Document pBGE08M02ozJ2MNY6ZEgjk2aE

Risk Assessment and Comparisons: An Introduction Richard Wilson and E. A. C. Crouch Risk assessment is presented as a way of examining risks so that they may be better avoided, reduced, or otherwise managed. Risk implies uncertainty, so that risk assess ment is largely concerned with uncertainty and hence with a concept of probability that is hard to grasp. The results of even the simplest risk assessments need to be compared with similar assessments of commonplace situ ations to give them some meaning. We compare and contrast some risk estimates to display their similarities and differences. ment is the exercise itself, in which (ideally) all aspects of some action arc explored The results, goals, and values ofperforming the nsk assessment must be sharply contrasted with the cultural values assigned to the results. Such cultural values will presumably be factors influencing societal decisions and may differ even for nsk estimates that are identical in probability. Risk and Uncertainty The concept of nsk and the notion of uncertainty are closely related. We may say that the lifetime risk of cancer is 25%, meaning that approximately 25% of all people develop cancer in their lifetimes Once an individual develops cancer, we can no longer talk very day we take risks and avoid others. It starts as about the nsk of cancer, for it is a certainty. Similarly if a man lies Esoon as we wake up. One of us lives in an old house that had dying after a car accident, the nsk of his dying of cancer drops to old wiring. Each time he turned on the light, there was a near zero. Thus estimates of nsks, insofar as they are expressions of small risk of electrocution. Every year about 200 people uanrecertainty, will change as knowledge improves. electrocuted in the United States in accidents involving hone wiringDifferent uncertainties appear m nsk estimation in different ways or appliances, representing a nsk ofdeath ofabout 10~* per year, or (2). There is dearly a nsk that an individual will be killed by a car if 7 x 10~3 per lifetime. To reduce this nsk, he got the wiring that person walks blindfolded across a crowded street One parr of replaced. When we walk downstairs, we recall that 7000 people die this risk is stochastic; it depends on whether the individual steps off cadi year in falls in U.S, homes. But most are over 65, so we pay the curb at the precise moment that a car amves. Another pan ofthe hide attention to this nsk since bath of us are younger than that. risk might be systcmanc; it will depend on the nature ofthe fenders How should we go to world Walking is probably safer than using and other features ofthe car. Similarly, iftwo people are both heaw a bicycle, but would take five times as long and provide less healthful cigarette smokers, one may die of cancer and the other not, we exercise. A car or, better, public transport would be both safer and cannot tell m advance However there is a systematic difference in faster. Expediency wins out, and the car comes out of the garage. Fortunately, the choice nowadays is not between horse or canoe-- both ofwhich are much more dangerous. The day has just begun, tins respect between being, for instance, a heavy smoker and a gluttonous eater of peanut butter, which contains aflatoxin Al though aflatoxin is known to cause cancer (quite likely even in and already we are aware of several risks, and have made decisions about them. Most of us act semi-automancally so minimize our risks. We also humans), the risk ofcancer from eating peanut butter is much lower than that from smoking cigarettes. Exactly how much lower is uncertain, but it is possible to make estimates of how much lower expea society to minimize the risks suffered by its members, subject and also to make estimates of how uncertain we are about the to overriding moral, economic, or other constraints. In some cases difference these constraints will dominate, m others there will be trade-offs Some estimates ofuncertainties are subjective, with differences of between the values assigned to risks and the constraints. Risk assessments, except m the simplest of draimstances, are not de signed for making judgments, but to illuminate them (1). To opinion arising because there is a disagreement among those assessing the nsks. Suppose one wishes to assess the risk (to humans) of some new chemical being introduced into the environ effectively illuminate, ana men to minimize, risks requires knowing what they are and how big they are. This knowledge usually is gained through experience, and die essence of risk assessment is the ment, or of a new technology Without any further mfbrminon, all we can say about any measure ofthe nsk is that it lies between zero and unity. Extreme opinions might be voiced, one person might sav application of this knowledge of past mistakes (and deliberate actions) in an attempt to prevent new mistakes m new situations. The results ofrisk assessments will necessarily be m the form ofan that we should initially assume a risk of unity, because wc do not know that the chemical or technology is safe; another might take the opposite extreme, and argue that we should initially assume that estimate of probabilities for