When considering the effects on
Health of exposures to radiation, or other carcinogens, it is
important to distinguish:
(1) Victims for whom it is highly probable that radiation was
the
cause (Risk Ratio greater than two -RR>2; more likely than not;
Porbability of Causation -POC >50%)
(2) Victims with an exposure such that one can
find a group with
similar exposures where the increase in rate is statistically
significant although POC<50% and individual causation is less likely
than not. Often called attributable cancers or an attributable
risk
(3) Victims with lower exposures - comparable to
background - where
only statistical calculation is possible. In this category are
about
7,000 cancer cases per year in the USA from natural radiation,
30,000
lung ailments in USA from particulate air pollution, or 10,000 cancers
per year in Bangalesh from arsenic. These numbers are
often
misunderstood. Two crucial references are:
"Fundamental
Carcinogenic Processes and their
Implications for Low Dose Risk Assessment," K.S Crump, D.G. Hoel, C.H.
Langley and R.Peto, Cancer Research 36, 2973-2979 (1976). (E)"Low-Dose Linearity: the Rule or the Exception?,"
M. Crawford and R. Wilson, Human and Ecological Risk Assessment, 2(2),
305-330 (1996).
In
the first of these papers, Crump et al. point out that
whatever the basic biological process relating a dose to cancer, a
differential
linearity results provided that the radiation dose and the background
act
on the biological system in the same way. Since the cancers produced by
radiation and those produced by background are indistinguishable, this
is an assumption that has not been refuted - although of course it is an
assumption whose validity must constantly be questioned.
Crawford and Wilson
went further and pointed out that the argument is a general one and can
apply to other outcomes than cancer, such as respiratory problems
caused
by air pollution or cigarette smoking. This in our view makes it
mandatory
for any discussion of low dose behavior (meaning as is usual these days
doses lower than background) to include a discussion of what causes the
natural background of cancers. Unfortunately this is rarely done.
Richard Wilson
and Radiation