A detailed explanation and listing of measurements of radiation is at:
http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/part020-1004.html

For a detailed explanation of the linear no-threshold radiation-hypothesis, see this article by the Canadian Nuclear Safety Commission:
http://nuclearsafety.gc.ca/eng/mediacentre/perspectives/linear_no_threshold_model.cfm

To see how our annual dose of radiation is growing, see:
http://radiology.rsna.org/content/253/2/293.full