Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Biological Effects of Ionizing Radiation

As a science educator, I find it frustrating that nowhere in the massive amount of journalism devoted to the Chernobyl disaster does one ever find any numerical statements about the amount of radiation to which people have been exposed. Anyone mentally capable of understanding sports statistics or weather reports ought to be able to understand such measure-ments, as long as something like the following explanatory text was inserted somewhere in the article:

Radiation exposure is measured in units of millirems. The average person is exposed to about 100 millirems each year from natural back-ground sources.

With this context, people would be able to come to informed conclusions based on statements such as, "Children in Finland received an average dose of ___________ millirems above natural background levels because of the Chernobyl disaster."

A millirem, or mrem, is, of course, a thousandth of a rem, but what is a rem? It measures the amount of energy per kilogram deposited in the body by ionizing radiation, multiplied by a "quality factor" to account for the different health hazards posed by alphas, betas, gammas, neutrons, and other types of radiation. Only ionizing radiation is counted, since nonioniz-ing radiation simply heats one's body rather than killing cells or altering DNA. For instance, alpha particles are typically moving so fast that their kinetic energy is sufficient to ionize thousands of atoms, but it is possible for an alpha particle to be moving so slowly that it would not have enough kinetic energy to ionize even one atom.

Notwithstanding the pop culture images of the Incredible Hulk and Godzilla, it is not possible for a multicellular animal to become "mutated" as a whole. In most cases, a particle of ionizing radiation will not even hit the DNA, and even if it does, it will only affect the DNA of a single cell, not every cell in the animal's body. Typically, that cell is simply killed, because the DNA becomes unable to function properly. Once in a while, however, the DNA may be altered so as to make that cell cancerous. For instance, skin cancer can be caused by UV light hitting a single skin cell in the body of a sunbather. If that cell becomes cancerous and begins reproducing uncontrollably, she will end up with a tumor twenty years later.

Other than cancer, the only other dramatic effect that can result from altering a single cell's DNA is if that cell happens to be a sperm or ovum, which can result in nonviable or mutated offspring. Men are relatively immune to reproductive harm from radiation, because their sperm cells are replaced frequently. Women are more vulnerable because they keep the same set of ova as long as they live.

A whole-body exposure of 500,000 mrem will kill a person within a week or so. Luckily, only a small number of humans have ever been exposed to such levels: one scientist working on the Manhattan Project, some victims of the Nagasaki and Hiroshima explosions, and 31 workers at Chernobyl. Death occurs by massive killing of cells, especially in the blood-producing cells of the bone marrow.

Lower levels, on the order of 100,000 mrem, were inflicted on some people at Nagasaki and Hiroshima. No acute symptoms result from this level of exposure, but certain types of cancer are significantly more common among these people. It was originally expected that the radiation would cause many mutations resulting in birth defects, but very few such inherited effects have been observed.

A great deal of time has been spent debating the effects of very low levels of ionizing radiation. A medical x-ray, for instance, may result in a dose on the order of 100 mrem above background, i.e. a doubling of the normal background level. Similar doses in excess of the average background level may be received by people living at high altitudes or people with high concentrations of radon gas in their houses. Unfortunately (or fortunately, depending on how you look at it), the added risks of cancer or birth defects resulting from these levels of exposure are extremely small, and therefore nearly impossible to measure. As with many suspected carcinogenic chemi-cals, the only practical method of estimating risks is to give laboratory animals doses many orders of magnitude greater, and then assume that the health risk is directly proportional to the dose. Under these assumptions, the added risk posed by a dental x-ray or radon in one's basement is negli-gible on a personal level, and is only significant in terms of a slight increase in cancer throughout the population. As a matter of social policy, excess radiation exposure is not a significant public health problem compared to car accidents or tobacco smoking.

Discussion Questions

A Should the quality factor for neutrinos be very small, because they mostly don't interact with your body?
B Would an alpha source be likely to cause different types of cancer depending on whether the source was external to the body or swallowed in contaminated food? What about a gamma source?

Last Update: 2009-06-21