Have you ever heard of a Röntgen Equivalent Man (rem)? It is a unit that scientists use to measure the biological effect from exposure to radiation. In other words, how much damage is it going to cause?
A typical commercial flight from New York to L.A. will expose you to 2-5 millirem (mrem).
REM In Perspective
A chest x-ray exposes you to about 10 mrem.
It is estimated that you will absorb about 620 mrem a year. About half of this comes from natural "background radiation" that is hitting us from space.
Heightened Exposure
Although a lot of our radiation exposure comes from space, we are protected from most of those harmful solar rays. Even if you aren't wearing sunscreen, our own atmosphere serves as a great protective shield. The thicker the atmosphere, the stronger the protection.
That is why when we fly at high altitudes, we are exposed to more radiation! There is less atmosphere between us and outer space.
Insecure About Security?
2-5 mrem may not seem like that much during a flight, but it is actually 2-5 thousand times more radiation than you get passing through security.
The x-ray technology used at more airport security stations only exposes you to .001 mrem.
It would take 25,000 of these x-ray scan over the course of a year to reach the safe limit for rem, according to FDA standards.