various events, usually injurious. The there is zero nsk, because nothing has been proven dangerous Here goal in performing a risk assessment is to obtain such estimates, and elsewhere, we argue that it is the task of the nsk assessor to use although we consider the major value in performing a nsk assess- whatever infbrmaaon is available to obtain a number between zero and one for a nsk estimate, with as much precision as possible, The sudbon ate n the Department ofPhyva and the Energy and Environmental Pofaey together with an estimate of the imprecision. In this context, the Center, Hamid Uiuvtmty* Cambridge, MA 02138. statement "I do not know* can be viewed only as procrastination 17 APRIL 1987 CBY 1912370 ARTICLES 267 i I 4 ! I 1 1 1 1 \ RSV0034176 A and not responsive to the request for a risk estimate (although this TabU 2. Some commonplace nsks (mean values with uncertainty) should not be read as condemning procrastination in all circum stances) Action Annual nsk Uncertainty The second extreme mentioned, the assumption ofzero risk, can Motor vehicle accident (total) 2 4k ltr* 10% arise because people and government agencies have a propensity to ignore anything that is not a proven hazard. We argue that this attitude is inconsistent if the objective is to unprove the public Motor Vehicle accident (pedestrian only) Home accidents Electrocution 42x icrs 10% 11 X 1CT* 5% 5 3 X JO'* 5% health, may also lead to economic inefficiencies, and often leads to Air pollution, eastern United States 2 x 10" Factor of 20 unnecessary contention between experts who disagree strongly downward only Fortunately, if risk assessors have been diligent in searching out Cigarette smoking, one pack per dav 36 x 10" Factor of 3 i A hazards to assess, few hazards posing large risks will be missed in this way, so that there may be minor direct danger to human health from Sea-levd background radiation (except radon) Ail cancers 2 * 10" Factor of 3 2.3 x 10" 10% I a continuation of die attitude. Four tablespoons peanut butter per day 8 x 10" Factor of 3 i Drinking water with EPA limit of 6 x 10" Factor of 10 chloroform Drinking water with EPA limit of 2 x 10" Factor of 10 Risk Estimation Based on Historical Data trichloroethylene Alcohol, light drinker 2 x 10" Factor of 10 The way in which risks are perceived is strongly correlated with Police killed in tine of duty (total) 2.2 x 10" 20% the way in winch they are calculated Risks based on historical data are particularly easy to understand and are often perceived reliably It is therefore easy to illustrate a risk calculated horn historical data Police killed m hne ofduty (by (dons) 1.3 x ID** 10% Frequent flying professor 5 x 10" 50% Mountaineering (mountaineers) 6 x 10" 50% to understand some characteristics of risk estimation. There are plenty of data an automobile accidents (although never enough to measured, an upper limit may be calculated, if one assumes somd make risk assessors happy). One dung that these data can tell us is son of model). If there is no historical database for the hazard (i the frequency of such accidents in the past and their trend through new powet plant or industrial finality, for instance), one approach u time. To make predictions, however, we must use a model. The to consider it in separate parts, calculating the risks from each pan simplest modd is that there will be as many accidents next year as and adding them together to estimate a risk for the whole. Foi last, to within a statistical error ofthe square root of the number. A example, all possible chains of events from an initiator to a final slightly more complicated, but perhaps more accurate, model might accident are followed in an "event tree,' with die probabilities oi be to fit a mathematical function to numbers from previous yean each event in the nee being esumared from historical data n and to argue that next year's accidents will follow the trend given by different situations. tins fonenon. A possibly better and possibly more accurate model A particularly well-known example is the calculation of the sell might use all available information that might influence accident probability of a severe accident at a nuclear power plant (3) Thai trends. For example, an oil embargo with a concomitant rise ui oil this procedure has at least a partial validity is due to the fare that the price and reduction in automobile travel would be likely to reduce design of nuclear power planes proceeded in approximaccly this the risk of accident In any event, it becomes dear that it is factorable way; attempts were made to imagine all major aoadeni impossible to calculate any nsk without a model ofsome sort, even possibilities, "maximum credible accidents' or "design basis acci die simple one that tomorrow will be like today. dents,' and then to add an independent device to prevent thu accident from having severe consequences. To the extent that th< added safety device is independent, the failure probability is inde Risks of New Technologies pendent, and the small overall accident probability is the product of individual failure probabilities which are larger. We can only use the historical approach to estimating risks when die hazard (for example, technology, chemical, or simply some action) has been present for some ome and the risk is Urge enough Risks by Analogy: Carcinogenic Risks to be directly measured (although when it is not Urge enough to be Some carcinogenic nsks may be estimated from historical data! TabU 1. Comparison of several common radiation nsks But this is complicated by the time delay between the insult and tha final cancer, one reason why causality is hard to prove if the nsk uj Action Dose (mmtV year) Cancen if all U S population exposed (assuming linearity) Medical x-rays Radon gas (l.