If you fly you know the drill: Check your luggage. Pass through security. Board the plane and fasten your seatbelt. Try to pay attention to instructions from the flight attendant while you fiddle with the over-head vents to improve airflow in your seating area.

Knowing that many flights re-circulate cabin air, have you ever wondered about the potential health risks of that air?

Just how likely are you to fall sick after cruising at 36,000 feet with folks who are sneezing or wheezing? Many health experts have speculated that using re-circulated air, while fuel-efficient, might spread germs that cause colds.

A study published in the Journal of the American Medical Association suggested that passengers aboard planes that recycle air aren’t more likely to contract a cold than their counterparts on flights that ventilate with fresh air. But flight passengers did come down with substantially more colds than expected, compared with non-fliers.

Scientists questioned more than 1,000 passengers traveling between San Francisco and Denver. A week after flying, 21 percent of those on fresh-air flights and 19 percent of those breathing re-circulated air had colds. In contrast, only three percent of non-travelers developed a cold.

Why the high number of colds?

You can breathe a sigh of relief. Many airlines filter the air as it is re-circulated, so that is taking some germs out of commission. More likely, stress, fatigue, drying of mucus membranes or poor eating habits… all associated with travel… are most likely to blame for airborne colds. In addition, many colds viruses are spread by touch… which is a key factor in close spaces, like airplanes.