Understanding Radiation Exposure

How much radiation is too much? What are the sources of the radiation we face on a daily basis? Get the facts on radiation exposure.

| August/September 2011

Nuclear Power Plant

Radiation exposure from nuclear power plants has been a heightened concern worldwide following the earthquake and subsequent tsunami that shut down several nuclear power plants in Japan earlier this year and led to higher-than-usual levels of 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.

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 and 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)

* 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. 

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