Electromagnetic radiation comes all over the electromagnetic spectrum. Frequencies above a certain point are ionizing radiation; below it are nonionizing radiation. Ionizing radiation causes cancer and other cellular damage. Nonionizing radiation has heating effects, unless it's at athermal levels. There is ongoing debate about whether athermal doses of nonionizing radiation (e.g. from cellular phones or high-tension power lines) can still cause cancer. See also microwaves.

Surprisingly little is known about the effects of ionizing radiation on the human body. There are various theories such as the linear no-threshold hypothesis, most of which are likely bogus. The only thing that can be said for sure is that you'll die if you receive a dose in excess of several hundred rem (which, by the way, stands for roentgen equivalent mammal).

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