Does everyday background radiation raise cancer risk?

This page is for anyone wondering whether everyday background radiation causes cancer. It covers where that radiation comes from, what EPA says about the risk, and where attention is best spent.

Not dramatically, and not because anything has gone wrong. Radiation is a permanent feature of living on this planet. It comes from several places:

• Up through the ground, from uranium and thorium in soil
• Down from space, as cosmic rays
• Out of the ground and building materials
• From inside your own body, where naturally radioactive minerals sit in your bones and tissues

The EPA puts the average yearly dose for a person in the United States at 6.2 millisieverts (a unit of radiation dose). That is 620 millirem in the older units. About half of that comes from natural background sources. The other half comes from medical procedures. These figures come from the EPA’s “Radiation Sources and Doses.” The source I opened gives the split only as roughly half and half.

EPA's own wording is careful and worth reading in full.

Ionizing radiation has enough energy to affect the atoms in living cells and damage their genetic material. Cells usually repair that damage well. But a repair that goes wrong can lead to cell death or to cancer. That is the mechanism EPA describes.

And then: exposure to low levels of radiation encountered in the environment does not cause immediate health effects, but is a minor contributor to our overall cancer risk.

Minor contributor. Not zero, not major. That phrase is precise, and it is the most accurate summary available.

EPA sets its protective standards using the linear no-threshold model. It assumes that cancer risk from a low dose rises in step with the dose. There is no threshold below which risk disappears. Halve the dose and you halve the risk.

People encounter this and conclude that any radiation at all is dangerous. That is not what it means.

The model is a deliberately cautious tool for regulation. Measuring the effect of very small doses directly is close to impossible. Any effect is buried under the huge number of cancers that happen for every other reason. Faced with that uncertainty, the protective choice is to assume risk continues in a straight line down to zero, rather than to assume it stops.

Assuming the cautious case for the purpose of writing rules is not the same as claiming it is proven.

EPA gives a figure that puts personal risk in perspective: about 99 percent of individuals would not get cancer as a result of a one-time uniform whole-body exposure of 100 millisieverts or lower. That is a dose many times the annual average.

Then it gives the other side. Among one million people, even a one percent increase could result in 10,000 additional cancers.

Both are true. The first is why you personally should not lose sleep over background radiation. The second is why an agency still writes rules about it. Thinking about a whole population and thinking about one person give different answers here. Neither is wrong.

If this topic concerns you, the useful action is not worrying about cosmic rays — it is radon. EPA lists radon and thoron (radioactive gases from the ground) first among natural background sources. Radon is a part of your background dose you may be able to check at home. Ask your state radon office or see the radon page on this site.

The second is medical imaging, which EPA says makes up about half of the average dose. That does not mean refusing scans. It means it is reasonable to ask whether a particular scan is needed and whether an alternative exists. Good clinicians are used to that question.

Everything else in the 6.2 millisieverts is essentially the price of existing on Earth, and it is not a price you can negotiate.