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.