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.

The debate took place inside a spacious atrium at Case Western Reserve University, in Cleveland, and ventilation—how often air is circulated, how thoroughly it is filtered, and where the vents are—would have affected Biden’s risk of catching viruses from Trump. These factors are unclear. Under advice from the Cleveland Clinic, the university detailed several steps to protect the health of participants, including limited attendance, disinfectant measures, and extra space between seats. “But what was notably absent was any mention of healthy building strategies, like filtration and ventilation,” Allen says. “These are things that should be thought about all the time, and certainly in this case.”

Second, there’s the matter of Trump himself. One of the most crucial questions—and the biggest unknowns—is whether he was contagious during the debate.