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April 16, 2008 > TechKnow Talk: How much radiation is too much?

TechKnow Talk: How much radiation is too much?

On Thanksgiving Day, 2006, 44-year-old Alexander Litvinenko lay dying of radiation poisoning in a London hospital bed. A former Russian spy and outspoken critic of the Putin government, he had unknowingly ingested a massive dose of polonium-210, possibly at the hands of a murderer.

Such acute radiation exposure is rare, though some victims of the Nagasaki and Hiroshima atomic bombs in 1945 and the Chernobyl nuclear reactor accident in 1986 suffered similar levels of exposure resulting in severe, in some cases fatal, radiation sickness. More pertinent for most of us is a basic understanding of the various forms of radiation, the ways in which we encounter it in our daily lives, and how much long-term exposure is safe.

Radiation refers to the entire breadth of the electromagnetic spectrum. Low frequency radiation, such as radio waves, microwaves, and visible light are not damaging to living beings in the same way as high frequency radiation such as X-rays and gamma rays. While microwaves or visible light may cause burns in large concentration (such as an oven or a laser), these low frequency waves do not have sufficient energy to break atoms into smaller pieces.

Radiation capable of stripping electrons and other components from atoms is called ionizing radiation, because an atom that has lost an electron is an ion. When most people talk about the dangers of radiation exposure, they are referring to this high frequency, high-energy ionizing radiation. Because it is capable of causing this fundamental change in the atoms that comprise human tissue, including cellular DNA, it poses a health danger. Hereafter, the term "radiation" refers to ionizing radiation.

There are three types of ionizing radiation: alpha, beta, and gamma. Like visible light, gamma radiation is pure energy; it has no mass. It is very energetic and penetrating, similar to an X-ray. It is likely to pass completely through a human, potentially doing some damage in the process. Alpha and beta particles do have mass (alpha particles are thousands of times heavier than beta). They are not as fast or as penetrating as gamma, but because they carry electrical charge, they also ionize atoms. Instead of passing through the body as gamma does, giving up little bits of energy along the way, they are likely to dump all their energy into whatever they first encounter.

These three forms of radiation are natural byproducts of the decay of radionuclides, or unstable atoms, as they transform themselves to more stable configurations. We are continuously bombarded with radiation from a variety of sources. Gamma rays are generated in our sun and many other sources throughout the universe. The earth's magnetic field and atmosphere intercept most of this cosmic radiation, but some of it gets through, and unless we are deep in a cave, we are bathed in a shower of gamma rays every day.

There are also natural sources of radiation on earth. Rocks and soil contain radionuclides that generate alpha, beta, and gamma radiation. These radionuclides find their way into our air, food, and water, and we all carry some of these radiation sources in our bodies.

All this naturally occurring radiation, taken together, is called background radiation. The level of background radiation varies substantially from place to place on the surface of the earth. For example, someone living at an elevation of 6,000 feet is exposed to more cosmic radiation than a seaside dweller. Radiation from the earth, primarily from a gas called radon, also varies dramatically with location.

In addition to background radiation, we are all exposed to manmade radiation, from X-rays and other medical treatments, the production and transport of nuclear fuel, smoke alarms, tobacco, televisions, and a host of other sources. Roughly 15-20 percent of the average person's exposure is attributable to these manmade sources.

When living cells are damaged, they attempt to repair themselves. But they are sometimes damaged beyond their ability to make effective repairs, or the chemical mechanisms controlling their growth or rate of replacement are disrupted. When this happens, a cell may continue to reproduce damaged versions of itself. Such unchecked growth is, in simple terms, cancer.

Thus, long-term exposure to radiation can increase the chance of developing cancer. High levels of radiation exposure are suspected to increase the risk of various blood and bone cancers, as well as cancers of the thyroid, lung, liver, bladder, stomach, colon, and other organs. Due to their greater rate of natural growth, fetuses and young children are at higher risk of radiation-induced cancer than adults.

Since many radionuclides are unstable forms of elements the body requires, it may be fooled into storing them in organs such as the thyroid, liver, etc. Once a radionuclide that generates alpha or beta particles enters the body, typically with food or water or by inhalation from the atmosphere, it may be placed in an organ where it can continue to irradiate a localized area, increasing the risk of cancer in that region. The polonium-210 that killed Litvinenko produces alpha particles. Had he survived the radiation poisoning, he would likely have developed cancer.

By the way, some scientists believe that a certain level of radiation exposure is healthy. It exercises the body's ability to repair itself, just as exposure to viruses and germs exercises the immune system. In addition, radiation causes mutations when it strikes a reproductive cell and the damaged DNA is passed to offspring. Though this may not be a good thing for any specific individual, it is one of nature's methods of ensuring the variability of genetic material and spurring evolutionary development of the species as a whole.

For most of us, the background (naturally-occurring) radiation comprises at least 80 percent of our total exposure. This background exposure has probably not changed dramatically for thousands of years. About 50 percent of total radiation exposure, on average, derives from radon, though individual levels of radon exposure vary greatly based on geographical location. Radon is a naturally-occurring gas that can cause lung cancer. While radon concentrations tend to be highest in northern and Midwestern states, some localized areas here in the Bay Area may have unhealthy levels.

So we are all subjected to constant radiation from above, from below, and even from within our own bodies. Is our total amount of radiation exposure "safe?" It seems to be, in the sense that moderately increasing exposure doesn't appear to increase cancer risk.

For example, the Nuclear Regulatory Commission (NRC) allows workers in the nuclear industry to receive more than 10 times (1,000 percent) the total radiation to which the average person is exposed. This would logically place these workers at higher risk of cancer than the rest of us. But there is no clear statistical evidence that this is the case. The NRC is relying on studies that indicate there is a "threshold" of radiation exposure below which the average person will not see a measurably increased risk.

However, not all scientists agree with these studies. Suppose in a population of 10,000 nuclear workers, 35 would die naturally of cancer each year. But because of their increased radiation exposure, 37 died of cancer one year, 34 the next, and 38 the next. It is very difficult to attribute those extra deaths to radiation, since they are well within a normal statistical variance.

To further complicate the research, there are no "average" people. Some of us are genetically pre-disposed to certain types of cancer. Why some people get cancer and others do not is far from well understood, but radiation exposure is clearly only one of many factors, including genetics and diet, and is probably not a very significant factor in most cases.

While no amount of radiation exposure can be considered completely safe, there are probably many more important things to worry about. Certainly, we can avoid unnecessary X-rays and consider testing our homes for radon. But we have limited control over our total exposure levels, and the amount of radiation most of us live with today seems to pose health risks not much different than those our ancestors faced.

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