Answer to Question #806 Submitted to "Ask the Experts"

Q

What are some of the effects of ingesting alpha, beta, or gamma rays? Can you please be specific on the effect caused by alpha rays and the amount of counts need for damage to occur?

A

You have posed an interesting and biologically important question. Unfortunately, the answer is very complex. The simple answer is that all three kinds of radiations produce the same effects on the body. If the dose from ingested material is sufficiently large, indeed very large, observable effects such as reduced blood counts, diarrhea, various cytogenetic effects (for example, chromosome breaks), or interference with function may occur. [In fact, extremely large doses of radiation are medically used to treat a number of diseases including malignancies and certain forms of thyroid disease to destroy or ablate malfunctioning tissue.]

Lower doses would not cause any directly observable effects but could increase the person's likelihood of developing cancer by a small degree many years—20 years, 30 years, or even more—after the exposure.

The specific type of cancer depends on what tissue(s) is(are) irradiated as well as the dose. The slight additional cancer risk is related to the dose, the dose rate, and the type of cancer. Thus it is not possible to say how many counts or alpha particles are needed to produce an effect; what we can say is that each increment of dose increases the chance of developing a radiation-induced cancer, but that the risk overall is very small and very much less than the risk of developing the cancer from other sources. So that is the simple answer.

The more complex and scientific answer requires that we exactly define some terms to eliminate any chance of confusion. Alpha, beta, and gamma are radiations emitted by radioactive substances or, to use the scientific term, radionuclides. Technically, one does not ingest the radiations but rather the radioactive material that produces these radiations. An analogy from everyday life would be onions or garlic, foods which have powerful odors. The odors are analogous to the radiations emitted by a radioactive substance. The onions or garlic which give rise to the odors are analogous to the radionuclide or radioactive substance.

Radioactive elements taken into the body are known as internal emitters because the radiations associated with them are produced directly inside the body. Gamma rays produced from internal emitters by and large escape from the body and only deposit a very small fraction of their energy in the tissues of the body.

Beta particles, which are simply high-speed electrons, deposit all of their energy in a very small volume of tissue; they travel up to perhaps a centimeter or so (about a quarter of an inch) in soft tissue.

Alpha particles have a range or travel distance of about one cell diameter, and thus deposit all of their energy in a very small volume of tissue. The deposition of energy is called absorbed dose and is one of the factors that determines the biological effect.

Another is what is called the linear energy transfer, a technical term that refers to how much energy is deposited along the track of the radiation. For gamma rays, the linear energy transfer is very low, for as already mentioned, they mostly escape from the body leaving behind a very little bit of their energy. Alpha particles have the highest linear energy transfer and, for any given absorbed dose, are about 20 times as effective at producing biological effects as are gamma rays.

So, in order to determine the biological dose, which has the technical name dose equivalent, health physicists multiply the absorbed dose from alpha radiation. Since the risk of cancer is different for different tissues, they also look at what tissues are being irradiated and use this information to determine what they call the effective dose.

The effective dose is a measure of the risk of producing cancer over the entire lifetime of the person. Only certain types of cancers are known to be radiation induced, and the probability or chance of developing a radiation-induced cancer over one's lifetime is very small, indeed very much smaller than the so-called natural incidence of these cancers in the general population.

In summary, response to radiation is determined by the following factors:

1. the magnitude of the absorbed dose
2. the rate at which the absorbed dose is delivered
3. the biological effectiveness of the radiation which is determined by its linear energy transfer
4. the specific tissue(s) irradiated
5. other factors specific to the individual

As you see, the answer is very complex, but I hope it has fully answered your question.

Ronald Kathren, CHP
Professor Emeritus
Washington State University