Severe Acute Respiratory Syndrome (SARS) and avian flu have sickened thousands and reaped headlines in recent years — spiking fears that these and other viral diseases could spread quickly in the enclosed environment of airplanes, where passengers and crew are packed closely together.
Fears of lethal bacteria also went skyward a few years ago, when the Environmental Protection Agency found that some water used to make coffee and tea on commercial airliners was contaminated. Germs in the sky may not be as scary as snakes on a plane, but it’s close enough as far as an apprehensive flying public is concerned.
However, fears of airborne superbugs may be overblown, if new research tracking bacteria on commercial jetliners is correct. Not only were concentrations of nasty microbes low, but aircraft filtration systems used at cruising altitude to keep the air free of contaminants worked effectively, according to the findings.
In fact, the airport air breathed by passengers before boarding and supplied to planes from ground units before the aircraft leave the ground could pose the highest risk to passenger and crew health, the research suggested.
The study was headed by Dr. Lauralynn McKernan of the National Institute for Occupational Safety and Health (NIOSH). Researchers from Harvard University and the University of Massachusetts also took part in the study, which was published in the "Annals of Occupational Hygiene".
McKernan said the research wasn’t done to study the transmission of disease per se, but to assess risk factors in the aircraft environment for aviation professionals — chiefly pilots and flight attendants.
Cabin air in cruise is healthful
The researchers, who boarded 12 Boeing 767 flights operated by two unnamed U.S. carriers, measured bacteria on randomly chosen flights lasting 4 ½ to 6 ½ hours. All told, they took 513 airborne bacterial samples. The results were reassuring in one sense, said McKernan: The cabin air in the middle of flights, when the aircraft reached peak cruising altitude and top speed, was healthful.
For members of the traveling public — worried about the transmission of infectious diseases by breathing “recycled” air on aircraft and remembering colds they picked up from a flight — the findings are somewhat counter-intuitive: Cabin air three or four hours into a long-haul flight may not smell especially fresh but it’s evidently safe. The flights studied used a mix of 50 percent recirculated air and 50 percent fresh air.
McKernan and her team did find higher concentrations of bacteria at two other times: when passengers were boarding the aircraft, and when they were deplaning. The concentrations didn’t pose immediate threats to health, she said.
Possible problems in packed planes
However, said McKernan, the results do suggest that when passengers pick up a bug on their flights, they do it when they’re literally rubbing elbows with other travelers at their seats, or shedding microbes from clothing and skin when they wake up, bustle about and grab bags from the overhead compartment as the aircraft taxis to the airport gate.
Microbe-rich environments that can spur transmission of disease aren’t limited to planes, of course. Similarly crowded moments — the big game, the big dance, the birthday party, the political caucus — pose similar sets of circumstances, and similar risks of getting sick. The flights studied ranged from 67 percent full to 100 percent full. Contamination could prove an especially big problem on the packed planes U.S. carriers are flying now, with their unavoidably close quarters.
The much-maligned air aboard commercial aircraft is the same air that passengers breathe in airport terminals, said Elizabeth Merida, a spokeswoman for the Air Transport Association, which represents major U.S. airlines. The terminal air is pumped into the plane on the ground and filtered in the aircraft.
Ground air is more contaminated
“We have been working with NIOSH for over a decade,” Merida said. “They have some very long-term projects on the occupational health of cabin crews. This research was carried out last summer. What this tells us is where contamination comes in: from the ground air. Ground air is much more contaminated than the air at cruising altitude. The filtration and ventilation systems are very, very effective.”
Lead researcher McKernan agrees.
“When you get up to cruise, then the system works pretty well,” she said. Airports, she said, use ground air units to supply air “to the aircraft without utilizing the filtration capacity of the B767 aircraft. Researchers may wish to focus future efforts on exposures during these ground-based operations.”
The recent study, concentrating on bacteria, largely ignored viruses — the microbes that cause deadly SARS and bird flu — and the annoying but not so deadly common cold. Actually, McKernan’s study did measure concentrations of the cold virus, she said, but by doing so in summer, researchers missed the peak cold seasons when travelers and others are likely to come down with the sniffles.