A University of California, San Francisco study has found that recycled air on short-term flights is no more likely to cause colds than flights using fresh air. The researchers interviewed 1,100 passengers before and one week after an approximate two-hour flight and reported that the levels of those with colds and colds with runny noses were virtually the same between recycled air and fresh air flyers.

In the last two decades, airplane manufacturers have elevated re-circulation levels to as high as 50% of all cabin air in an effort to reduce the engines' workload, thus increasing the planes' fuel-efficiency. Recycled air is filtered before being mixed with fresh air, but the UCSF researchers concede such filters are limited in their ability to capture viruses, and completely ineffective if the filters become clogged.

Their findings may ease common passenger concerns about recycled air containing viruses, though other factors on flights not included in this study may contribute to infectious disease, such as stress, lack of sleep, unhealthy eating, and confinement to a small, crowded space.

The Journal of the American Medical Association 2002;288:483-486