For months, experts have hammered home this message: The riskiest place to be during the COVID-19 pandemic is a poorly-ventilated indoor environment with lots of other people, particularly if those people are unmasked. If even one person in such circumstances is infected, an innocent gathering can quickly turn into a super-spreading event.
In a worst-case scenario, Tuesday’s presidential debate could turn into just such a catastrophe, following news of President Donald Trump’s coronavirus diagnosis.
“This incident here with the President is illustrative of what can happen,” says David Edwards, a biomedical engineering expert at Harvard. “This is precisely the kind of thing that we need to be concerned about, and we need to be protecting ourselves better.”
The debate’s host facility, Case Western Reserve University, did a lot of things right, thanks to guidance from the Cleveland Clinic. The candidates—Trump and former Vice President Joe Biden—stayed six feet away from one another and did not shake hands. Spectators’ chairs were spaced apart, capacity was limited to about 100 people and onlookers were screened for the coronavirus before the debate. Attendees were also asked to wear masks, though some—including First Lady Melania Trump, who has also tested positive for COVID-19—removed theirs after entering.
But even with those precautions in place, there’s still the issue of airborne transmission.
Research now strongly suggests people can catch COVID-19 not only through direct contact with large respiratory droplets—such as those expelled when a sick person coughs or sneezes—but also by inhaling tiny aerosol particles that can linger in the air for hours after they’re exhaled. The risks of airborne transmission are highest in crowded indoor environments where people are unmasked and expelling lots of aerosols by talking, singing or yelling. Bars fit that bill—as do, say, debate halls where people are loudly talking over one another, unmasked, for 90 minutes straight.
The big question now, Edwards says, is whether Trump was contagious on Tuesday. Trump adviser Hope Hicks, who accompanied the President to the debate, tested positive for COVID-19 shortly before Trump did, but it’s not clear when the President was exposed. Research has shown that people tend to be most contagious a few days before they develop symptoms. If Trump and his circle were already infectious on Tuesday, his staff and everyone in the room for the debate, from the stage crew to moderator Chris Wallace, is feasibly at risk, Edwards says. (If the room was well-ventilated, the risk to onlookers is substantially lower.)
Biden tested negative for COVID-19 on Friday morning, which is a good sign. However, several days can pass between the time a person is exposed and the time their test will come back positive. A Biden campaign aide told CNN that the risk of transmission was minimal because “we were never near them.”
But that’s not entirely true, says Linsey Marr, a civil and environmental engineering professor at Virginia Tech and an aerosols expert. Enforcing capacity limits and social-distancing measures certainly helps reduce the risk of disease transmission, mainly by lowering the chances of coming into contact with large respiratory droplets. But tiny, airborne particles can disperse through the air like cigarette smoke.
“The idea that distancing alone will protect us is incorrect,” Marr says. “The virus is transmitted in the microscopic droplets. We need to be thinking about that.”
It’s not yet clear whether future presidential debates will go forward as planned. Representatives from the Commission on Presidential Debates did not immediately respond to TIME’s request for comment.
Joe Allen, an aerosol expert who directs the Healthy Buildings program at the Harvard T.H. Chan School of Public Health, says Trump’s test result shows future debates, if they happen, need to rely on more than testing and physical distancing. “We’ve been recommending a layered defense approach, knowing that one strategy in and of itself is not enough,” Allen says. Clearly, testing isn’t perfect. If Trump—or anyone else at the debate—was exposed to the virus shortly before being tested, it may not have shown up in their results. False negative rates are also fairly high for coronavirus tests.
Allen believes future debates can be held safely, but he says organizers need to employ rigorous testing and social distancing protocols, require everyone to stay masked and implement enhanced ventilation and filtration systems to keep air clean. For the general public, he says, the situation should be a reminder that indoor gatherings aren’t a great idea right now.
“Time spent indoors where people are speaking loudly without masks leads to higher risk,” Allen says. “For the general public, these are the exact scenarios that we’ve been urging people avoid.”
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