SAN FRANCISCO—The skies are pretty friendly after all—at least as far as the quality of cabin air is concerned. A recent study found no increase in the incidence of upper respiratory infections (URIs) among passengers traveling on commercial flights that used recirculated air ventilation.[1]

RECIRCULATED VERSUS FRESH AIR

In the early 1980s, to increase fuel efficiency, aircraft manufacturers began building ventilation systems that recirculated cabin air. Newer airplane models recirculate as much as 50% of cabin air, which decreases the engines’ work.

The current study evaluated the use of recirculated air as a predictor of postflight URIs. The investigators recruited passengers flying from San Francisco or Oakland, California, to Denver between January and early April of 1999.

Boeing 737s and 727s were chosen for the study because these aircraft have similar seating density, cabin setup, airflow patterns, and fuselage size. However, 737s recirculate about 50% of cabin air, whereas 727s use 100% fresh air. DC-10s were also included in the study, with equal samples of planes using fresh or recirculated air.

Participants completed a questionnaire in the boarding area before flying, and they agreed to participate in a telephone interview five to seven days after flying. The postflight survey asked about possible URI symptoms during the week after flight; it also inquired about the participants’ knowledge and beliefs regarding air recirculation on airplanes and the risk of URIs during flight.

Three different definitions of a URI were used in the study. The least restrictive definition was self-report of a cold. The next most restrictive was self-report of a cold and a runny nose. The most restrictive was a score higher than 13 on the Jackson questionnaire. (The Jackson score is the sum of the scores for eight respiratory symptoms; each symptom is rated on a scale of 0 to 3.)

Data were available for 1,100 people. A total of 516 traveled on airplanes with fresh air ventilation and 584 traveled on airplanes that used recirculated air. Passenger density did not differ significantly between aircraft types.

NO DIFFERENCE IN URI

There were no significant differences for any outcomes. Self-reported colds occurred in 21% of passengers in airplanes with fresh air and in 19% of passengers in airplanes with recirculated air. Likewise, self-reported colds with a runny nose occurred in 11% and 10% of participants, respectively. Jackson scores higher than 13 occurred in 3% of both groups; this incidence is consistent with previous reports of URI in people who did not fly.

Jessica Nutik Zitter, MD, MPH, Assistant Clinical Professor of Medicine at the University of California, San Francisco, and the principal author of the study, said that the findings were unanticipated. “We were expecting to see an increase in URIs.”

NO EVIDENCE OF A PROBLEM

Russell B. Rayman, MD, MPH, Executive Director of the Aerospace Medical Association in Alexandria, Virginia, said in an interview, “In my opinion, the transmission of contagious diseases like URIs is a problem of person-to-person contact. This kind of contact occurs in an enclosed space. I do not believe the ventilation system is the cause of the transmission,” he said. “It is caused by proximity, and that can happen anywhere.”

The American Medical Association issued a report in 1998 on airborne infection during commercial flights. According to the report, several studies conducted by Consolidated Safety Services and the Department of Transportation in the late 1990s found that levels of bacteria and fungi on airplanes were lower than those found in public buildings.[2]

In a 1997 review,[3,4] Dr. Rayman cited several studies of air quality aboard commercial aircraft: One study of 92 flights revealed that levels of respiratory particulates, carbon monoxide, and ozone were well below acceptable industry standards. “A lot of studies have been performed, and most of them show that there is nothing to get alarmed about,” Dr. Rayman remarked. “By and large, there is no evidence that the cabin air is not clean.”

Likewise, the Air Transport Association studied 35 flights from eight US airlines that had airplanes using both types of ventilation. The organisms detected in this study were normal skin flora; no respiratory pathogens were found.[3]

In its brochure “Useful Tips for Airline Travel,”[5] the Aerospace Medical Association notes that recirculated air in airplanes passes through a high-efficiency particulate air (HEPA) filter before reentering the cabin. There is a total change of air in the cabin every two to three minutes. The HEPA filter has a 99% rate of efficiency in filtering out particles larger than 0.3 µm. Most airborne particles are larger than this, and although viruses can be smaller than 0.3 µm, they usually form colonies that cannot pass through the HEPA filter.[3]

WHY THE BAD IMAGE?

There is a negative image of the air quality on airplanes, admitted Dr. Rayman. Various factors may contribute to it. “Passengers might feel that the air is stale when they enter the cabin, or they think they smell something. Then they have a headache when they get off the plane. There is an inclination to assume a cause and effect.”

The attention given to the air quality issue by the media hasn’t helped, either. “There have been a lot of news stories about flight attendants who are complaining of an array of symptoms,” he continued. “I am not saying their symptoms aren’t real—but everyone assumes it is due to something in the air.”

Concern about aircraft cabin air quality and the transmission of pathogens is a fairly recent phenomenon, said Dr. Rayman. “Fifteen or 20 years ago, no one even talked about it.”

Transmission Isn’t Impossible

Despite the low levels of bacteria and fungi on airplanes, there have been cases of airborne transmission of infectious diseases among airline passengers. In 1977, a strain of influenza (A/Texas) was transmitted to 72% of interviewed passengers and crew—all of whom became ill—from a single passenger who had developed flu symptoms shortly after boarding the airplane.[1] It is noteworthy, however, that because of a mechanical problem, the plane had been on the ground for three hours, during which time the passengers had not been permitted to deplane, and the ventilation system had been turned off. The high rate of transmission was attributed to the presence of a contagious person in an enclosed, unventilated space for a prolonged period. There have also been several investigations of tuberculosis (TB) transmission aboard aircraft. Risk factors for transmission included duration of the flight, proximity to the infected person, and infectiousness level of that person. In one study, the lack of passengers with TB skin test conversion in cabin sections other than the one in which the person with TB was seated provided evidence that TB was not transmitted through the aircraft’s air recirculation system.[1] —Gale Jurasek

Reference 1. AMA Council on Scientific Affairs. Report 10. Airborne infections on commercial flights. June 1998. Accessed August 12, 2002.

—Gale Jurasek

References
1. Zitter JN, Mazonson PD, Miller DP, et al. Aircraft cabin air recirculation and symptoms of the common cold. JAMA. 2002;288:483-486.
2. AMA Council on Scientific Affairs. Report 10. Airborne infections on commercial flights. June 1998. Accessed August 12, 2002.
3. Rayman RB. Passenger safety, health, and comfort: a review. Aviat Space Environ Med. 1997;68:432-440.
4. Air Transport Committee of the ASMA. Medical Guidelines for Airline Passengers. Alexandria, Va: Aerospace Medical Association; 1997.
5. Aerospace Medical Association. Useful Tips for Airline Travel. 2001. Accessed August 12, 2002.

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