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Science and Its Lunits.
The Regulator's Dilemma
ALVIN M. WEINBERG
In his essay "Risk, Science, and Democracy," William D. Ruckelshaus
has expressed very clearly what might be called the regulator's dilemma:
During the past 15 years, there has been a shift in public emphasis from visible and
demonstrable problems such as smog from automobiles and raw sewage, to
potential and largely invisible problems, such as the effects of low concentrations
of toxic pollutants on human health. This shift is notable for two reasons. First, it
has changed the way in which science is applied to practical questions of public
health protection and environmental regulation. Second, it has raised difficult
questions as to how to manage chronic risks within the context of free arid
democratic institutions. [Ruckelshaus, 1985; see also Bayer, Bond, and Whipple,
in this volume.]
When the concerns were obvious-like smog in Los Angeles-science
could and did give unequivocal answers. For example, smog comes from
liquid hydrocarbons, and the answer to smog lay in controlling emissions of
these substances. The regulator's course was rather straightforward because
the science upon which the regulator based his judgment was operating well
within its power. But when the concern was subtle-How much cancer is
caused by 10 percent of background radiation? science was being asked a
question that lay beyond its power to answer; the question was trans-
scientific. Yet the regulator, by law, was expected to regulate, even though
science could hardly help in the process. This is the regulator's dilemma.
A slightly different version of this paper appears in Issues in Science and Technology vol. 2,
no. 1 (Fall 1985):59-72.
9
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10
ALVIN .
WEINBERG
Though this essay is subtitled "The Regulator's Dilemma," many of the
same issues arise in the adjudication of disputes over who is to blame, and
who is to be compensated, for damages allegedly caused by rare events. The
regulator's dilemma is faced also by the toxic tort judge indeed the regula-
tor's dilemma could equally be called the "toxic tort dilemma."
If my car injures a pedestrian, I am liable to be sued but at issue is not
whetherI have injured the pedestrian; instead, the question is whetherI am at
fault for running into him. If the lead from my car's exhaust is alleged to
cause bodily harm, the issue is not whether my car emitted lead but whether
the lead actually caused the alleged harm. The two situations are quite
different: in the first, the relation between cause and injury is not at issue; in
the second, it is the issue.
This paper, therefore, is an attempt to delineate more precisely those limits
to science that give rise to the regulator's dilemma; I shall speculate on how
these intrinsic limits to science seem to have catalyzed a profound attack on
science by some sociologists and public interest activists; and I shall offer a
few ideas that might help harried regulators finesse these trans-scientific
limits of science.
SCIENCE AND RARE EVENTS
Science deals with regularities in our experience; art deals with singulari-
ties. It is no wonder that science tends to lose its predictive or even explana-
tory power when the phenomena it deals with are singular, unreproducible,
and one of a kind that is, rare rather than regular, reproducible, and
recurring. Though science can often analyze a rare event after the fact (say,
the Cretaceous-Tertia~y extinction), it has great difficulty predicting when
such an uncommon event will occur.
Let us distinguish between two sorts of rare events-"accidents" and
"low-level physical insults." Accidents are large-scale malfunctions whose
etiology is not in doubt but whose a priori likelihood is very small. The
occurrences at Three Mile Island in 1979 and at Bhopal, India, in 1984 are
examples of accidents. The precursors to these events and the way in which
the accidents unfolded are well understood. Estimates of the likelihood of
the particular sequence of malfunctions are on less solid ground. As the
number of individual accidents increases, prediction of their probability
becomes more and more reliable. We can predict very well how many
automobile fatalities will occur in 1986; we can hardly claim the same
degree of reliability in predicting the number of serious reactor accidents in
1986.
Low-level insults are rare in a sense different from "rare" as applied to
accidents. We know that about 100 reds of radiation will double the mutation
OCR for page 11
SCIENCE AND ITS LIMITS: THE REGU~OR'S DILEMMA
1
1
rate in a large population of exposed mice. How many mutations will occur
in a population of mice exposed to 100 millirems of radiation? Here the
mutations, if induced at all by such low levels of exposure, are so rare that to
demonstrate unequivocally an effect with 95 percent confidence would
require the examination of many millions of mice. Though in principle this is
not impossible, in practice it is. Moreover, even if we could perform so
heroic a mouse experiment, the extrapolation of such findings to humans
would still be fraught with uncertainty. Thus, the effects of very low level
insult in human beings are rare events whose frequency again is beyond the
ability of science to predict with accuracy.
