The limits of epidemiology
Authors: Doll Sir Richard 
Published: August 2002

ICRF/MRC/BHF Clinical Trial Service Unit and Epidemiological Studies Unit, Harkness
Building, Radcliffe Infirmary, Oxford OX2 6HE, UK
The National Registry for Radiation Workers, which has enabled Muirhead and his
colleagues to provide important new data on the risks associated with chronic
exposure to very low doses of ionising radiation in this issue, owes its origin to
recommendations made by the Royal Commission on Environmental Pollution (1976) under
the Chairmanship of Sir Brian Flowers, as Lord Flowers then was. The Commission was
concerned, inter alia, that a report on the mortality of Windscale
workers who had been exposed to plutonium included only the numbers of deaths
observed and expected among those who were actively employed (Dolphin 1976). There
was a need, the Commission believed, for a complete national register of radiation
workers to be kept by the National Radiological Protection Board, with a section
comprising those exposed to plutonium and the higher actinides, which would include
those who had retired or left the industry.
The Board differed then from the one we have had for the last 15 years, and when the
register was set up on 1 January 1976, it was limited to workers who opted to be
included on it, a limitation that, to my knowledge, has never been imposed on any
other industrial cohort that was the subject of research to establish the extent of
the hazards to which its members were exposed. That the research should be known to
and approved by representatives of the workers is important, but that it should
require workers to opt in individually could seriously reduce the numbers available
for study and might also introduce a potential source of bias into the findings. The
restriction was subsequently changed to the provision of an opportunity for
individuals to opt out - still an undesirable and unnecessary condition - but one
which, as it has been operated, has generally resulted in such a low refusal rate
that only a few units, in which the rate was greater than 10%, have had to be
excluded from the study.
The register has now been in use for over 20 years and includes over 120 000 men and
women, some of whom were first employed as long ago as 1955, but even so follow-up to
the end of 1992 has recorded less than 13 000 deaths (10.4%) and, in view of the
small doses to which the workers were exposed (the mean cumulative dose having been
30.5 mSv), this is far too few to establish the precise effects that low doses
received at low dose rates are likely to cause. This is not a criticism of the study.
On the contrary, it is a good reason for supporting its continuation and its
participation in the international study of radiation workers that the International
Agency for Research on Cancer (Cardis et al 1995, Cardis and
Martuzzi 1998) is now carrying out.
With the current findings it is perhaps understandable that some might argue that
those who were conducting these studies were pursuing a will-o'-the-wisp, that the
ultimate object will always be just out of reach, and that the conclusion will always
be that larger numbers and more research are needed. In a sense this is true. Sooner
or later we have to base our estimate of risk at very low doses, such as those (say)
from natural background radiation, on some model, whether derived from laboratory
experiment, epidemiological evidence of the dose-response relationship at higher
doses, or, better, a combination of the two. But we have not reached that point yet.
The new data from the registry of radiation workers, reported on pages 3-26, give
some assurance that the limitations of epidemiology are less restrictive than the
French Academy of Sciences would imply (Académie des Sciences 1997, Tubiana 1998) and
they strengthen our confidence that estimates of the effect of doses less than 200
mSv, based on the dose-response relationships observed with higher doses in the
survivors of the atomic bomb explosions, are not far out. They don't in themselves
provide a good estimate of what the effect is; but it is reasonable to expect that
they will, when combined with the data from other countries that are being collected
by the International Agency for Research on Cancer (Cardis and Martuzzi 1998) and the
experience of a few more years.

References

Académie des Sciences 1997 Problems Associated with the Effects of Low
Doses of Ionizing Radiation Rapport de l'Académie des Sciences No 38
(Paris: Technique and Documentation)
Cardis E and Martuzzi M 1998 Improving the estimates of radiation induced cancer
risk Seguridad Nuclear
6 31--7
Cardis E et al 1995 Effects of low doses and low dose rates of
external ionizing radiation: cancer mortality among nuclear industry workers in three
countries Radiat. Res.
142 117--32
Dolphin G W 1976 A Comparison of the Observed and Expected Cancers of the
Haematopoietic and Lymphatic Systems among Workers at Windscale NRPB
Report R54 (Chilton: NRPB)
Royal Commission on Environmental Pollution 1976 Sixth Report: Nuclear
Power and the Environment (London: HMSO)
Tubiana M 1998 The report of the French Academy of Sciences: `Problems associated
with the effects of low doses of ionising radiation' J. Radiol.
Prot.
18 243--8