By Ed Hewitt

updated 11:39 a.m. ET Oct. 8, 2007

Many travelers would swear that they get sick after every trip or vacation. They wonder if it was the food, the water, the pina coladas — or, like me, the airplane ride. While I don't think you can count out the pina coladas (or that burrito you bought on the street), it turns out you would be right about airplanes.

Airline carriers are also formidable carriers of the common cold; a recent study says you may be more than 100 times as likely to catch a cold on a plane as in your normal daily rounds, according to a study publishing in the Journal of Environmental Health Research: Common cold transmission in commercial aircraft: Industry and passenger implications.

Why this is the case isn't perfectly clear, but the publishers of the study investigate a panoply of possible causes, including close quarters, shared air and, as I will explain, the most likely culprit: extremely low cabin humidity.

On to the numbers
The study found that "When the scenarios of 6 days, 24 hours, or 5 hours were taken as the relevant flight exposure times to colds, passenger transmission rates for colds of 5, 23, and 113 times the normal daily ground level transmission rate were obtained." (Ominously, transmission rates for tuberculosis were also found to be dramatically increased as well.)

Thus, the common perception that flying causes colds seems to be based in fact — maybe even 113 times over.

The culprit: Low cabin humidity
The study runs through several potential sources of higher transmission, but settles primarily on a single likely cause: extremely low cabin humidity caused by low humidity at high elevations. (A review of the study reveals the conclusion that aircraft that actively recirculated air actually showed slightly lower transmission rates than those that did not.)

Most commercial airlines fly in an elevation range of 30,000 to 35,000 feet, where humidity typically runs at 10 percent or lower. At very low levels of humidity, the "natural defense system" of mucus in our noses and throats dries up and is crippled, creating a much more tolerant environment for germs to infect us.

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This protective system, called the Mucociliary Clearance System, is your first line of defense against harmful germs and bacteria. To wit, if the common cold is pounced on by a sufficiently moist and percolating proboscis and throttled by your throat, you remain uninfected. Shut down those systems, and you'll be suffering within days.

Tips to avoid the airline carrier cold
1. Stay hydrated. It turns out that drinking plenty of water will not only counter the overall dehydrating effects of air travel, which can lead to headaches, stomach problems, cramps, fatigue and more, but can actually fortify your preemptive natural immune mechanisms to function considerably better. As your body dries out, your nose and throat go with them. Of course, this is the case in normal daily life — when exercising, during prolonged sun exposure, etc.; even caffeine and alcohol consumption can dry you out. However, in an airplane, where your nose and throat are on the front lines of the war with exceedingly dry air, these are the first places to suffer; most travelers have felt the sting of a brutally arid mouth, throat and nose in flight.

Sipping water or some other fluid regularly throughout the flight may be more effective than drinking a lot of water at one time before or during the flight; this will keep your protective system from long dry spells. (And we do mean to single out water here — as noted above, alcohol and caffeinated drinks such as coffee or sodas can actually dehydrate you.)

Nasal mists have been found to be very effective in keeping this system working in your nose. Additionally, hot drinks are a good way to keep your protective mucous membranes working —first, to assist in keeping you generally hydrated; second, by triggering the system into gear; and third, by directly providing moisture in the form of steam. Note that this is not a treatment per se; rather, it just keeps your defenses strong and functioning.

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