When dealing with events of this sort, science resorts to the language of
probability that is, instead of saying that this accident will happen on that
date or that a particular person exposed to a low-level insult will suffer a
particular fate, it tries to assign probabilities for such occurrences. Of
course, where the number of instances is very large or where the underlying
mechanisms are fully understood, the probabilities themselves are perfectly
reliable. In quantum mechanics there is no uncertainty as to the probability
distributions. But in the class of phenomena being discussed here, even
though the likelihood of an event's happening or of a disease's being caused
by a specific exposure is given as a probability, the probability distribution
itself is very uncertain. One can think of a somewhat fuzzy demarcation
between what I have called science and trans-science: the domain of science
covers phenomena that are deterministic, or the probability of whose occur-
rence can itself be stated precisely; trans-science covers the domain of
events whose probability of occurrence is itself highly uncertain.
"Scientif;ic "Approaches to Rare Events
Despite the difficulties, science has devised mechanisms for estimating,
however imperfectly, the probability of rare events. For accidents, the tech-
nique is probabilistic risk assessment (PRA); forlow-level insults, a variety
of empirical and theoretical approaches have been used.
Though probabilistic risk assessment had been used in the aerospace
industry for a long time, it first sprang into public prominence with Norman
C. Rasmussen's Reactor Safety Study in 1975 (U.S. Nuclear Regulatory
Commission, 1975~. Probabilistic risk assessment seeks to identify all
sequences of subsystem failures that may lead to a failure of the overall
system; it then tries to estimate the consequences of each system failure so
identified. The output of a PRA is a probability distribution, P(C); that is,
the probability, P. per reactor-year (RY), of a consequence having magni-
tude C. Consequences include both material damage and health effects. The
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12
ALVIN M. WEINBERG
probability of accidents having large consequences is usually less than the
probability of accidents having small consequences.
A probabilistic risk assessment for a reactor requires two separate esti-
mates: first, an estimate of the probability of each accident sequence and,
second, an estimate of the consequences particularly the damage to human
health caused by the uncontrolled effluents released in the accident. An
accident sequence is a series of equipment malfunctions or human miscalcu-
lations: a pump that fails to start, a valve that does not close, an operator
confusing an "on" with an "off" signal. For many of these individual
events, we have statistical data for example, enough valves have operated
for enough years so that at least in principle we can make pretty good
estimates of the probability of failure.
But uncertainties still remain, since we can never be certain that we have
identified every relevant sequence. Proof of the adequacy of PRA must
therefore await the accumulation of operating experience. For example, the
median probability of a core melt in a light-water reactor (LWR), according
to the original Rasmussen report, was 5 x 10-s/RY; the core melt at Three
Mile Island's number 2 reactor (TMI-2) occurred after only 700 light-water
reactor-years. However, TMI-2 differed from the reactors treated by Ras-
mussen and, in retrospect, one could rationalize most of the discrepancy
between the Rasmussen estimate and the seemingly premature occurrence at
TMI-2 (Rasmussen, 19811. Since TMI-2, the world's LWRs have accumu-
lated some 1,500 years of reactor operation without a core melt. This perfor-
mance places an upper limit on the a priori estimate of the core-melt proba-
bility. Thus, if this probability were as high as 10-3/RY (as had been
suggested by D. Okrent, 1981), then the likelihood of surviving 1,500
reactor-years would not be more than 22 percent; or, we can say with 78
percent confidence that the core-melt probability is not as high as 1 in 1,000
reactor-years. With 500 LWRs on line in the world, should we survive until
2000 without another core melt, we could then say with 95 percent confi-
dence that the core-melt probability is not higher than 1 in 3,000 reactor-
years. In the absence of such experience, one is left with rather subjective
judgments.
