The study involved more than 1,000 passengers flying between the San Francisco Bay area and Denver during the winter and early spring of 1999. About 19 percent of passengers on planes with recirculated cabin air reported colds one week after the flight. In comparison, 21 percent who flew in planes using only fresh air reported colds. The study was published in the July 24 issue of JAMA, the Journal of the American Medical Association.
"I think there's a fairly universal feeling that flying increases your risk of getting colds and other infections, but air circulation may not be the key issue," said John Balmes, MD, UCSF professor of medicine and chief of the division of occupational and environmental medicine at the UCSF-affiliated San Francisco General Hospital Medical Center (SFGHMC). Balmes is senior author on the JAMA paper.
"Filters used on planes with recirculated air are designed to filter out infectious particles, and they may be doing a good job. It may be that just being on a plane, packed with a lot of people is the primary factor in transmission of colds, or maybe traveling is the issue: changing time zones and losing sleep have been documented to increase rates of viral infections. My best advice is to wash up after shaking hands, and avoid touching your nose."
In the study, passengers filled out a pre-flight questionnaire and were interviewed by phone five to seven days after the flight. The researchers were unable to study passengers taking flights longer than two hours, since planes that do not recirculate air are used almost exclusively on shorter routes.
"It could be that longer flights would result in more exposure and more colds," Balmes said. "We couldn't study that."
Balmes points out that the study measured the rates of colds reported by the passengers. The researchers did not culture viruses to prove the passengers actually had cold virus infections. Analysis of the results revealed that people who believed air travel increases the risk of colds did not report more post-flight colds. Females reported colds more frequently than males.
In the early 1980s, to increase fuel efficiency, aircraft manufacturers began to build ventilation systems that recirculated cabin air. Older systems had used 100 percent fresh air, compressed, humidified and cooled by the engines in an energy-demanding process. Although air recirculation had been shown to increase rates of transmission of cold viruses in army barracks, the issue had not been studied in airplanes.
The study focused on a comparison between Boeing 737s and Boeing 727s, as well as between different DC-10 models. The two Boeing craft have similar seating arrangements, cabin airflow patterns and fuselage size, but 737s recirculate about half of the cabin air, while 727s use 100 percent fresh air. Similarly some DC-10s do and some do not recirculate cabin air.
Passengers were accepted for the study only if they had not flown during the previous week, including a connecting flight on the interview day; they had no plans for additional air travel before the follow-up interview; they did not have a cold when the initial questionnaire was distributed.
Lead author on the study was Jessica Zitter, MD, MPH, who was a post-doctoral fellow in pulmonary medicine and occupational and environmental medicine working with Balmes when the research was carried out.
Co-authors are Peter Mazonson, MD, MBA and Dave Miller, MS, both with The Lewin Group, a healthcare consulting group; Stephen Hulley, MD, MPH, professor and chair of epidemiology and biostatistics at UCSF.
The research was funded by the National Institutes of Health and the National Institute for Occupational Safety and Health/CDC and The Lewin Group.