$ pCi/litcr, equivalent dose)* Potassium in own body Cosmic radiation at sea level Cosmic radiation at Denver Dose to avenge resident near Chernobyl mist year One traiucommemal round tnp by arr Average within 20 nuks ofnuclear plant 40 500 30 40 65 5000 5 0 02 1100 13,500 1000 1100 1800 Not relevant 135 >1 Die radon exposure * m the fangs and cannot be directly compared to whole body external exposure. The comparison hoc n on the ben of me same magnitude of rub. The uncertainty of die radon number is at kaat a factor of 3 small. This is the difficult field of epidemiology. I Although some of the largest cancer nsks have been identified through the use of epidemiology (4), preventive public healtn suggests that we endeavor to estimate risks even where no historical l data exist and tix nsk is small. This is often done by analogy with the cancer nsks to animals, usually rodents, which are deliberate^ exposed to large enough quantities of pollutant so that an effect is observed. To use these data to estimate the nsk at low doses in people involves (to oversimplify marten) two difficult steps the companion of carcinogenic potency in animal and man (5-7) and the extrapolation from a high dose ro a low dose. Because bath steps require a certain amount of theory, they are controversial. Indeed, there are those who regard the uncertainty as so great that they prefer not to provide numerical estimates of nsk (8, P), although they may order materials in carcinogenic potency The difference a6* CBY 1912371 SCIENCE, VOL. IM fr** RSV0034177 between this and providing a numerical estimate is important, but is perspective about the meaning of the magnitude of the nsk. L one of presentation rather than substance. If there are no animal data, or if in an animal experiment there is no stansncally significant effect, it docs not necessarily mean that Comparisons can be useful. We are not bom with an mstinctne feeling for what a nsk of one in a million per lifetime means, although we do learn that some nsks are small and others large It is there is no risk. If the experimenters have been diligent, the nsk is particularly helpful to compare nsks that are calculated in a similar probably small, although never zero, even though that may be the way For example, the risk oftraveling by automobile can be compared best estimate. Various attempts are made to use data even less direct to that of traveling by hone with the use of historical data than the animal bioassays to estimate risks in such cases These Another common procedure is to compare exposures only Table include simple analogies based on chemical similarity {Iff}, and I shows a list of radiation exposures in typical situations (17) The comparison with outcomes other than cancer--for example, muta dose-response relation for radiations with similar energy deposition genesis (11) and acute toxicity (12, 13). Nor surprisingly, these pa unit track length will be similar, although there may be some more indirect procedures arouse even more controversy than the correction required for dose-rate effects, so that ordering by expo animal bioassays. sure should be similar to ordering by nsk. In estimating the number There have been few attempts to perform risk assessments for of lethal cancers on a linear hypothesis, we have here assumed biological aid points other than cancer. However, it is known that approximately 8000 man-rems per cancer (at low doses), in itself the pollutants in cigarette smoke cause at least as many deaths uncertain by 30% or more. through heart problems as by cancer (14), and we should not be An example of comparison of nsks that are similarly calculated is surprised ifocher carcinogens were to produce chronic effects ocher die comparison of nsks of various chlorinated hydrocarbons in than cancer For now, die cancer risk assessment has to act as drinking water. The nsks to humans are estimated from carcinogen surrogate for these other risks also. bioassays in rodents (rats and mice). Since these are similar maten- ais, we might expect that the dose-response rdaDOtuhtps have the same shape. Chloroform, which is produced by interaction of Risk Value Versus Certainty of Information chlorine with organic matter during the chlorination of surface waters to kill bacteria, produces cancer in animals 20 nines as readily After risks of a number ofsituations have been assessed, we often as does trichloroethylene, an industrial solvent that is occasionally want to order them in order to decide which should command our found in well waters as a result of accidental pollution. Although attention. It is not always the order ofincreasing nsk that is used for neither is known to cause