Although the Lewis critique (U.S. Nuclear Regulatory Commission,
1978) of Rasmussen's study asserted that it could not place a bound on the
uncertainty of PRA, Rasmussen has argued that his estimate of core-melt
probability might be in error by about a factor of 10 that is, the probability
may be as high as 1 in 2,000 reactor-years or as low as 1 in 200,000 reactor-
years. As we see, we can, after 1,500 reactor-years of operation without a
core melt, say with about 50 percent confidence that Rasmussen's upper
limit (1 in 2,000 reactor-years) is not too optimistic. And if we survive to
2000 without a core melt, the confidence level with which we can make this
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SCIENCE AND ITS LlMlTS: THE REGU~OR'S DILEMMA
13
assertion rises to 95 percent. Our confidence in probabilistic risk analysis
can eventually be tested against actual, observable experience. But until this
experience has been accumulated, we must concede that any probability we
predict must be highly uncertain. To this degree our science is incapable of
dealing with rare accidents, but time, so to speak, annihilates uncertainty in
estimates of accident probability.
Unfortunately, time does not annihilate uncertainties over consequences
as unequivocally as it does frequency of accidents. A large reactor or chemi-
cal plant accident can cause both immediate, acute health effects and
delayed, chronic effects. If the exposure either to radiation or to methyl
isocyanate (MIC) is high enough, the effect on health is quite certain. For
example, a single exposure of about 400 rems will cause about half of those
exposed to die. On the other hand, in a large accident there will also be many
who are exposed to smaller doses, indeed to doses so low that the dose-
response is indeterminable. At Bhopal, 200,000 people who were exposed
to MIC recovered. We cannot say positively whether or not they will suffer
some chronic disability.
The worst accident envisaged in the Rasmussen study, with a probability
of 10-9/RY, would lead to an estimated 3,300 early fatalities, 45,000 early
illnesses, and 1,500 per year delayed cancers among 10 million exposed
people. Almost all of the estimated delayed cancers are attributed to expo-
sures of less than 1,000 millirems per year, a level at which it is very difficult
to estimate the risk of inducing cancer. Similarly, the critique by the Ameri-
can Physical Society (1975) ofthe Rasmussen study attributed an additional
10,000 deaths over 30 years among 10 million people exposed to cesium-
135 from a large accident. The average exposure in this case was 250
millirems per year, again, a level at which our estimates of dose-response are
extremely uncertain.
Has the nuclear community, particularly its regulators, figuratively shot
itself in the foot by trying to estimate the number of delayed casualties
resulting from these low-level exposures? In retrospect, the Rasmussen
study would have been on more solid ground had it confined its estimates
only to those health effects that resulted from exposures at higher levels,
where science makes reliable estimates. For the lower exposures the conse-
quences could have been stated simply as the number of man-reins of expo-
sure of individuals whose total exposure did not exceed, say, 5,000 mil-
lirems, without trying to convert this number into numbers of latent cancers.
Thus, health consequences would be reported in two categories: (1) for
highly exposed individuals, the number of health effects; (2) for slightly
exposed individuals, the total man-reins, or even the distribution of expo-
sures accrued by the large number of individuals so exposed. Perhaps some
scheme such as this could be adopted in reporting the results of future
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14
~LVINM. WEINBERG
probabilistic risk assessments: it at least has the virtue of being more faithful
to the state of scientific knowledge than does the present convention.
Low-Level Exposure
In both examples of accidents (Bhopal and TMI-2) cited above, many
people are exposed to low-level insult. The uncertainties inherent in estimat-
ing the effects of such low-level exposure are heaped on top of uncertainties
in estimating the probability of the accident that might lead to the exposure in
the first place.
While science has exerted great effort to ascertain the shape of the dose-
response curve at low doses, very little, if anything, can be said with cer-
tainty about the low dose-response. Thus, to quote the 1980 report The
Effects on Populations of Exposure to Low Levels of Ionizing Radiation
(known as the BEIR-III report)-of the National Research Council's Com-
mittee on the Biological Effects of Ionizing Radiations, "The Committee
does not know whether dose rates of gamma or x rays of about 100 mrads/yr
are detrimental to man.... It is unlikely that carcinogenic and teratogenic
effects of doses of low-LET [linear energy transfer] radiation administered
at this dose rate will be demonstrable in the foreseeable future" (National
Research Council, 1980, p. 31. This prompted Philip Handler, then presi-
dent of the National Academy of Sciences, to comment in his letter transmit-
ting the report to the Environmental Protection Agency, "It is not unusual
for scientists to disagree . . . (and) . . . the sparser and less reliable the data
base, the more opportunity for disagreement.... This report has been
delayed . . . to permit time . . . to display all of the valid opinions rather than
distribute a report that might create the false impression of a clear consensus
where none exists" (National Research Council, 1980, p. iii).
