More than years ago, scientists discovered that many elements commonly found on Earth occur in different configurations at the most basic atom level. These various configurations called isotopes have identical chemical properties, but different physical properties. In particular, some isotopes known as radioisotopes are radioactive, meaning that they emit energy in several different forms.
This energy emission is what we call radiation. Over time, we have come to think of radiation in terms of its biological effect on living cells. For low levels of radiation exposure, these biological effects are so small that they may not even be detectable. In addition, the human body has defense mechanisms against many types of damage induced by radiation. Consequently, radiation may have one of three biological effects, with distinct outcomes for living cells: 1 injured or damaged cells repair themselves, resulting in no residual damage; 2 cells die, much like millions of body cells do every day, being replaced through normal biological processes; or 3 cells incorrectly repair themselves, resulting in a biophysical change.
The exact effect depends on the specific type and intensity of the radiation exposure. In general, however, a 3-millirem exposure imposes the same chance of death — 1 in a million — as each of the following common life experiences:. Search NRC. Report a Safety Concern.
CT scanning and nuclear imaging have revolutionized diagnosis and treatment, almost eliminating the need for once-common exploratory surgeries and many other invasive and potentially risky procedures. The benefits of these tests, when they're appropriate, far outweigh any radiation-associated cancer risks, and the risk from a single CT scan or nuclear imaging test is quite small.
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But are we courting future public health problems? The radiation you get from x-ray, CT, and nuclear imaging is ionizing radiation — high-energy wavelengths or particles that penetrate tissue to reveal the body's internal organs and structures. Ionizing radiation can damage DNA, and although your cells repair most of the damage, they sometimes do the job imperfectly, leaving small areas of "misrepair.
We're exposed to small doses of ionizing radiation from natural sources all the time — in particular, cosmic radiation, mainly from the sun, and radon, a radioactive gas that comes from the natural breakdown of uranium in soil, rock, water, and building materials.
How much of this so-called background radiation you are exposed to depends on many factors, including altitude and home ventilation. But the average is 3 millisieverts mSv per year.
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A millisievert is a measure of radiation exposure; see "Measuring radiation. Exposure to ionizing radiation from natural or background sources hasn't changed since about , but Americans' total per capita radiation exposure has nearly doubled, and experts believe the main reason is increased use of medical imaging. If you mention the measurement of radiation, many people will recall the classic Geiger counter with its crescendo of clicks.
But Geiger counters detect only the intensity of radioactive emissions. Measuring their impact on human tissues and health is more difficult. That's where the sievert Sv and millisievert mSv come in. These units, the ones most commonly used in comparing imaging procedures, take into account the biological effect of radiation, which varies with the type of radiation and the vulnerability of the affected body tissue.
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Taking these into account, millisieverts describe what's called the "equivalent dose. We've long known that children and teens who receive high doses of radiation to treat lymphoma or other cancers are more likely to develop additional cancers later in life.
But we have no clinical trials to guide our thinking about cancer risk from medical radiation in healthy adults. Most of what we know about the risks of ionizing radiation comes from long-term studies of people who survived the atomic bomb blasts at Hiroshima and Nagasaki. These studies show a slightly but significantly increased risk of cancer in those exposed to the blasts, including a group of 25, Hiroshima survivors who received less than 50 mSv of radiation — an amount you might get from two or three CT scans.
See "Imaging procedures and their approximate effective radiation doses. The atomic blast isn't a perfect model for exposure to medical radiation, because the bomb released its radiation all at once, while the doses from medical imaging are smaller and spread over time. Still, most experts believe that can be almost as harmful as getting an equivalent dose all at once. Source: Mettler FA, et al.
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Most of the increased exposure in the United States is due to CT scanning and nuclear imaging, which require larger radiation doses than traditional x-rays. A chest x-ray, for example, delivers 0. And that's not counting the very common follow-up CT scans. In a study from Brigham and Women's Hospital in Boston, researchers estimated the potential risk of cancer from CT scans in 31, patients over 22 years.
For the group as a whole, the increase in risk was slight — 0.
But for patients who had multiple CT scans, the increase in risk was higher, ranging from 2. Unless you were exposed to high doses of radiation during cancer treatment in youth, any increase in your risk for cancer due to medical radiation appears to be slight. But we don't really know for sure, since the effects of radiation damage typically take many years to appear, and the increase in high-dose imaging has occurred only since