It is not known whether there is a threshold to the dose required to produce genetic effects, since the data is very scant. It is difficult to obtain

53 DNA / >4 в—sr (©) r—————- —’O’ B—SC "^P B—SZ_ ^P в—sr P An incoming photon (y) collides with an orbital electron (-) of one of the atoms of tissue. A DNA molecule (BSP BSP. . J rests nearby. (The letters stand for subunits of the large DNA mole­cule: В = a base, S = a sugar, deoxyribose, and P — a phosphate J

An incoming electron (-) (either a primary beta ray or the secondary product of an X or gamma ray) ricochets among tissue atoms, ionizing some of them.

results at very low dose rates, although present evidence would seem to infer that some threshold exists (3a, b).

In the meantime a linear relationship must be assumed between damage and dose. This is pessimistic and is likely to overestimate damage, although popular science writers would take the opposite view.

Radiation affects genes and gives rise to mutations, a large number of which may be undesirable (99% has been suggested). A normal person naturally experiences mutation of genes due to natural background radio­active and chemical disturbances, at a rate of about 2% to an individual. Thus for each child there is a natural 1 in 50 chance that it carries a new mutation. Extra radiation would increase this chance slowly.

Figure 5.2 shows the range of damage effects as a function of dosage. For a whole body exposure of 2 rem, the normal rate of mutation is increased by between 1.5 and 6.5% of the original value. For a whole body dose of 5 rem the normal rate is increased by between 7 and 17%, and not until a whole body exposure of over 30 rem is experienced, is the normal rate of mutation is likely to double to a total of 4% per individual (4).