This forthright admission that science can say little about low-level insults
is admirable. It represents an improvement over the unjustified assertion in
the BEIR-II report of 1972 that 170 millirems per year over 30 years, if
imposed on the entire U.S. population, would cause between 3,000 and
15,000 cancer deaths per year (National Research Council, 19721. I do not
quarrel with the estimated upper limit which amounts to 1 cancer per 2, 500
man-reins; however, I regard the lower limit's being different from zero as
unjustified and as having caused great harm. The proper statement should
have been, at 170 millirems per year, we estimate that the upper limit for the
number of cancers would be 15,000 per year; and the lower limit might be
zero.
Since the appearance of the BEIR reports, two other developments have
added to the burden ofthose who mustjudge the carcinogenic hazard of low-
level insults: (1) natural carcinogens and (2) ambiguous carcinogens.
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SCIENCE AND ITS LIMITS: THE REGU~OR'S DILEMMA
Natural Carcinogens
15
Is cancer "environmental" in the sense of being caused by technology's
effluents, or is cancer a natural consequence of aging? In the past few years
we have seen a remarkable shift in viewpoint: whereas 15 years ago most
cancer experts would have accepted a primarily environmental etiology for
cancer, today the view that natural carcinogens are far more important than
man-made ones has gained many converts. In his famous Science article
illustrated by Robert Indiana's modern painting Eat-Die, Bruce N. Ames
(1983) marshaled powerful evidence that many of our most common foods
contain carcinogens. Indeed, John R. Totter (1980), supported by the late
Philip Handler, has offered epidemiological evidence for the oxygen radical
theory of carcinogenesis: that we grow older and eventually get cancer
because we metabolize oxygen, and oxygen radicals can play havoc with our
DNA. As such views of the etiology of cancer acquire scientific support, the
trans-scientific question of how much cancer is caused by a tiny chemical or
physical insult likely will be recognized as irrelevant. One does not swat
gnats in the face of a stampeding elephant.
Ambiguous Carcinogens
To further complicate the cancer picture, certain agents, such as dioxin,
various dyes, and even moderate levels of radiation, seem to diminish the
incidence of some cancers at the same time that they increase the incidence
of others; the lifespan of animals treated with such substances on average
exceeds that of untreated animals (Weinberg and Storer, 19851. A most
striking example, given by Haseman (1983), is that of yellow dye #14: given
to leukemia-prone female F344 rats, the dye completely suppresses leuke-
mia, which is always fatal, but causes liver tumors, most of which are
benign.
These two findings or, perhaps, points of view illustrate an underlying
point: with regard to low-level insult to human beings, we can say very little
about the cancer dose-response curve. Saying that so many cancers will be
caused by so much low-level exposure to so many people, a practice that
terrifies many people, goes far beyond what science actually can say.
How Science Reacts to Intrinsic Uncertainty
Does the scientific community accept the notion that there are intrinsic
limits to what it can say about rare events? That as events become rarer, the
uncertainty in the probability of occurrence of a rare event is bound to grow?
Perhaps a better way of framing the question is this: To what use can we put
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16
ALVIN M. WEINBERG
the tools of scientific investigation of rare events say, probabilistic risk
assessment and large-scale animal experimentation as surrogate for epide-
miological inquiry if we concede that we can never get definitive answers?
An uncertainty as high as a factor of 10 is often useful in probabilistic risk
assessment, especially if one uses the PRA for comparing risks. For exam-
ple, the 1,500 reactor-years already experienced since the TMI-2 accident
suggest that a reactor core-melt probability is likely to be less than 10-3/yr
and may well be as low as the PRA predicts, less than 10-4/yr. This is to be
compared with dam failures, whose probability, based on many hundreds of
thousands of dam years (and where time has annihilated uncertainty), is
around 10~4/yr. Even with this uncertainty, we can judge roughly how safe
reactors are compared to dams.
