Topic 4 Biological effects of nuclear radiation

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An understanding of the biological effects of nuclear radiation is important for evaluating many potential uses and dangers.
The biological effect of nuclear radiation on humans is measured in terms of equivalent dose, whose unit is the sievert (Sv). A sievert takes into account the amount and type of radiation, and amount of human tissue absorbing the energy. The net result is an indication of the amount of biological damage caused to the organ or tissue.

Some types of nuclear radiation are more damaging than others. Alpha radiation is the most damaging to human tissues because its particles are 'large' and strongly ionising. Luckily, alpha radiation is easily blocked (eg by skin), but if an alpha-emitting substance enters the body, the results can be serious. Neutrons are also damaging to cells, because they interact very easily with body tissue (which contains a lot of water). They are a highly penetrating form of radiation, and stopped only by thick layers of concrete or water. Beta and gamma radiation are the least damaging forms of nuclear radiation but they are able to penetrate deeper into the body than alpha radiation. Gamma radiation passes through the body easily. The most energetic beta radiation penetrates about one centimetre.

Background radiation

You are constantly being exposed to a certain amount of background nuclear radiation: gamma rays from space and from the Earth, and natural radon in the air. In fact, even your own body is slightly radioactive. Radioactive potassium-40 atoms are naturally present in your body and undergo several thousand nuclear disintegrations every second. The total dose from natural background radiation averages 2.4 mSv per year worldwide. In Australia the average is about 1.5 mSv. By comparison, the annual dose of medical radiation exposures averages globally between 0.02 mSv to 1.3 mSv per year.

Figure 4.1 Sources of background radiation, averaged over world populations.

At a cellular level nuclear radiation may cause a number of effects. First, the passage of radiation through a cell may cause direct physical damage to molecules and cellular structures. More importantly, nuclear radiation can cause damage by ionising a cell's molecules. This ionising effect is particularly strong with alpha and beta particles, due to their charge. These effects of nuclear radiation can cause cells to malfunction or even die. Radiation can also cause mutations (changes) in a cell's DNA, which may result in cancer or birth defects in the next generation. We need to know how common or how rare these effects are likely to be in our lives. This is explored in the next two sections.

Dose and effect

To a significant extent, damaged cells can repair themselves, and dead cells can be replaced. Most organs can even recover from losing large numbers of cells. However, if too much damage is caused all at once (an acute dose), the body's tissues are so damaged that radiation sickness results, leading to hair loss, headaches, vomiting and bleeding. Death may occur and, depending on the dose, it can take months, weeks or just days! These strong radiation effects are a certain consequence of high level radiation exposure, so they are called 'deterministic' effects.

There are many safeguards to prevent high-level exposure to nuclear radiation, so deterministic radiation effects are rare. They would occur only when something goes terribly wrong, such as in the explosion of a nuclear weapon or a reactor meltdown. After the accident at the Chernobyl nuclear power reactor, about 130 workers suffered from radiation sickness, of whom 30 died over the next three months.

Scientists are also attempting to measure the longer term impact of Chernobyl and the nuclear attacks on Hiroshima and Nagasaki during World War II. These long-term effects include cancers that can result from radiation-induced mutations. These are known as 'stochastic' effects, because they may or may not arise, depending on chance. These effects are difficult to study. Even if an individual does develop cancer, it will not be certain that the cancer was caused by radiation. Much of our scientific understanding of these stochastic effects comes from comparing natural cancer rates in large populations that have not been exposed to a nuclear attack or accident to rates among the victims of Chernobyl, Hiroshima and Nagasaki.

A safe threshold?

Scientists are unsure whether exposure to very low levels of nuclear radiation causes any health effects at all, and some studies even suggest that low doses of radiation may be beneficial. Because of low probabilities, long time scales, and complicated disease patterns, scientists have not found a way to identify (or rule out!) related causes and effects. For example, the United Nations estimates that about 5 million Europeans were exposed to radioactive contamination from the Chernobyl accident. For most, however, the level of exposure was comparable to natural background radiation. So far, determining whether any of the cancers since 1986 were caused by Chernobyl has proved to be beyond scientific possibility, other than 1,700 thyroid cancers. These mostly non-fatal cancers could (and should) have been prevented by the prompt issue of (non-radioactive) iodine tablets after the accident, to block uptake of radioactive iodine-131 by the thyroid gland.

Figure 4.2 Accurately mapping the radiation exposure of European populations due to Chernobyl is just the first difficult task in estimating the long-term effects of that disaster.

Some scientists believe that human cells can completely recover from very small amounts of radiation, or that they even benefit from it because living things have been exposed to background radiation throughout their evolution. If this were true, then large amounts of money are wasted on overly conservative nuclear safety regulations. However, many other scientists, including international agencies such as the United Nations and the International Atomic Energy Agency support the continuation of the more conservative 'no safe threshold' concept. This assumes there is no safe threshold for nuclear radiation, and that any level of radiation carries the risk of causing irreparable mutations. Consequently, international safety standards are based on the assumption that even the smallest amount of radiation is potentially harmful. This includes natural background radiation, though there is nothing we can do about this, and medical radiation, although in this case the benefits are generally accepted as worthwhile. There are other applications that we also consider to be worthwhile such as the use of radioactive americium-241 in smoke detectors and radioactive sources in industry.

Figure 4.3 Health effects of nuclear radiation.

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