We all know that a sneeze or a cough in a crowded bus or train car triggers a quick vector calculation: Am I far enough from the hacker/sneezer to be safe from contagion? Or have I just been showered in germs, with more likely to come?
Based on those calculations, the germ-averse switch Metra seats or shove through clogged CTA bus or train aisles to get as far as possible from the sneezer/cougher.
The same is true for office mates of people who bravely trudge to work with a cold or flu in full bloom, and then insist — insist! — that their doctors said they weren’t contagious. Who doesn’t edge away when these ambulatory germ factories sniffle in a meeting?
This month came more fret fuel for the germ-wary: MIT researchers say that a cough or a sneeze releases a cloud of invisible gas that keeps germ-laden droplets aloft over much greater distances than previously thought.
The researchers used high-speed imaging of coughs and sneezes, lab experiments and mathematical modeling to reach this conclusion in a study published in the Journal of Fluid Mechanics: Those cloud-borne droplets emitted during a sneeze travel five or more times farther — in fact, up to 200 times farther! — than previously believed. Also, small droplets generally travel farther than larger ones.
Bottom line: The next time you hear that sneeze several rows away, and think OK, I’m out of range … think again. And step back. Way back.
You could be quite a way from the offender and still be showered in germs, Lydia Bourouiba, a physical applied mathematician and lead author of the study, tells us. A cough or sneeze — a “multiphase turbulent buoyant cloud” in MIT’s term — mixes with surrounding air “before its payload of liquid droplets falls out, evaporates into solid residues, or both.” Bourouiba says droplets containing pathogens from a single sneeze can span a bus, a train car or an office, depending on air flow and other factors.
The researchers suggest these findings should prompt architects and engineers to re-examine the design of ventilation in hospitals, offices and airports, among other places that airborne pathogens are released and transmitted.
No word yet on whether MIT suggests that Chicago’s Metra designate no-hacking cars, just as it has set aside quiet cars for rush hour. Or, conversely, how about a sneeze-please car for cold sufferers so they can feel free to spew germs. Another possibility: Restaurants, hotels and other establishments may want to set up no-sneeze zones to protect the unsuspecting healthy from their sneezy fellow patrons.
As Mom advised, we should sneeze into tissues (or into the crook of the arm) to cut down on the airborne germ load. Yes, we’re talking about you, Metra and CTA hackers.
Bourouiba says her next project will seek to determine the precise size of droplets emitted in a sneeze, to better estimate how much pathogen is contained in each droplet. That should help scientists better understand how epidemics spread and how people can protect themselves.
We already know one answer: If you’re sick, stay home.
