Randall Munroe created this chart, which puts radiation exposure in perspective. He looks at how much radiation humans receive from eating a banana (.1 μSv), having a dental x-ray (5 μSv), or flying from New York to Los Angeles (40 μSv), as well as spending an hour outside of Chernobyl (6 mSv).
Since nearly all our foods are (slightly) radioactive, you have to wonder why scientists, or whomever, chose bananas? Maggie Koerth-Baker explained on BoingBoing that the so-called "banana equivalent dose" was created to put radiation exposure in perspective. Unfortunately, she writes:
The Potassium-40 in bananas is a particularly poor model isotope to use, [Geoff Meggitt, the author of Taming the Rays,] says, because the potassium content of our bodies seems to be under homeostatic control. When you eat a banana, your body's level of Potassium-40 doesn't increase. You just get rid of some excess Potassium-40. The net dose of a banana is zero.
Much of the radiation exposure after the Chernobyl accident came from iodine-131 in contaminated milk—and officials in Japan are not only withholding milk from around Fukushima, they're also administering non-radioactive potassium iodide (KI) to inhibit that radioactive iodine from being taken up by the thyroid gland. So, clearly, as Munroe points out in his chart, "If you're basing radiation safety procedures on an internet PNG image and things go wrong, you have no one to blame but yourself."