Many worry about radiation exposure — particularly after Japan’s 2011 Fukushima Daiichi nuclear plant disaster released significant amounts of radionuclides, such as iodine-131 and cesium-137 into the atmosphere and ocean. Increased exposure to radiation may also come form living near a nuclear power plant, flying in an airplane, or from undergoing certain medical procedures, such as X-rays, mammograms or CT scans.
It’s important to remember that radiation is a natural factor in the environment. You are exposed to background radiation in the air, in the soil, in the food we eat, even in our own bodies. In your home, radiation comes from typical construction materials, such as bricks and wallboard, granite countertops and tile floors, through cat litter, or even from the soil or bedrock beneath your house.
On average, 82% of your total annual exposure to radiation is from natural sources, most of that from radon. A wonderful graphic (shown to the left) is from the xkcd website. Note that we’re talking about ionizing radiation, the type of exposure absorbed by body tissue that can potentially damage human cells.
One measure that has been proposed to illustrate natural radiation exposure is the Banana Equivalent Dose (more technically the biologically effective dose) or BED. This refers to the radiation exposure from eating one banana. In fact, all foods are slightly radioactive, but bananas are particularly rich in potassium, which is the major source of natural radioactivity in plant matter. And 0.1% of potassium consists of the radioactive isotope Potassium-40 (40K), which decays with a half-life of 1.25 billion years. For an average banana, that yields about 14 decays per second. This translates into a microscopic amount of radiation, which is quantified as 0.1 micro Sievert or mSv.
Obviously bananas won’t kill you. They won’t set off a Geiger counter; they don’t glow in the dark. There are rumors that crates of bananas have set off radiation detectors at customs — but these are most likely urban legends. Trace levels of radiation are also found in foods such as beans, nuts, seeds and potatoes. Even human bodies are naturally radioactive. Sleeping next to someone for eight hours yields a dose of 0.05 mSv. This works out to half a BED – which seems quite appropriate!
To put it in perspective, fifty bananas would be the equivalent of the radiation dose of a dental X-ray; eating 70,000 bananas would equal one chest CT scan. The yearly dose per person from food is estimated at 400 micro sieverts . The maximum yearly dose permitted for U.S. radiation workers is 50,000 mSv.
Is the BED a valid measure of radiation exposure? Maggie Koerth Baker writes that the potassium-40 in bananas is a poor choice, “because the potassium content of our bodies seems to be under homeostatic control. When you eat a banana, your body’s level of Potassium-40 doesn’t increase. You just get rid of some excess Potassium-40. The net dose of a banana is zero.” But it takes time for the body to remove excess potassium, during which time doses can accumulate.