Flying involves inherently dangerous elements—complex mechanical equipment, harsh environments, crowded airspace. Air & Space reader Jon Day asks about an additional risk for frequent fliers: radiation exposure.

Because Earth's atmosphere shields us from cosmic radiation—the denser the atmosphere, the greater the protection—flying in the upper, thinner portion of the atmosphere increases your radiation exposure. The amount doubles with every 6,000 feet of altitude.

Day asks: "For typical altitudes in international flying, how much flying is too much?" Two organizations—the U.S. National Council on Radiation Protection and Measurements and its global equivalent, the International Commission on Radiological Protection—have established the radiation tolerance limits for air crews, the general public, and fetuses. A single, long international flight will expose you to a week's worth of natural background radiation. That's far from a health concern, even for pregnant women. But aviation workers can easily exceed the groups' recommended limits. For example, a pregnant flight attendant working a London-to-Chicago route for just 100 hours (about 12 trips) would exceed the safe exposure for her fetus. For air crews, the limit is 20 millisieverts (mSV) a year. (For scale, a person at ground level gets about 2.4 mSv of natural background radiation a year.)

The Federal Aviation Administration says these maximums are suggested, not mandated. The FAA's Advisory Circular 120-61, from 1994, recommends these limits for an air crew member: a five-year average dose of 20 mSv per year, with no more than 50 mSv in a single year. The European Union, using nearly the same standards, by law requires tabulations of exposure for air crew and limits the exposure to 20 mSv per year and 1mSv for fetuses.

In 1997, researchers from NASA's Langley Research Center in Virginia flew a modified Lockheed ER-2, a variant of the U-2 spyplane, with a spectrograph to measure the radiation exposure at altitudes between 52,000 and 70,000 feet. "Even though the exposure levels are higher at the higher cruise altitude, the typical flying public will actually receive less radiation exposure than on today's subsonic transports because of the higher speed," the researchers said.

The Concorde was the only commercial aircraft equipped with radiation dosimeters. It was an abundance of caution—the typical exposure can be calculated before the supersonic transport takes off. France's Civil Aviation Department takes the guesswork from the levels by operating an online calculator that can assess the level of exposure to those on board.

A solar flare can raise radiation levels. U.S. Department of Energy scientists extrapolated the exposure of a high-flying supersonic aircraft during the strongest known solar event—a 1956 solar flare—and concluded that the flare likely would have boosted exposure to 10 mSv per hour. If you're worried, consult with the solar weathermen at the Space Environment Center before flying.

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