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Before it can be
determined in any quantitative way what risk exists, it is necessary to determine whether ANY risk (hazard) exists. This is usually done through a systematic evaluation of the available human and animal
Explanation: For asbestos exposure and lung cancer, for
example, there is evidence that INDUSTRIAL PROCESS
affects the slope of the risk. Thus, those employed in asbestos mining and milling had measureable, but
shallow risk (lower risk at highest exposures), while
those who made asbestos textiles had highest risks.
If the black line
intersecting zero is “correct”, there is theoretically some “risk” at any level of exposure (although the probability of that risk may be effectively none at zero).
This is almost a convention in cancer risk assessment,
but may be wrong if the GREEN line is correct, which suggests that there is ZERO theoretical risk below the exposure at which the
green line intersects the X (exposure) axis.
The problem is thus that
the studies do not actually identify any points which are
low on the dose level; whether human or (especially) animal. The resulting
PROJECTIONS may be largely either guesswork, convention (as in the linear no-threshold model), or both.
Remember that this is
called Risk MANAGEMENT by EPA but Risk CHARACTERIZATION
by many others. Since management is a
whole other ballgame, I prefer characterization: this is simply a mathematical modelling of risk.
This is an example from
Camus et al. in which in an ecological study it was shown
that the numbers of expected lung cancers around a chrysotile mine among a female population were grossly overestimated when the
actual estimated exposures were plugged into one
particular risk assessment model (the one developed by
Nicholson for EPA in 1986).
A similar, preliminary
analysis of mesothelioma risk applied in the same way comes
to the same conclusions.
This slide strictly for
the mathematically unchallenged!
The “plausible range”
should not be confused with a confidence interval; it was
simply estimated by an expert panel and the authors that the estimates of exposure, based on many sources, COULD have a range this wide
dependent on many factors. To put these numbers in context, remember that it is generally accepted that asbestosis risk does not exist below 25 to 50 fibre-years
Although difficult to
interpret, this slide indicates that the EPA predicted rate based on the midpoint of the “plausible” exposures would have
predicted a hundred-fold more cases of this disease in
this population (or thirty-fold if the risk assessment
had been limited to the high-tremolite Thetford-Mines area).
Moving to a NEW
TOPIC: This is not part of “risk
assessment” but some observers object to classical
risk assessment activities in that they feel these do not
adequately take this “precautionary principle” into account (although clearly they could, and often do, if enough of a margin of safety
These authors find that
there are advantages to scientists in applying the precautionary
principle, IF certain aspects of that principle are understood at the outset.
Restating the general
principles; the quote is from the RIO 1992 document.