Q. I’m a frequent flyer. Is it true that airplane flights can expose me to a lot of radiation?

A. When you're on the ground, and especially at sea level, the earth’s atmosphere does a pretty good job shielding you from exposure to atmospheric radiation. But when you're flying at 30,000 to 40,000 feet, the thinner atmosphere at that altitude does increase radiation exposure, according to the Federal Aviation Administration. But no one has determined whether frequent flyers face any increased health risks from that exposure.

“There is no definitive proof of any harm, although no real data exists” on cancer rates in frequent flyers versus similar people who don’t travel by air, according to radiological physicist Robert Barish, Ph.D., who has authored several papers on the topic.

And a 2010 report suggested that even if you fly more than 85,000 air miles per year, although your radiation exposure might be more than the average person’s, it won’t exceed safety limits for nuclear power plant workers.

A variety of factors influence radiation exposure when flying, including the flight's altitude and its route. According to NASA, flights over the poles are exposed to the most radiation. The amount of radiation is also affected by the number of sunspots on the face of the sun and the intensity of solar storm activity.

According to the U.S. Nuclear Regulatory Commission, about half of the radiation we’re exposed to comes from natural background radiation and half is from man-made sources.

Natural background radiation includes radiation from radon, a colorless, odorless radioactive gas that’s linked to 21,000 lung cancer deaths each year. It's found in soil and as well as some water and even the air. The other main source is ultraviolet radiation from the sun, which is linked to skin cancer. (See our reviews of radon test kits and sunscreens.)

Most all of the man-made radiation we’re exposed to is from medical procedures such as CT scans and X-rays. Cell phones also expose you to a small amount of radiation, though the effect of exposure to cell-phone radiation is unclear.