The cells of the lining of the intestine are continually lost through the action of stomach acids and digestive juices. Consequently after high radiation doses (5 Sv), insufficient parent cells may be able to divide to maintain the lining. Intake of fluids and nutrients is then reduced and body fluids and salts may be lost. At very high doses the results may be fatal.

The Foetus

Following fertilisation, the growing foetus contains large numbers of rapidly dividing cells. During this period of growth the foetus is very sensitive to radia­tion damage. This leads to the additional restrictions on the radiation dose which may be received by wo­men of child-bearing age.

Bone

Bone is a network of protein fibres with insoluble inorganic salts (mainly calcium phosphates) deposited in them. Certain isotopes chemically similar to cal­cium, can, when ingested, be absorbed onto bone surfaces and heavily irradiate the surrounding tissue. Such isotopes include those of strontium, radium and plutonium.

The Thyroid

The thyroid gland is small and soft and located either side of the Adam’s apple. The hormone produced by the thyroid is a compound of iodine and an amino — acid. As the thyroid is the body’s store for iodine it is particularly ‘sensitive’ to inhaled or ingested isotopes of iodine or chemically similar elements.

The Eye

New cells are continually produced in the lens of the eye. Irradiation produces cell damage or death with a clouding of the lens (cataract). The effect is of greatest significance when low energy X-rays are used and is not observed at doses of less than 5 Gy.

The Testis

The most sensitive cells in the testis are those which continually produce sperm. A single dose of 0.1 Sv to the testis may lower the sperm count for up to a year, 4 Sv may produce permanent sterility. Of greater significance is the possibility that damage to the pa­rent cells may lead to a transformation in the gene­tic information carried by the sperm cell, which may lead to spontaneous abortion of the fertilised ovum or possibly the birth of an abnormal child. Conse­quently, men who have received high doses of ra­diation, e. g., from radiotherapy, are advised against fathering children for a period dependent on the dose received.

The Ovary

Unlike the testis, the ovary does not continually pro­duce new cells. At birth it has a full complement of

undeveloped ova which gradually differentiate and are shed. A dose to the ovary of 3 Sv will result in sterilisation. The same considerations with regard to genetic damage apply as in the case of the testis ex­cept that, as all the ova are present from birth, any damage is permanent.