Understanding radiation exposure.

In the days after an 8.9-magnitude earthquake rocked Japan on March 11, the world's attention turned to the state of the Fukushima Daiichi nuclear power plant. For the first time since Chernobyl, the mainstream media has been talking seriously about radiation levels. Here are a few basics to help you put the relative risks of radiation exposure into perspective.

First, when we talk about human radiation exposure, we're specifically discussing "ionizing radiation." This is high-energy radiation that's powerful enough to break or modify chemical bonds, and cause tissue and chromosome damage in the human body.

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While many of the sources of radiation in our lives produce lower-energy, or "non-ionizing radiation" (such as microwaves and radio waves), we're also exposed to very low levels of ionizing radiation every day. That's why, when calculating risk, experts talk about exposures above "background levels." These include radiation doses from environmental radon, as well as cosmic radiation reaching Earth from space.

Here's a short list that shows how selected radiation exposures compare.

Radiation Source                          mrem *

Traveling 1,000 miles by plane                 1 (one time)
Natural cosmic radiation at sea level         24 (annual)
  Natural cosmic radiation in Denver          50 (annual)
  Environmental radon                        200 (annual)
Average total exposure of U.S. resident      620 (annual)
Natural background sources                   310 (annual)
Medical procedures & consumer products       310 (annual)
CT scan (head and body)                    1,100 (one time)
Limit for U.S. nuclear workers             5,000 (annual)
Japan's limit for nuclear workers         25,000 (annual)
Radiation levels measured near
  Fukushima evacuation zone                1,600 (annual)
Peak radiation inside damaged
  Fukushima reactor                       40,000 (per hour)

For more information, go to www.epa.gov/radiation. You'll also find a
helpful visual depiction of the various exposures at
xkcd.com/radiation.

* Radiation exposure to humans is often measured by a unit known as
"roentgen equivalent man," or rem. A millirem (mrem) is 1/1000th
of a rem.

Sources: Agence France Presse, Scientific American, U.S. Department of
Energy, U.S. Environmental Protection Agency, U.S. Nuclear Regulatory
Commission, Wall Street Journal