Japan's Fukushima Daiichi nuclear reactor accident has focused new attention on how much ionizing radiation people are exposed to from different sources (see list below). By far the largest source is medical imaging technology (see "Graphic Science: Exposed" in the May 2011 issue). Americans, on average, are exposed to 3.1 millisieverts of radiation a year from natural background factors such as radon gas from the Earth and cosmic rays from the universe. Safety experts recommend the public receive less than one millisievert a year beyond that level, although they do not include medical procedures in that limit because the procedures may bring health benefits. Here's a list of common sources.

Average Radiation Dose to Entire Body (millisieverts)

Natural background (U.S.) per year: 3.1

Airport scanner (backscatter method): 0.0001

Natural gas cooking per year: 0.0004

Arm x-ray: 0.001

Bone density x-ray: 0.001

Highway travel per year: 0.004

Dental x-ray: 0.005

Domestic airline flight (five hours): 0.017

Smoking one pack of cigarettes per day for a year: 0.36

Mammogram: 0.4

Fukushima emergency workers per hour: 1.0

Brain CT scan: 2.0

Thyroid scan (nuclear medicine): 4.8

Brain scan(nuclear medicine): 6.9

Pelvis CT scan: 10

Coronary CT angiography: 16

Astronaut on space station for one year: 72

Sources: National Council on Radiation Protection and Measurements; RadiologyInfo.org