There are two ways to measure radiation; the Gray, which measures the amount absorbed, and the sievert, which is used to calculate the risk. As the sievert is a very large measure, risk is usually calculated in millisieverts (msv).

Of course there is radiation in the environment naturally, but the levels are usually harmless. To give some idea, if you get an X-ray from your dentist, that is about 0.01 msv, and you need a measure of 100 msv per year to have an increased risk of cancer (the recommended limit for workers is 100 per year). The radiation levels at the Fukiyama nuclear plant, however, at one point reached 400 msv per hour. This sounds deadly, but a single dose of 1,000 msv, while it would cause radiation sickness, would not usually be fatal. The workers who died after the Chernobyl disaster in 1985 often recorded levels of 6,000.