SARS-CoV-2 dose and aerosols

"For SARS, also a coronavirus, the estimated infective dose is just a few hundred particles. For MERS, the infective dose is much higher, on the order of thousands of particles.

The new coronavirus, SARS-CoV-2, is more similar to the SARS virus and, therefore, the infectious dose may be hundreds of particles, Dr. Rasmussen said.

A higher dose is clearly worse, though, and that may explain why some young health care workers have fallen victim even though the virus usually targets older people.

And coronavirus patients are most infectious two to three days before symptoms begin, less so after the illness really hits.

Coughing, sneezing, singing, talking and even heavy breathing can result in the expulsion of thousands of large and small respiratory droplets carrying the virus.

“It’s clear that one doesn’t have to be sick and coughing and sneezing for transmission to occur,” said Dr. Dan Barouch, a viral immunologist at Beth Israel Deaconess Medical Center in Boston.

Larger droplets are heavy and float down quickly — unless there’s a breeze or an air-conditioning blast — and can’t penetrate surgical masks. But droplets less than 5 microns in diameter, called aerosols, can linger in the air for hours.

“They travel further, last longer and have the potential of more spread than the large droplets,” Dr. Barouch said.

Three factors seem to be particularly important for aerosol transmission: proximity to the infected person, air flow and timing.

A windowless public bathroom with high foot traffic is riskier than a bathroom with a window, or a bathroom that’s rarely used. A short outdoor conversation with a masked neighbor is much safer than either of those scenarios.

Recently, Dutch researchers used a special spray nozzle to simulate the expulsion of saliva droplets and then tracked their movement. The scientists found that just cracking open a door or a window can banish aerosols.

“Even the smallest breeze will do something,” said Daniel Bonn, a physicist at the University of Amsterdam who led the study.

Observations from two hospitals in Wuhan, China, published in April in the journal Nature, determined much the same thing: more aerosolized particles were found in unventilated toilet areas than in airier patient rooms or crowded public areas.

This makes intuitive sense, experts said. But they noted that aerosols, because they are smaller than 5 microns, would also contain much less, perhaps millions-fold less, virus than droplets of 500 microns.

“It really takes a lot of these single-digit size droplets to change the risk for you,” said Dr. Joshua Rabinowitz, a quantitative biologist at Princeton University.

Apart from avoiding crowded indoor spaces, the most effective thing people can do is wear masks, all of the experts said. Even if masks don’t fully shield you from droplets loaded with virus, they can cut down the amount you receive, and perhaps bring it below the infectious dose."