It is possible in this manner to determine that background radiation accounts for considerably less than 1% of the spontaneous mutations that take place. The other mutations must arise out of chemical misadventures, out of the random heat-jiggling of molecules, and so on. These, it can be presumed, will remain constant when the radiation dose is increased.
This is a hopeful aspect of the situation for it means that, if the background radiation is doubled or tripled for mankind as a whole, only that small portion of the spontaneous mutation rate that is due to the background radiation will be doubled or tripled.
Let us suppose, for instance, that fully 1% of the spontaneous mutations occurring in mankind is due to background radiation. In that case, the tripling of the background radiation produced in the United States by man-made causes (see [Table]) would triple that 1%. In place of 99 non-radiational mutations plus 1 radiational, we would have 99 plus 3. The total number of mutations would increase from 100 to 102—an increase of 2%, not an increase of 200% that one would expect if all spontaneous mutations were caused by background radiation.
| RADIATION EXPOSURES IN THE UNITED STATES[7] | ||||
|---|---|---|---|---|
| Millirems[8] | ||||
| Natural Sources | ||||
| A. External to the body | ||||
| 1. From cosmic radiation | 50.0 | |||
| 2. From the earth | 47.0 | |||
| 3. From building materials | 3.0 | |||
| B. Inside the body | ||||
| 1. Inhalation of air | 5.0 | |||
| 2. Elements found naturally in human tissues | 21.0 | |||
| Total, Natural sources | 126.0 | |||
| Man-made Sources | ||||
| A. Medical Procedures | ||||
| 1. Diagnostic X rays | 50.0 | |||
| 2. Radiotherapy X ray, radioisotopes | 10.0 | |||
| 3. Internal diagnosis, therapy | 1.0 | |||
| Subtotal | 61.0 | |||
| B. Atomic energy industry, laboratories | 0.2 | |||
| C. Luminous watch dials, television tubes, radioactive industrial wastes, etc. | 2.0 | |||
| D. Radioactive fallout | 4.0 | |||
| Subtotal | 6.2 | |||
| Total, man-made sources | 67.2 | |||
| Overall total | 193.2 | |||
Dosage Rates
Another difference between the genetic and somatic effects of radiation rests in the response to changes in the rate at which radiation is absorbed. It makes a considerable difference to the body whether a large dose of radiation is absorbed over the space of a few minutes or a few years.
When a large dose is absorbed over a short interval of time, so many of the growing tissues lose the capacity for cell division that death may follow. If the same dose is delivered over years, only a small bit of radiation is absorbed on any given day and only small proportions of growing cells lose the capacity for division at any one time. The unaffected cells will continually make up for this and will replace the affected ones. The body is, so to speak, continually repairing the radiation damage and no serious symptoms will develop.
Then, too, if a moderate dose is delivered, the body may show visible symptoms of radiation sickness but can recover. It will then be capable of withstanding another moderate dose, and so on.
The situation is quite different with respect to the genetic effects, at least as far as experiments with Drosophila and bacteria seem to show. Even the smallest doses will produce a few mutations in the chromosomes of those cells in the gonads that eventually develop into sex cells. The affected gonad cells will continue to produce sex cells with those mutations for the rest of the life of the organism. Every tiny bit of radiation adds to the number of mutated sex cells being constantly produced. There is no recovery, because the sex cells, after formation, do not work in cooperation, and affected cells are not replaced by those that are unaffected.
This means (judging by the experiments on lower creatures) that what counts, where genetic damage is in question, is not the rate at which radiation is absorbed but the total sum of radiation. Every exposure an organism experiences, however small, adds its bit of damage.