When one compares the relative intrinsic safety of two very similar
devices e.g., two water-moderated reactors probabilistic risk assess-
ment is on much more solid ground. Here one is not asking for absolute
estimates of risk, but rather for estimates of relative safety. If the reactors, A
and B. differ in only a few details say that reactor A has two auxiliary feed
water (AFW) trains whereas B has only one the ratio of core-melt probabil-
ities should be much more reliable than their absolute values, since the ratio
requires an estimate of failure of a single subsystem, in this case, the extra
AFW on reactor A.
Not only can one say with reasonable assurance how much safer reactor A
is than reactor B. but one can, as a result of the detailed analysis, identify the
subsystems that contribute most to the estimated failure rate. Even if PRA is
inaccurate, it is very useful in unearthing deficiencies: one can hardly deny
that a reactor in which deficiencies revealed by PRA have been corrected is
safer then one in which they have not been corrected, even if one is unwilling
to say how much safer.
Somewhat the same considerations apply to low-level insult. An agent
that does not shorten lifespan at higher dose will not shorten lifespan at lower
dose. An agent that is a very powerful carcinogen at high dose is more likely
to be a carcinogen at low dose than is an agent that is a less powerful high-
dose carcinogen. Thus, animal experiments surely are useful in deciding
which agents to worry about and which not to worry about. And of course the
Ames test has made at least some preliminary screening of carcinogens more
feasible. The difficulty today seems to be not so much identifying agents that
at high dose may be carcinogens as it is prohibiting exposures far below
levels at which no effect can be, or ever will be, demonstrated. lThe regulator
and the concerned citizen are inclined to go so far as to approve the Delaney
Clause [21 U.S.C. 348(c)], which forbids in interstate commerce any car-
cinogenic agent in food, without ever saying anything about allowable levels
or relative risks of, say, cancer induction by nitrosoamines and digestive
disorders caused by meat untreated with nitrites!
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SCIENCE AND ITS LIMITS: 'THE REGUL f OR 'S DILEMMA
17
The Delaney Clause is the worst example of how disregard for an intrinsic
limit of science can lead to bad policy by overenthusiastic politicians. Physi-
cist Harvey Brooks has often pointed out that one can never prove the
impossibility of an event that is not forbidden by a law of nature. Most will
agree that a perpetuum mobile is impossible because it violates the laws of
thermodynamics. That one molecule of a polychlorinated biphenyl (PCB)
may cause a cancer in humans is a proposition that violates no law of nature:
hence, many, even within the scientific community, seem willing to believe
that this possibility is something to worry about! It was this error that led to
the Delaney Clause.
THE ATTACK ON SCIENCE FROM THE SOCIOLOGY
OF KNOWLEDGE
When is an event so rare that the prediction of its occurrence forever lies
outside the domain of science, that is, within the domain of trans-science?
Clearly we cannot say. Perhaps as science progresses, this boundary
between science and trans-science recedes toward events of lower fre-
quency. But at any stage the boundary is fuzzy, and much scientific contro-
versy revolves around deciding where that boundary lies. One need only
read the violent exchange between Edward P. Radford and Harald H. Rossi
(National Research Council, 1980) over the risk of cancer from low levels of
radiation to recognize that, where the facts are obscure, argument even ad
hominem argument blossoms. Indeed, Alice Whittemore (1983), in an
article entitled "Facts and Values in Risk Analysis for Environmental Toxi-
cants," has pointed out that at this "rare event" boundary between science
and trans-science, facts and values are always intermingled. A scientist who
believes that nuclear energy is evil because it inevitably leads to proliferation
of nuclear weapons (which is a common basis for opposition to nuclear
energy) is likely to form judgments about the data on induction of leukemia
from low-level exposures at Nagasaki that are different from the judgments
of a scientist whose whole career has been devoted to making nuclear power
work. Cognitive dissonance is all but unavoidable when the data are ambigu-
ous and the social and political stakes are high.
