J. F. LOUTIT 



It is when the fission-products reach the troposphere that the biologist 

 becomes involved. Figure 1 in the paper of Bryant, Chamberlain et al.^'*, 

 shows the chain in which we are interested. ''"Sr can be inhaled directly 

 from the air and be absorl^ed through the lung. From the average concen- 

 tration of -'"Sr in air, it can be calculated, however, that the hazard from 

 this route of entry is less by several orders of magnitude than from the gut. 

 In this connexion let us first consider rainwater which has been shown to be 

 the main vehicle whereby the radio-activity is deposited — Stewart's recent 

 data indicate 2 to 7 [j(.[j,C '•"'Sr per litre of rainwater, depending on the season 

 in the United Kingdom. If rainwater were used directly as drinking water, 

 this would constitute a significant fraction of the average daily intake of ^•'Sr, 

 now estimated to be about 5 to 6 [j.[jt.C per day^. However, in the United 

 Kingdom drinking water is obtained from reservoirs, rivers and bores. The 

 rainwater in contact with the earth has every opportunity to be purified by 

 the absorption of fission-products on to clay-minerals and by exchange of 

 fission-products with other ions in the soil. Thus drinking waters ai'e in 

 fact from 10 to 100 times less radio-active than rainwater. The main con- 

 tamination at the present time is of food. Radio-activity in rain is deposited 

 directly on to food-stuffs such as cereals and vegetables, which constitute an 

 important part of the diets of man and domestic animals. It also falls on to 

 grass which is the most efficient trap. It is true that the next shower of rain 

 may wash off" a considerable proportion of the radio-activity, but in turn it 

 deposits a new consignment. Whereas man derives his vegetable food from 

 perhaps a square foot or so of ground a day, grazing animals, such as the cow 

 and sheep, browse off" areas anything up to a thousand times greater. These 

 ruminants are thus great accumulators, but also, as far as human diet is 

 concerned, filters. 



The rain also of course falls directly on to soil, or is washed off" vegetation 

 into soil, from which all plants derive their macro-nutrients and a large 

 part of their micro-nutrients. It has been known for some time that many 

 trace elements, which we now know to include strontium and caesium, can 

 be absorbed directly from the leaves. Due to Scott Russell and his colleagues 

 we are becoming increasingly aware that the entry of these materials into 

 plants is not only a matter of foliar uptake and absorption from the true soil 

 by the feeding roots, but that there is a third mechanism which Scott Russell 

 has called 'stem-base absorption'. This is of particular importance in the 

 case of pasture grasses, especially those which are not well maintained. This 

 will be discussed in more detail later. 



From the animal biologist's point of view, the cow derives its nutrients in 

 large part from grass ; the strontium, being a chemical analogue of calcium, 

 is metabolized by the cow in much the same way as calcium, and some of it 

 appears in the milk. Under normal conditions in the United Kingdom, milk 

 provides at least 50 per cent of the dietary calcium for the human population. 

 Caesium, being an alkali metal, is treated metabolically in some ways as 

 potassium. Thus it is generally distributed in soft tissues, but the biological 

 turnover appears to vary quite widely between species, being some two 

 weeks in the rat, some four months in man. 



Since 1954 the Health Physics and Chemical Divisions of the Atomic 

 Energy Research Establishment at Harwell have put an increasing eff"ort 



211 



