Studying How Germs Spread on a Plane
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(Inside Science) -- It’s flu season and the passenger four rows behind you is coughing. How likely is that you or other people on the plane will catch the flu?
The answer is: not very.
A widely held belief about flying is that everyone’s germs circulate in the stagnant air of a jetliner and there is a good chance many people on board will catch something. But research done by scientists at Emory University and the Georgia Institute of Technology, both in Atlanta, suggests the fear is greatly overstated.
There have been at least a dozen cases of people on transoceanic flights infecting other passengers with serious diseases, including multiple cases involving SARS, which is, like flu, a respiratory disease. In January, an Indiana University student infected with measles may have infected people traveling from the Memphis, Newark and Detroit airports.
But, unless you are sitting within one or two rows of the sick passenger, the chances of you catching anything is about 3 percent, so most of the passengers are safe, according to a study published March 19 in the journal Proceedings of the National Academy of Sciences. If you are next to that passenger, it’s more like 80 percent.
The Georgia researchers concentrated on influenza, and most of the flights in the experiment were during the flu season, said Howard Weiss, a statistician at Georgia Tech and Emory.
Influenza is an airborne respiratory disease, now fading after a long season in the U.S. It is transmitted mainly by droplets from coughs, sneezes, or simply breathing. Most people are infectious for days before the symptoms break out. The infectious zone around infected people is about a meter, or three feet.
“Someone called it the two-by-two rule,” said Charles Gerba, an environmental microbiologist at the University of Arizona in Phoenix, who has also done research on passenger jets and was not involved in the new study. “Two seats in front of you and two people on either side.”
In the experiment, ten public health or nursing students sat in the economy section of 10 flights, mostly Boeing 757 narrow-body, single aisle aircraft, for flights between Atlanta and West Coast cities.
The researchers reasoned that passengers could catch the flu in three main ways: sitting within the infection zone, walking by the subject on their way to the bathroom, or having the subject walk by them. To account for these potential sources of spread, the students tracked the movements of cabin crew and passengers.
To address the risk to passengers that stayed seated throughout the flight, the researchers estimated that people sitting in the same aisle or in a row or two in front or behind an infected passenger would be in a roughly one meter zone where infection transmission is most likely. They also looked for passengers who were coughing.
“Of 1,500 passengers on the 10 flights, only one person was coughing,” Weiss said. “For some reason, that surprised me.”
One student sat in the back of each flight, where the air is driest, with two air monitors, sampling cabin air.
Thirty-eight percent of all the passengers never left their seats during the flight; 38 percent left once, 13 percent twice, and 11 percent more than twice. Passengers in the window seats moved scarcely at all, while aisle seat passengers moved the most, probably a reflection of which seats the passengers chose. The lavatory was the main destination.
The researchers also tracked one cabin attendant per flight. Flight attendants who are ill are unlikely to show up for work. The likelihood that they could spread diseases if they worked while ill is a good reason for flight attendants who are not feeling well to stay home, the researchers wrote.
While the air right next to a sick person may be infectious, the health danger of the air in the cabin as a whole is an urban myth, according to the researchers. The air inside a modern jet is constantly being drawn from outside the aircraft, circulated, and filtered, and it is completely replaced 20 times an hour. And the airflow is ceiling-to-floor, so you are not getting the air from people in other parts of the plane, Gerba said.
One weakness in the system is that the humidity in most planes is kept at 20 percent, which makes it more habitable to pathogens. One exception is the newer Boeing 787 Dreamliner, which has moister air.
In addition to breathing in infectious droplets, passengers could be getting sick from touching seat backs, tray tables or faucets in the bathroom. Experts agree that the dirtiest part of an airliner is the tray table, which is why many passengers bring disinfecting tissues and wipe them off. Lavatories are next. Gerba’s studies showed fecal matter on the door handles.
The Georgia researchers did not find germs on the tray tables as Gerba’s research has, but they only sampled parts of the tables while Gerba sampled the whole table.
Weiss said their model did not apply to diseases such as tuberculosis, measles, or chicken pox, which ride on tiny droplets too small for flu viruses. But for the flu, their model may have actually over-estimated the risk of infection, because it did not include the possibility that seat backs provide a barrier to the virus, he said.
The scientists also wrote that the longer the flight, the more people moved around, so the odds may change somewhat.
If you are still worried, the new study and past research suggests that your best bet is to sit by the window, not get up, sterilize the tray table and avoid the toilet.