No one would dispute that judgments of scientific truth are much affected
by the scientist's value system when the issues are at or close to the boundary
between science and trans-science. On the other hand, as the matter under
dispute moves away from that border into the domain of science, most would
claim that the scientist's extrascientific values intrude less and less. Soviet
scientists and American scientists may disagree on the effectiveness of a
ballistic missile defense, but they agree on the cross section of uranium-235
or the lifetime of the pi-meson.
This all seems obvious, even trite. Yet in the past decade or so, a school of
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ALVIN M. WE1NB~G
sociology of knowledge has sprung up in the United Kingdom, claiming that
"scientific views are determined by social (external) conditions, rasher then
by the internal logic of scientific tradition and inherent characteristics of the
phenomenal world" (Ben-David, 1978), or that "all knowledge and knowl-
edge claims are to be treated as being socially constructed: genesis, accep-
tance, and rejection of knowledge [are] sought in the domain of the Social
World rather than . . . the Natural World" (Pinch and Bilker, 19841.
The attack here is not on science at the border, in particular, the prediction
of the frequency of rare events. At least the more extreme of the sociologists
of knowledge claim that the traditional ways of establishing scientific
truth by appealing to nature in a disciplined manner are not how science
really works even in situations very far from the border between science and
trans-science. Scientists are seen as competitors for prestige, forpay, and for
power, and it is the interplay between these conflicting aspirations, not the
working of some underlying scientific ethic, that defines scientific "truth."
To be sure, these attitudes toward science are not widely held by practicing
scientists at the center of scientific activity; however, they are taken seri-
ously by many political activists who, though not in the mainstream of
science, nevertheless exert important influence on other institutions the
press, the media, the courts which ultimately influence public attitudes
toward science and its technologies.
If one takes such a caricature of science seriously, how can one trust an
expert? If scientific truth, even at the core of science, is decided by negotia-
tion between individuals in conflict because they hold different nonscientific
beliefs, how can one say that this scientist's opinion is preferred to that
one's? And if the matter at issue moves across the science/trans-science
boundary, where all we can say with certainty is that uncertainties are very
large, how much less able are we to distinguish between the expert and the
charlatan, between the scientist who tries to adhere to the usual norms of
scientific behavior and the scientist who suppresses facts that conflict with
his or her political, social, or moral preconceptions.
It will not do to define a new branch of science, "regulatory science," in
which the norms of scientific proof are less demanding than are the norms in
ordinary science. A far more honest and straightforward way of dealing with
the intrinsic inability of science to predict the occurrence of rare events is to
concede this limitation and not to ask of science or scientists more than they
are capable of providing. Regulators, instead of asking science for answers
to unanswerable questions, ought to be content with less far-reaching
answers; where uncertainty bands can be established, they should regulate
on the basis of uncertainty; where uncertainty bands are so wide as to be
meaningless, they need to recast questions so that regulation does not
depend on answers to the unanswerable. And, since these same limits apply
:-
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SCIENCE AND ITS LIMITS: THE REGUL'OR'S DILE1IIMAt
19
to litigation, the legal system ought to recognize, much more explicitly than
it has heretofore, that science and scientists often have little to say, probably
much less than some scientific activists would admit.
The bona tides of scientific adversaries often is at the heart of litigation
over personal injury alleged to be caused by subtle, low-level exposures.
Each side presents witnesses whose scientific credentials are regarded as
impeccable by the side the witnesses are supporting. Since the issues them-
selves tend to be trans-scientific, one can hardly decide the validity of the
"scientific" assertions of either side's witnesses. Under the circumstances,
one is probably justified in regarding a scientific witness no differently from
any other witness: his or her credibility is judged by past record, behavior,
and general demeanor, as well as by self-consistency of testimony. Such, at
least, was the way in which Judge Patrick Kelley settled the Johnston v.
United States case (U.S. District Court, District of Kansas, Wichita, filed
Nov. 15, 1984, #81-1060), by impugning, on grounds no different from
those one would invoke in an ordinary lawsuit, the competence if not the
integrity of one side's scientific witnesses.
FINESSING UNCERTAINTY
Various approaches for finessing uncertainty can be identified. Two of
these the technological fix and invoking the principle of de m~nimis are
described briefly below without claim that these are the most important, let
alone the only, approaches.
