The advent this spring of severe acute respiratory syndrome (SARS) struck fear into air travelers everywhere. Passengers have long complained of catching colds on planes, but concerns about SARS made an old debate about the healthiness of cabin air take on new importance.
It's true that the cabin environment is quite unlike most places in which you spend time on the ground. Most significantly, planes are pressurized, "for your comfort," to an altitude of 8,000 feet. In the early years of commercial aviation, air was brought from outside the plane and compressed by its engines. In the process, the air became very hot, and cooling it burned a considerable amount of fuel. In the late 1960's, jet manufacturers developed a new generation of engines along with a more fuel-efficient air system that mixed outdoor air with air recirculated from the cabin. Today, air on most newer jets is continually recirculated, at a rate of 25 to 30 times an hour, and passed through high-efficiency particulate air (HEPA) filters.
The medical consensus is that there's nothing about the quality of cabin air itself that makes you more susceptible to illness—whether the common cold, the flu, or even tuberculosis—on a plane than you would be in any other shared, confined space. "A number of studies over the years tend to indicate there aren't major health issues associated with cabin air," says Dr. Jon Jordan, federal air surgeon and head of the Federal Aviation Administration's Office of Aerospace Medicine.
Among those studies is one published last year in the Journal of the American Medical Association that found no evidence that recycling cabin air increases passengers' risk of developing an upper respiratory infection. The research team surveyed 1,100 people flying from the San Francisco area to Denver in early 1999. About half the group (47 percent) flew on planes using outdoor air ventilation, and the other half (53 percent) traveled on aircraft that mixed outdoor and recirculated air. Both groups reported similar rates of suspected illness: 19 percent on the flights using outdoor air believed they had developed a cold, compared with 21 percent on the mixed-air flights. What's more, because the actual rate of severe symptoms for both groups, 3 percent, corresponded to the incidence of such symptoms in the overall population at that time, the results also suggest that, in general, flying may not significantly increase one's risk of catching a cold.
Why?In part it's because planes are climatized to a low level of humidity, between 5 and 15 percent, which isn't conducive to bacterial or viral survival. Second, many common illnesses are not airborne —that is, spread through normal breathing—so infectious droplets aren't usually floating around the cabin like invisible miniature hot-air balloons. Even if airborne viruses or bacteria were introducedon board, they would most likely be caught by the plane's filtration system. According to Boeing, its HEPA filters capture between 94 and 99.9 percent of microbes. So there's little chance that a sick passenger in, say, Row 10 could infect a passenger in Row 20.
Nevertheless, the Association of Flight Attendantswants the level of outdoor air in planes increased. "With this amount of recirculated air, you have a higher amount of carbon dioxide because of exhaled air," says Christopher Witkowski, the AFA's director of air safety, health, and security. Although there is no demonstrable link between disease transmission and a lower level of oxygen, some suggest that it may aggravate existing medical conditions such as pulmonary disorders.
In December 2001 the National Research Council (NRC), an arm of the National Academy of Sciences, released a study of airline cabin environments, conducted at the request of Congress. One of its recommendations was that the FAA require airlines to deboard passengers when the ventilation system on a grounded plane is off for more than 30 minutes.
As anyone who has sat in the dead air of a delayed plane can attest, the atmosphere is exceedingly uncomfortable. It could also present a health hazard, since the air is not being recycled and no fresh air is coming into the plane. The FAA issued an advisory to airlines in January, asking them to follow the same guidelines proposed by the NRC.
The NRC also recommended that the FAA require the airlines to monitor cabin-air quality on an ongoing basis. Johns Hopkins University is developing equipment for this purpose, and prototypes may be tested on planes later this summer. The American Society of Heating, Refrigeration & Air Conditioning Engineers is also establishing a set of cabin-air standards that it hopes the FAA will use as a benchmark.
Despite the good news that cabin air itself does not pose major risks for spreading illnesses, the fact remains that travelers come into close contact with a wide variety of people on flights, sometimes over long periods of time. Indeed, this proximity is the culprit in the vast majority of in-flight disease transmissions, according to Dr. Harriet Burge, a professor at Harvard University's School of Public Health and co-investigator of an unpublished study of irritants, allergens, and pathogens on commercial aircraft.