Checking the CDC’s Math
This week, the Centers for Disease Control (CDC) admitted that Ebola can travel through the air in aerosols, but claims that it can never go more than 3 feet.
Let’s check their math …
But the CDC itself admits that flu droplets can travel 6 feet.
Mythbusters demonstrated that sneezes can nail people some 17 feet away:
“[The study] changes our current ideas of how far germs can spread in aerosols such as coughs or sneezes,” Mary B. Farone, Ph.D., associate professor of biology at Middle Tennessee State University, told weather.com. “We used to think if we could see the spray, that was the limit of the dissemination, but this study shows that tiny particles, such as bacteria and viruses, can be spread much further on gas clouds.”
A novel study by MIT researchers shows that coughs and sneezes have associated gas clouds that keep their potentially infectious droplets aloft over much greater distances than previously realized.“When you cough or sneeze, you see the droplets, or feel them if someone sneezes on you,” says John Bush, a professor of applied mathematics at MIT, and co-author of a new paper on the subject. “But you don’t see the cloud, the invisible gas phase. The influence of this gas cloud is to extend the range of the individual droplets, particularly the small ones.”
Indeed, the study finds, the smaller droplets that emerge in a cough or sneeze may travel five to 200 times further than they would if those droplets simply moved as groups of unconnected particles — which is what previous estimates had assumed. The tendency of these droplets to stay airborne, resuspended by gas clouds, means that ventilation systems may be more prone to transmitting potentially infectious particles than had been suspected.
The researchers used high-speed imaging of coughs and sneezes, as well as laboratory simulations and mathematical modeling, to produce a new analysis of coughs and sneezes from a fluid-mechanics perspective. Their conclusions upend some prior thinking on the subject.
The study finds that droplets 100 micrometers — or millionths of a meter — in diameter travel five times farther than previously estimated, while droplets 10 micrometers in diameter travel 200 times farther. Droplets less than 50 micrometers in size can frequently remain airborne long enough to reach ceiling ventilation units.
A cough or sneeze is a “multiphase turbulent buoyant cloud,” as the researchers term it in the paper, because the cloud mixes with surrounding air before its payload of liquid droplets falls out, evaporates into solid residues, or both.
The study notes:
Our key findings are as follows. The turbulent multiphase cloud plays a critical role in extending the range of the majority of pathogen-bearing drops that accompany human coughs and sneezes. Smaller droplets (less than 50 µm diameter) can remain suspended in the cloud long enough for the cough to reach heights where ventilation systems can be contaminated (4–6 m).
6 meters equals 19.685 feet.
(“µm” means micrometers … i.e. one millionth of a meter. Ebola viruses are only 80 nanometers in diameter, and up to 14,000 nanometers long. A nanometer is one billionth of a meter. As such, Ebola virus filaments can easily fit within a 50 micrometer size droplet.)
While Slate may have gotten the numbers wrong, they made an entertaining video about the MIT study: