Sources of information on biological effects
Man-made sources of radiation have been in existence since the turn of the century. Radiation doses have since been administered to patients as a contribution to medical diagnosis and therapy. These doses are variable, but in most developed countries they represent a significant amount of the additional radiation already received over and above that from the natural background. As an example, one chest X ray may give about 1/50th of 1 mSv (0.02 mSv). Other procedures such as fluoroscopy will give considerably higher doses and patients treated by radiotherapy may be given doses many times higher than that.
Before effective radiation protection practices were generally adopted, certain occupational groups such as hospital radiologists received high doses, and eventually showed deviations from normal health patterns that could be attributed to radiation.
The most comprehensive experience of the health effects of high doses of radiation stems from the atomic bombing, in 1945, of the Japanese cities of Hiroshima and Nagasaki. These irradiated groups and many more have been monitored for health effects over many decades. Additionally, there have been extensive programmes of other types of biological research. As a result, there is considerable knowledge as to what kinds of health effects may occur and what doses may produce them. There is also information about how long after exposure these effects may appear.
All these facts are kept under constant review by permanent international bodies of scientists which continually assess radiation risks in the light of existing and new knowledge and experience. Among these bodies are the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and the International Commission on Radiological Protection (ICRP).
If the whole body is exposed to a very high level of radiation, death may occur within a matter of days or weeks. Other effects such as local skin burns and sterility can occur if only part of the body is heavily irradiated. If, however, the exposure is more spread out in time, the effects are less severe.
In the case of the population studied by the project, such acute effects were not to be expected nor were they found.
The table below lists the principal harmful radiation effects, the condition for their occurrence and the source of information about them.
The most important late effect of radiation is cancer, which is always serious and often fatal. Tumors appear after what is called the "latent period", which is normally several years. The higher the exposure, the greater the risk of an individual subsequently developing cancer, but only a relatively small number of people exposed will contract cancer because of this. It is important to recall that cancer also has many causes. The situation is analogous to smoking, where those who smoke most run the highest risk of lung cancer, but not all of them will contract it. Cancer is a very common disease; something like 1 in every 4 or 5 deaths is attributable to it. Therefore, unless the number of new cancers caused by the excess of radiation is large, it may be difficult to be certain that they are actually attributable to radiation. It is for that reason that very careful statistical studies have been recommended in various parts of the report.
Condition Circumstances Source Acute radiation effects 1 Sv+ (1000 mSv+)
Human data from accidents
Late radiation effects Any exposure Human experience --cancer
Biological studies Developmental changes Irradiation of embryo
manifested after birth
Limited human data