Technological Fix
Science cannot predict exactly the probability of a serious accident in a
light-water reactor, or the likelihood that a radioactive waste canister in a
depository will dissolve and release radioactivity to the environment. Can
one design reactors or waste cans for which the probability of such occur-
rences is zero or at least which depend, for the prevention of such mishaps,
on immutable laws of nature that can never fail, rather than on the incom-
pletely reliable intervention of electromechanical devices? Surprisingly, this
approach to nuclear safety has come into prominence only in the past five
years. K. Hannerz (1983) in Sweden and H. Reutler and G. H. Lohnert
(1983) in Germany have proposed reactor systems (an intrinsically safe
light-water reactor and the modular high-temperature gas-cooled reactor,
respectively), whose safety does not depend on active interventions but on
passive, inherent characteristics. Though one cannot say that the probability
of mischance has been reduced to zero, there is little doubt that the probabili-
ties are several, perhaps three, orders of magnitude lower than the probabili-
ties of mischance for existing reactors. To the extent that such reactors
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~LVINM. WFINBERG
embody the principle of inherent safety, their adoption would avoid much of
the controversy over reactor safety, the Price-Anderson Act, repetition of
the Three Mile Island accident, and so forth. In short, such a technical fix
enables one largely to ignore the uncertainties in any prediction of core-melt
probabilities.
The idea of incorporating inherent or passive safety in the design of
chemical plants had been proposed, unbeknownst to the nuclear community,
by TrevorA. Kletz (1984) ofthe Loughborough University of Technology in
England in 1974, shortly after the disaster at the Flixborough cyclohexane
plant, which killed 28 people. One of the main consequences of the Bhopal
disaster may well be the incorporation of inherent safety into new chemical
plants, which is, again, a way of finessing uncertainty in predicting failure
probabilities.
The De Minimis Principle
A perfect technical fix, such as a totally safe reactor or a crash-proof car, is
usually not available, at least at affordable cost. Some low levels of exposure
to materials that are toxic at high levels are inevitable, even though we can
never accurately establish the risk of such exposures. One way of dealing
with this situation is to invoke the principle of de minimis. This principle, as
Howard Adler and I showed in 1978, argues that for insults that occur
naturally and to which the biosphere has always been exposed, and presum-
ably to which it has adapted, one should not worry about any additional man-
made exposure as long as the mans de exposure is small compared to the
natural exposure (Adler and Weinberg, 1978~. The basic idea here is that the
natural level of a ubiquitous exposure (like cosmic radiation), if it is deleteri-
ous, cannot have been very deleterious since in spite of its ubiquity, the race
has survived. Moreover, we concede that we do not know and can never
know what the residual effect of natural exposure really is. An additional
exposure that is small compared to the natural background ought to be
acceptable; at the very least, its deleterious effect, if any, can never be
determined.
Adler suggested that for radiation whose natural background is well
known, one might choose a de minimis level as the standard deviation of the
natural background, which is about 20 percent of the mean background-
that is, about 20 millirems per year. This value has been used as the Environ-
mental Protection Agency's standard for exposure to the entire radiochemi-
cal fuel cycle.
We know more about the natural incidence and biological effects of radia-
tion than we do for any other agent. It would be natural, therefore, to use the
standard established for radiation as a standard for other agents. This
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SCIENCE AND ITS LIMITS: THE REGU~OR'S DILEMMA
21
approach has been used by Westermark (1980) of Sweden, who has sug-
gested that for naturally occurring carcinogens such as arsenic, chromium,
and beryllium, one might choose a de minimis level to be, say, 10 percent of
the natural background.
Clearly, a de minimis level will always be somewhat arbitrary. Neverthe-
less, it seems that unless such a level is established, we shall forever be
involved in fruitless arguments, the only beneficiary of which will be the
toxic tort lawyers. Could the principle of de minimis be applied in litigation
in much the same way it might be applied to regulation that is, if the
exposure is below de minimis, then the blame is intrinsically unprovable and
cannot be litigated? The legal de minimis level might be set higher than the
regulatory de minimis; for example, the legal de minimis for radiation might
be the background (since the BEIR-III report concedes that there is no way of
knowing whether or not such levels are deleterious). The regulatory de
minimis couldjustifiably be lower, simply on grounds of erring on the side of
safety.
