The principal health hazard from exposure to ionizing radiation is the possible development of a cancer in the exposed tissue or organ at some point later in life. At low levels of exposure, the risk of this occurring is very low, but the risk increases in proportion to the amount of exposure. Based on epidemiological studies, the increased risk of developing a fatal cancer due to radiation exposure has been estimated to be about 5% per Sv. In Canada, the probability of developing a fatal cancer (from all causes) during a lifetime is about 25%.Therefore, a dose of 100 mSv results in an increased risk of about 0.5%, increasing the probability of developing a fatal cancer from all causes to 25.5%. Radiation is considered to be a relatively weak carcinogen.
For doses below 100 mSv it is more difficult to detect a difference between exposed and unexposed populations because of the high background incidence of cancer. The conservative approach accepted by many international organizations is to assume that the proportional relationship between radiation exposure and risk can be extrapolated down to extremely low doses; this is called the Linear No-Threshold (LNT) assumption.