Safety thresholds for radiation have not fully taken into account long-term effects, especially those resulting from low-level exposure for extended periods of time. Whereas the effects for higher levels of exposure, such as those from nuclear weapons have been monitored, far less attention has been accorded to effects which may, for example, only appear to result in a higher vulnerability to leukaemia.
There is a direct, proportional correlation between exposure to penetrating, or ionizing, radiation and cancer risk. In short, no radiation exposure can be considered free of risk.
Circumstantial evidence indicates that some forms of low level radiation may be beneficial. Some tests on laboratory animals exposed to low levels of radiation indicate that they tend to live longer. In regions where the natural radiation is high (India, China, Brazil) there is some indication that such levels of radiation offer protection against cancer.