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.