The world's filled with radiation. It's unavoidable. But at higher altitudes - say, in an aeroplane - you're hit with around 100 times more cosmic radiation from space. You can't dodge it, but you can measure what you're soaking in.
The FAA's nifty little Galactic Radiation Calculator is decently easy to use. Plug in your approximate flight date, origin and destination airport codes, cruising altitude and flight time, and it'll spit out the amount of space radiation you were hit with while reading SkyMall.
For example: a trip from DC to LA would rack up 27.05 microsieverts - about 27 times the amount you're zapped with when you get a bone x-ray. If you're a frequent traveler (say a dozen flights a year), you could end up with a quarter of your normal ground exposure just from being in the air.
This isn't anything to get in a panic about - again, it's unavoidable. It's nature. The only alternative is to wear a suit of lead armour every day of your life. But it's important to be informed. [FAA]