There are several kinds of ionizing radiation. The two first are so called particle radiation. These different kinds of radiation can all give rise to ions. They have different qualities and carry different health risks. It is used for X-ray examinations and can penetrate the human body.
Any damage depends on the interaction between radiation and living cells.
The general population is exposed to radiation from natural radioactive nuclides. The most important are potassium-40, nuclides of the uranium and thorium series. He works at department of Public Health Sciences, Karolinska Institutet, Stockholm, Sweden.