He says people like to think that there's some sharp, black-and-white distinction between "airborne" viruses that can linger and float in the air, and ones that only spread when embedded in larger moist droplets picked up through close contact, but the reality of transmission is far more nuanced.
"The epidemiologists say if it's 'close contact' then it's not airborne. That's baloney," he says.
When epidemiologists are working in the field, trying to understand an outbreak of an unknown pathogen, it's not possible for them to know exactly what's going on as a pathogen is spread from person to person, Milton says. "Epidemiologists cannot tell the difference between droplet transmission and short-range aerosol transmission."
He says these are hard questions to answer, and scientists still argue over how much of the transmission of influenza might be airborne. Some research shows that exhaled gas clouds from people contain a continuum of many droplet sizes and that a "high-momentum cloud" created by a cough or sneeze might carry droplets long distances.
What's more, one study of hospital rooms of patients with COVID-19 found that "swabs taken from the air exhaust outlets tested positive, suggesting that small virus-laden droplets may be displaced by airflows and deposited on equipment such as vents."
Another study in Wuhan hospitals found that most areas had undetectable or low levels of airborne virus.
In the face of this uncertainty, Milton thinks the WHO should follow the example of the CDC and "employ the precautionary principle to recommend airborne precautions."
"The U.S. CDC has it exactly right," he says, noting that it recommends airborne precautions for any situation involving the care of COVID-19 patients.
Of course, the world is struggling with a shortage of the most protective medical masks and gear. For the average person not working in a hospital, Milton says the recommendation to stay 6 feet away from others sounds reasonable.
He says if someone in a house is sick, it makes sense to have them wear a mask and to increase the ventilation in the room, if possible, by cracking open a window. People shouldn't cram into cars with the windows rolled up, he says, and officials need to keep crowding down in mass transit vehicles like trains and buses.
2/2
