The debate over whether something is “airborne” is particularly sensitive around pathogens that cause the most acute, deadliest outbreaks. But there’s not even agreement among experts as to how regular old influenza transmits through the air. Those who say the flu does this well point to a curious incident from the 1970s in which an airplane with 54 passengers was grounded on the tarmac for three hours because of engine issues during a takeoff attempt. There was one person who had been ill onboard; and within three days, three-quarters of the other people who had been on the plane showed symptoms of flu such as cough, fever and fatigue. The majority of those tested were positive for the virus. Donald Milton, whose research at the University of Maryland School of Public Health includes studies of infectious bioaerosols, says that all these years later he and his peers are still trying to convince other scientists that influenza is substantially airborne. He published a paper in 2018 asserting that, contrary to what some might think, sneezing and coughing are not required for influenza virus to be released in an aerosol form that can float around.

Meanwhile, the aerodynamics of more exotic pathogens have stirred controversy. One infectious disease expert warned, in 2014, that Ebola might become highly transmissible by air. This proved to be a false alarm. There is some evidence that coronaviruses such as SARS and MERS can travel in hospital air. Some researchers still dispute these data: the MERS research, for example, did not use a hospital room without infectious patients as a control. But others take it as a given that these coronaviruses were floating in their infectious form around parts of hospitals.

As for the airborne behavior of the new coronavirus, scientists are racing to obtain data. A study published in the Journal of the American Medical Association on Mar. 4 looked at the hospital isolation rooms of three patients in Singapore with Covid-19. The study offered some solace because it didn’t find evidence of the virus in air samples. However, the air vent blades in one patient’s room did test positive. A second study, described in a preprint paper published on Mar. 10, examined the hospital environments of Covid-19 patients in Wuhan, China. Although the levels of the microbe that causes Covid-19 in most rooms were undetectable or low, the study did find the presence of the virus in aerosol form. That there would be non-negligible amounts of virus in the air does not surprise Linsey Marr, a researcher at Virginia Tech who studies the dynamics of viruses in the air. “This is exactly what I suspected,” she says. Even before that paper came out, she’d told me it’s “unfortunate” that the WHO insists on saying that the new coronavirus “is not airborne.”

Crucially, the hospital studies only looked for the genetic signature of the virus, as opposed to mixing the viral material with animal cells to see whether it would wreak havoc. As such they could not know whether the viral material present in the ventilation system or the air was infectious. This is a critical point—virologists emphasize that the presence of residual RNA or DNA left by pathogens in no way guarantees that people might get sick from it. However, the question of whether the new coronavirus is infectious as an aerosol was explored in another paper posted as a preprint this week. In that study, scientists used a laboratory machine to force the virus into aerosolized form and then tracked it for 3 hours. They found the pathogen was still able to infect animal cells at the end of that timeframe, although there was substantially less of virus suspended in the air from one hour to the next.