Ed Yong writes:

“I don’t wear masks like him,” President Donald Trump said during Tuesday night’s presidential debate, deriding his opponent, former Vice President Joe Biden. “Every time you see him, he’s got a mask.” But at nearly 1 a.m. eastern time today, Trump announced that he had tested positive for the SARS-CoV-2 coronavirus, which causes COVID-19.

As president, Trump’s dire mishandling of the U.S. pandemic response has contributed to the deaths of about 200,000 Americans and at least 7.2 million infections. But now that he himself is sick, one has to wonder: How many people has Trump personally infected? And could Biden be one of them? Encouragingly, Biden tested negative for the virus today, but “a negative test doesn’t say he’s completely in the clear,” says Angela Rasmussen, a virologist at Columbia University. Several factors suggest that he could have been exposed to the virus during the debate, and should continue to take precautions for at least another week.

First, there are the conditions of the debate itself. The coronavirus mostly spreads through the air, traveling from the nose and mouth of an infected person in either large, wet particles (droplets) or smaller, drier ones (aerosols). Most droplets fall to the ground within six feet of their source, and Trump and Biden were clearly standing farther apart than that. But “aerosols behave like cigarette smoke and don’t stop at six feet,” says Linsey Marr, who studies airborne-disease transmission at Virginia Tech. “Imagine Trump was smoking the whole time. Would Biden have been exposed to some of that?”

When thinking about COVID-19 transmission, there are no absolutes, only probabilities. The distance between Trump and Biden lowered the odds of infection: The farther Trump’s aerosols traveled, the more dilute they would have become. But almost everything else about the debate increased the risk that those aerosols could have found their way into Biden’s nose. People release about 10 times more aerosols when talking than when breathing silently, Marr says, and even more when talking loudly. Trump certainly did that—for 90 minutes, in an enclosed space, without wearing a mask, and often in Biden’s direction. “It wasn’t a one-off cough by someone in the audience,” says Joseph Allen, an environmental-health expert at Harvard University. “It was one and a half hours of constant emissions.” These are the same conditions that make bars and restaurants such risky venues for COVID-19 transmission. [Continue reading…]