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.