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Harvard Commentaries
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Reviewed by the Faculty of Harvard Medical School

Medical Myths Medical Myths

Radiation and Your Health

February 27, 2013

By Robert H. Shmerling M.D.

Beth Israel Deaconess Medical Center

Last reviewed February 27, 2013

Radiation is not something to take lightly. It can be dangerous and even deadly.

The atomic bombs that were dropped on Hiroshima and Nagasaki in 1945 are a powerful indicator that excessive radiation can be horribly toxic. At one time, especially during the 1920s, people actually drank radioactive radium for a number of supposed health benefits. This, too, led to deaths.

The effects of exposure to radiation are complex and raise questions about just how serious a threat it is. How justified are people's fears about radiation from microwave ovens, X-rays and cell phones? Where are the real threats — and benefits — of radiation?

The Hazards of Radiation

Radiation is a form of energy that travels as high-speed particles or waves.

Most health concerns have to do with ionizing radiation. This is a type of radiation with enough energy to alter molecules or disrupt atoms, leading to the formation of charged particles (ions).

The most common sources of ionizing radiation are:

  • Natural sources (such as cosmic rays from outer space)
  • Medical tests or treatments
  • Military applications
  • Certain occupations, such as uranium mining, working in a nuclear plant, or health care occupations that involve X-ray tests or radiation treatments

Radar, microwave ovens and cell phones, for example, produce radiation, but they don't generally create ionizing radiation. (Even so, the safety of prolonged exposure to low-level, non-ionizing radiation remains somewhat controversial.)

Exposure to high levels of ionizing radiation is a clear hazard. It can cause radiation poisoning. Symptoms include nausea, hair loss, skin burns and organ damage. It can cause death within days to weeks.

Even exposure to low levels of radiation can cause trouble over the long term. (See below.) It can:

  • Damage cells and alter their ability to repair the damage normally
  • Alter the normal ability of cells to control their growth, which is a key feature of cancer cells. Studies going back to the early 1900s demonstrate that radiation can increase the risk of cancer.
  • Cause damage to DNA, leading to dangerous mutations that may be passed on to one's offspring

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How Is Radiation Measured?

Measuring radiation exposure is a bit complicated and confusing. Its effects depend on:

  • The amount of radiation involved
  • The exposure time
  • The concentration over an area of the body
  • The parts of the body exposed

Relatively high doses of radiation are measured in various units, including roentgen, rads, rem and sieverts (Sv). "Rem" stands for "roentgen equivalent in man," an amount of energy deposited in tissues adjusted for the amount of tissue damage caused by that type of radiation. It's a common way to describe radiation dose (though many experts prefer the international system unit of measure, the Sv). Smaller doses found in more common exposures are measured in thousandths, called millirads (mrad), millirems (mrem) and so on.

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How Much Is Too Much?

It may surprise you to learn that most of us can't avoid radiation exposure. That's because living on Earth carries with it some ionizing radiation exposure.

Natural sources of radiation include cosmic rays from outer space, which increase slightly during air travel, and radioactive substances in the soil.

To get some perspective on this, the average person is exposed to 360 mrem per year. Most of this comes from natural sources. But it's not uncommon for people to be exposed to much more than that, particularly if they have many medical tests. And, according to some international standards, the upper limit of safe occupational exposure is 5,000 mrem in a single year.

Consider these common radiation exposures:



Round-trip air travel from New York City to Los Angeles
3 mrem
Chest X-ray
5-10 mrem
70 mrem
CT scan
1,000 mrem

Now, compare that to these dangerous amounts:

  • 75,000 to 100,000 mrem – the dose that causes symptoms of radiation sickness, including vomiting, diarrhea, mouth soreness, bleeding, weakness and hair loss
  • 400,000 mrem – a dose that will kill 50% of those exposed
  • 600,000 mrem – a lethal dose after which 100% of those exposed die (as calculated after the atomic bombs were dropped on Japan)

While it's possible that some minimal exposure to radiation may be harmless, there is still no official safe amount of exposure that covers all circumstances. But it's impossible to avoid all radiation. And it's very difficult to study the health effects of low-level radiation. The best we can do is limit our exposure as much as possible.

I know people who avoid standing near their microwave oven. That's almost certainly a waste of effort. Unless the oven is malfunctioning or has been modified in some way, modern microwave ovens are remarkably safe and do not emit potentially harmful ionizing radiation. In fact, the biggest danger of a microwave oven is similar to that of other ovens: getting burned if you touch hot or steaming food.

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The Cumulative Effects of Low-Level Radiation

The lethal dose of radiation and doses that cause symptoms are well established and well accepted. But that's not true for the repeated effects of low-level radiation over time. What we know about the risks of tiny amounts of radiation is based on what we know about higher doses. For example, if a large dose of radiation is fatal in 100% of people and a lower dose is fatal in 50% of people,we may assume that a tiny exposure of radiation might kill a proportionally smaller number of people.

This is important for figuring out how much radiation is dangerous for most of us who aren't exposed to large amounts. For example, it's not clear how much radiation related to medical testing might be harmful. A study in 2007 suggested that the rising popularity of computed tomography (CT) scans could account for up to 2% of cancers. This estimate was based largely on the assumption that small amounts of radiation over time (as with multiple CT scans) may cause a small number of deaths.

But that may be wrong. It's largely unproven whether or not cumulative, low-level radiation is dangerous. As a result, the widely held belief that even the smallest amount of radiation may cause cancer or other ailments may be wrong. This is a controversial area among people most interested in the health effects of radiation, including doctors, patients, scientists, the Environmental Protection Agency, industrial producers of radiation, and people who live or work near nuclear power plants.

So what should you do? Anyone exposed to radiation from a medical treatment or a job should weigh the risks of such exposure against the benefits. For example, if a medical test might provide information that will guide lifesaving treatment, the small risk of an X-ray test may be outweighed by the potential benefit. We take particular care, for example, to avoid unnecessary or excessive radiation exposure of the testicles or ovaries as it could damage the DNA of a future fetus.

Low-level radiation over a prolonged period of time may — or may not — be as harmful as a one-time higher dose.

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Radiation as Therapy

Scientific evidence supports the use of radiation to treat certain cancers when used with chemotherapy, surgery or both. For example, radiation is commonly a part of the treatment plan for prostate and breast cancers. In one long-term study published in 2005 among women with high-risk breast cancers (those most likely to recur), adding radiation therapy to surgery and chemotherapy improved survival by more than 30%.

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Can Radiation Be Good for You?

Although it's a controversial, there is at least indirect evidence that low levels of radiation may actually have health benefits. For example, studies suggest that shipyard workers and people who live in western U.S. states — groups with higher than average radiation exposure — have lower cancer rates than people in other states with less exposure. While no one would recommend unnecessary and preventable radiation exposure, these studies call into the question the assumption that radiation is uniformly harmful.

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The Bottom Line

The effects of radiation are complicated and there is much that we do not know about them. Too much can certainly cause harm. But excessive fear of radiation is not warranted either. Some cancer patients turn down radiation therapy because of this fear. In these cases, concerns about the harm associated with radiation may be more harmful than radiation itself.

It makes sense to be careful with radiation. But it may not be as bad as you think.

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Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

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