It's a common complaint: Fly on a crowded plane and come home with a cold. What's in the air up there?

Air travelers suffer higher rates of disease infection, research has shown. One study pegged the increased risk for catching a cold as high as 20 percent. And the holidays are a particularly infectious time of year, with planes packed full of families with all their presents—and all those germs.

Air that is recirculated throughout the cabin is most often blamed. But studies have shown that high-efficiency particulate air (HEPA) filters on most jets today can capture 99.97 percent of bacterial and virus-carrying particles. That said, when air circulation is shut down, which sometimes happens during long waits on the ground or for short periods when passengers are boarding or exiting, infections can spread like wildfire.

One well-known study in 1979 found that when a plane sat three hours with its engines off and no air circulating, 72 percent of the 54 people on board got sick within two days. The flu strain they had was traced to one passenger. For that reason, the Federal Aviation Administration issued an advisory in 2003 to airlines saying that passengers should be removed from planes within 30 minutes if there's no air circulation, but compliance isn't mandatory.

Much of the danger comes from the mouths, noses and hands of passengers sitting nearby. The hot zone for exposure is generally two seats beside, in front of and behind you, according to a study in July in the journal Emerging Infectious Diseases, published by the U.S. Centers for Disease Control and Prevention.

A number of factors increase the odds of bringing home a souvenir cough and runny nose. For one, the environment at 30,000 feet enables easier spread of disease. Air in airplanes is extremely dry, and viruses tend to thrive in low-humidity conditions. When mucous membranes dry out, they are far less effective at blocking infection. High altitudes can tire the body, and fatigue plays a role in making people more susceptible to catching colds, too.

Also, viruses and bacteria can live for hours on some surfaces—some viral particles have been found to be active up to a day in certain places. Tray tables can be contaminated, and seat-back pockets, which get stuffed with used tissues, soiled napkins and trash, can be particularly skuzzy. It's also difficult to know what germs are lurking in an airline's pillows and blankets.

Research has shown how easily disease can spread. Tracing influenza transmission on long-haul flights in 2009 with passengers infected with the H1N1 flu strain, Australian researchers found that 2 percent passengers had the disease during the flight and 5 percent came down within a week after landing. Coach-cabin passengers were at a 3.6 percent increased risk of contracting H1N1 if they sat within two rows of someone who had symptoms in-flight. That increased risk for post-flight disease doubled to 7.7 percent for passengers seated in a two-seat hot zone.

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