One approach might be to concede that there is some level of exposure that
is "beyond demonstrable effect" (BDE). This defines a "trans-scientific"
threshold. A de minimis level might then be established at some fraction, say
one-tenth, of this BDE level. For example, if we take the previously quoted
value of 100 millirems per year of low LET (linear energy transfer) radiation
as the BDE level for somatic effects, then a de minimis for low LET might be
set at 10 millirems per year. Of course, such a procedure would evoke much
controversy as to what the BDE level is or whether 10 is an ample safety
factor. This example demonstrates, however, that at least in the case of low-
level radiation, a scientific committee was able to agree on a BDE level. The
safety factor of 10 cannot be adjudicated on scientific grounds. The most one
can say is that tradition often supports a safety factor of 10- forexample, the
old standard for public exposure (500 millirems per year) was set at one-
tenth of the tolerance level for workers (5,000 millirems per year).
Can a principle of de minimis be applied to accidents? The idea is that
accidents that are sufficiently rare might be regarded somehow in the same
category as acts of God, and compensated accordingly. We already recog-
nize that natural disasters should be compensated by the society as a whole.
One can argue that an accident whose occurrence requires an exceedingly
unlikely sequence of untoward events might also be regarded as an act of
God. Thus, the Price-Anderson Act (42 U. S. C. 2210) might be modified so
that, quite explicitly, accidents whose consequences exceeded a certain
level, and whose probability as estimated by PRA would be less than, say,
10-9 per year, would be treated as acts of God. Compensation in excess of
the amount stipulated in the revised act would be the responsibility of Con-
gress. The cutoff for compensation, or for probabilities, would be negotia
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22
ALVII!iM. WEINBERG
ble, and perhaps would be revised every 10 years or so. One not entirely
fanciful suggestion might be to set any probability of the order of 10-7 to
10-8 per year to be a de minimis cutoff, this being the frequency at which the
earth may have been visited by the cometary asteroids that may have caused
the geologic extinctions.
CONCLUSIONS
The reader must be aware that, as in most such questions, identifying and
characterizing the problem is easier than solving it. That the regulator's and
the toxic torts dilemma is rooted in science's inability to predict rare events
cannot be denied. How to get the regulator and the toxic tort judge off the
horns of the dilemma is far from easy, and my two suggestions are offered
tentatively and with diffidence.
Equally obvious is the intrinsic social dimension of the issue. In an open,
litigious democracy such as ours, any regulation, any judicial decision can
be appealed, and if the courts offer no redress, in principle Congress can; but
these mechanisms are ponderous. The result seems to me to be a gradual
slowing of our technological-social engine as it becomes more and more
enmeshed in fruitless argument over irresolvable questions.
Western society was debilitated once before by such fruitless tilting with
windmills. That was, of course, the devastating campaign against witches of
the fourteenth to the early seventeenth centuries. As William Clark (1981)
has put it so vividly, in this period society took for granted that death,
disease, and crop failure could be caused by witches. To avoid such catastro-
phes one had to burn the witches responsible and some million innocent
witches were burned as a result. Finally in 1610, the Spanish inquisitor
Alonzo Salazar y Frias realized there was no demonstrated connection
between catastrophe and witches. Though he did not prohibit their burning,
he did prohibit use of torture to extract confessions. The burning of witches,
and witch-hunting generally, declined precipitously.
This story seems to capture the essence of our dilemma: the connection
between low-level insult and bodily harm is probably as difficult to prove as
is the connection between witches and failed crops. That our society never-
theless has allowed this issue to emerge as a serious social concern is an
aberration, which in the modern context is hardly less fatuous than were the
witch hunts of the Middle Ages. That dark phase in Western society died out
only after several centuries. Let us hope our open, democratic society can
regain its sense of proportion far sooner and can get on with managing the
many real problems before us instead of wasting our energies on essentially
insoluble, and by comparison, intrinsically unimportant, problems.
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SCIENCE AND ITS LIMITS: THE REGU~OR'S DILL
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Representative terms from entire chapter:
probabilistic risk
