radon ingested in drinking water had a ‘mean life’ relation to somatic effects, the maximum permissible — level be taken as one thirtieth of the continuous by the lungs. On this assumption it is easy to cal- occupational value (168 hr./week) computed accordine to its basic rules. The value of maximum permissivle culate that during its stay in the body approximately concentration for soluble radium-226 in water sug0:75 per cent, that is, 60 upe. of radon and of each of its two «-emitting daughters, disintegrate com- gested by the International Commission for conpletely. Since I uuec. of radon-222 comprises 1:78 x . tinuous occupational exposure is 100 uye./htre, so 104 atoms of radon, this implies the production of that the value for large populations would be 3:3 uue./litre. This value is lower than our observed about 3 million «-particles and of a million or so values for British spa waters and two Cornish waters. atoms of lead-210 (radium D) in the body as the The activities of several other waters are more than result of each day’s intake of such water. In a future 10 per cent of this estimate. A similar situation has paper it is hoped to discuss certain aspeets of the radiation dosimetry resulting from these findings. been noted for some large populations in the United It will be seen from Table 3 that a number of States!9, waters at the time of sampling also contain radtium-224 However, direct observations by us?° on human skeletons from Cornwall show that the radioactivity (thorium X) and its three a-emitting daughters at levels of activity comparable with those of the longobserved in them is many times less than would be lived activity due to radium-226 and its daughters. expected on the basis of the assumptions made by the International Commission. The observed mean Since radium-224 is an isotope of radium-226 their level of activity of the skeletonsis a factor of at least fundamental chemistry would be identical, but the short mean life of radium-224 and its deeay products a thousand below the lowest body burden known to have resulted in a tumour. may result in a very different distribution of absorbed The apparent anomaly arises largely because the energy. of about | hr. in the body, being excreted principally biological parameters which have been used in the Daily Intakes of Alpha-Activity We now consider the mean daily intakes of a-activity per head of population resulting from drinking water. From information kindly supplied by the Ministry of Housing and Local Government we have estimated the number of consumers of each of the principal water supplies. Assuming a daily intake of 2-5 litres per head the mean daily intakes of a-activity from this source for various sections of the population are estimated in Table 4. Table 4, Mean DAILY INTAKE OF @-ACTIVITY FROM DRINKING WATER “uec. PER HEAD OF POPULATION Group of population | | Cornwall | London area Whole of Britain | Scotland Spa patients (1 litre per day) , | Long-lived activity Ra-226 and + danghters 4-5 1-0 0-5 Q-25 21-1 Activity radonand daughters 8,300 20 17 2 ~ 1,000 Activity + Ra-224 and daughters ~ 5-5 very small i ” * " : | : | It will be seen that the mean daily intakes of longlived «-activity for the four groups cover a range of 18 to 1, whereas from Table 1 it is clear that the range of individual intakes may exceed 400 to I. For the mean daily intakes of radon plus daughters we have a range of 4,000 to 1 between the groups, with the range for individuals exceeding 10,000 to 1. The low mean value of long-lived e-activicy in the Scottish waters illustrates, rather surprisingly, the low values which inay be encountered in areas of granitic formations. For comparison, Table 4 also shows the approximate daily intake of «-activity resulting from the consumption of 1 litre per day of British spa water having an activity equal to the mean value observed by us for this type of water. Maximum Permissible Levels of Natural Activity The report of the International Commission on Radiological Protection'® deliberately excludes ‘natural background’ from its scope. It is, however, recommended by the International Commission that, in considering the exposure of large populations in calculations were based on observations on human subjects having large radium burdens acquired many years previously and who were therefore excreting the radioactive material at a very low rate. In a previous article’? we reported investigations of the levels of natural q-activity present in the daily diets of the population of Great Britain and came to the conclusion that the daily intakes of such activity could vary from a minimum of 4 or 5 upe. to 400 or 500 unc. or even more, depending on the particular choice of diet. It is evident that in places where the drinking water has a level of «-activity at the higher end of our observed range and the individual intake from food happensto be at the lower end of the range of dietary activity, then the daily intake of natural a-activity will be dominated by the contribution from water. In other locations where the drinking water is derived from surface sources having much lower activity, the contribution from food will be predominant, regardless of the choice of diet. Whether the human body finds it easier to retain radium-226 and its daughters present in soluble form in drinking water than to retain the same elements, possibly in different chemical form, ingested in food, is not yet known. The relative contributions made by water and food to the real retention and utilization by the body of a-active substances and the factors which control retention in the two cases are the subject of present investigations, Comparison with Fission Product Activity in Waters Finally, perhaps we may consider these daily intakes of natural «-activity in water in relation to the published figures of the artificial 6- and y-activity due to the presence of strontium-90 and cesium-137 in drinking water in Great Britain”. The pattern of distribution of these fission products is quite different from that: of the natural «-activity, since they appear at their highest levels in surface waters. Ground waters have extremely low contents of fission products, but, as we have seen, high values of natural «-activity.