Ionizing radiation can be either acute, large dose, or low dose over time. In one there is acute radiation sickness, and in the other there is chronic exposure. In both molecular bonds in biological tissues may be disrupted and caused injury. Fetuses and young children are more sensitive to the effects because they are biologically very active.
There is a 5% of getting cancer if one is exposed to 1 Sievert. The effects are linear: twice the dose, twice the risk.
The EPA reports that cosmic radiation at sea level (from outer space) gives 26 mrem annually. From air, (radon) 200 mrem annually, From food and water, (e.g., potassium) 40 mrem annually, for a total of 260 mrem annually or 13 rem over 50 years.
Here is a table showing scale of different amounts of radiation.
Another EPA report suggests that the annual dose is more like 600mrem because of the exposure in the medical setting. The New England Journal of Medicine reports that as many as 4 million patients get more than 50 mS annually, the limit for health care workers.
The Fukushima disaster has been thought to lead to a 30 % increase in thyroid disorders on the West Coast among the newborn. There has been 200 fold increase in the amount of radioactive Iodine on the West Coast.
- New map of radioactive iodine released from Fukushima Daiichi (enformable.com)