in China earlier [this year], where clearly people became infected by breathing in virus from the air at distances greater than six feet. It’s highly likely that happened in the Skagit [County] choral outbreak. Cigarette smoke doesn’t stop at six feet. It keeps going, and it will spread out. And that’s how we should be thinking about the virus behaving in [the debate]. This emphasizes the importance of wearing a mask—even when you’re able to maintain social distance.

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