The lungs are important as inhalation represents one of the easiest ways for radioactive material to enter the body. The purpose of the lungs is to provide an efficient system for exchanging carbon dioxide for oxygen in the red cells of the bloodstream. This takes place in the deep lung through the walls of alveoli which are tiny cavities formed into compact groups so that a very large surface area is enclosed in a small volume. The total surface area of the lungs is about the same as that of a tennis court!

Particles inhaled and deposited in the lungs will have a residence time dependent upon the region in which they are deposited and also on their size. The larger the particles and the higher in the respira­tory tract they fall, the greater the probability that they will be rapidly removed by the action of hairlike structures (cilia) which carry mucous, secreted by cells in the lung, to a point where it can be swallowed. Small particles which find their way into the deep lung, where cilia do not exist, have long residence times (hundreds of days), although many such parti­cles are engulfed by white blood cells and then re­moved by coughing or via the lymphatic system.

The Skin

The cells of the surface layer of the skin are con­tinuously lost by abrasion. The parent cells below the surface are sensitive to radiation damage because of their high rate of division. The lower layers of skin comprise connective tissue with a good blood supply and nerve network. The outer layers are composed of several layers of cells. In the basal layer are found nucleated cells capable of division. As they grow up to replace ceils removed from the outermost layer they lose their typical ‘round’ shape and nucleus and become simply ‘dead’, flat cells whose job it is to form a protective covering over their parent cells. The first effect of irradiation of the skin, at about 4 Sv, is reddening or erythema. If a large number of basal cells are killed an inflamed wound ultimately results which will be very slow to heal. The dose required for this is about Ю Sv.

The term radiation burn is sometimes used to draw a parallel with thermal burns. The time course in the two cases is, however, very different and whereas a thermal blister appears within a few minutes the ana­logous radiation damage may take more than Ю days to evolve.