cancer ui people, we might expect that such purposes. There have been proposals to order potential chloroform would do so about 20 times as readily. carcinogens on other factors (8, 15), such as the certainty of Table 2 shows a variety of nsks calculated in vanous ways and our information. estimate of the uncertainly. They are deliberately jumbled to Vinyl chloride gas has been found to cause angiosarcomas both in provoke thought by juxtaposiuon. [Risk estimates quoted by the people and in rats. Since an angiosarcoma is a rare tumor, the risk Environmental Protection Agency (EPA) for carcinogens tend to be ratio (the ratio of the observed number ofcancers in those exposed greater than those shown in Table 2 by a factor approximately equal to the number expected by chance) is oforder 100 or more in some to riie uncertainty factor--this is not accidental (5,18).] cases. If an angiosarcoma is seen in a vinyl chloride worker, the attribution to vinyl chloride exposure is almost certain. On the other hand, the numba ofpersons who have been heavily exposed to vinyl chloride is small, so that only about 125 angiosarcomas have been Contrasting Bisks seen among vinyl chloride workers worldwide in the last 20 years. Objections have been raised to nsk comparisons on the ground Now that exposures in the workplace have been greatly reduced, no that they are misleading This would be true if all risks of the same angiosarcomas attributable to recent occupational exposure have numencal magnitude were treated in the same way. But they are not. been seen. We do nor know the dose-response relation, but it is In some cases it u useful to contrast risks to indicate the different generally believed that the response falls at least linearly as the ways in which they arc treated in society. In Table 3 we give an exposure is reduced, so that no more than one cancer is expected in example by comparing and contrasting the carcinogenic effects of several years. afiaroxin B1 and dioxin, both among the most carcinogenic chemi We can compare this with the possible cancer incidence that was cals known. The difference in treatment of these two materials is predicted by the Food and Drug Administration (FDA) in 1977 from use ofsaccharin (16). This was based on experiments with rats, leading to an additional uncertainty. More people ate saccharin than perhaps a reflection of different values assigned to vanous aspeas of rite problems caused by their presence. Afiaroxin and dioxin have similar toxiaucs and carcinogenic were exposed to vinyl chloride, and nearly 500 cancers pa year were potency (perhaps within a factor of 10, although both measures for estimated for the United Stares alone. For vinyl chloride we both chemicals vary substantially with species tested). The certainty therefore have die situation that the individual nsk is now low, yet of information for afluoxin is great. There is less information about there is considerable certainty that there is a risk For saccharin the carcinogenicity of dioxin Dioxin may be a promoter and pose a risk is higher, but there is more uncertainty about the value of the minuscule nsk at low doses, whereas afiaroxin is almost certainly an risk. Some persons, in some situations, may demand that more iruuator also. Nonetheless such standards as there are appear to be attention be given to the nsk from vinyl chkmde than to the nsk more stringent for dioxin, possibly because dioxin is an artificial from saccharin; for odia persons or situations the reverse may be chemical and possibly because it was a trace component of a the case. chemical mixture (Agent Orange) that was used in warfare. The small nsk ofa large accident in a nuclear power plant can also be contrasted with the more numerous small accidents or events chat Comparison of Risks occur every day in the mining, transport, and burning ofcoal One feature that is brought out dearly hoe is that we do not always The purpose of nsk assessmenr is to be useful in making decisions compare the risk averaged ova tune, but worry more about nsks about rite hazards causing nsks, and so it is important to gam some that are sharply peaked in time. 17 APRIL 19*7 CBY 1912372 ARTICLES x69 RSV0034178 Expression of Risks Just as a comparison ofrisks is an aid in understanding them, so is a careful selection of die methods of expression. It is hard to comprehend the scarisocal (stoehasne) nature ofrisk. There are ways to mitigate this difficulty in comprehension. We are almost all used to one such statistical concept--the expectation oflife When we talk about the expectation oflife being 79 yean (for a nonsmoking male in the United States) we all know that some die young and that many live to be over 80. Thus the expression of a risk as the reduction of life expectancy caused by the risky action conveys some ofdie statistical concept essential to its undemanding. One partial' 1st calculation ofthis type can be used as an anchor for many people, because it is easy to remember. The reduction oflife expectancy by smoking agaretres can be calculated from the risk, one in 2 million, ofsmoking one cigarette, muluplied by the difference ofthe avenge life-span of a nonsmoker and a lung cancer victim. This rums out to be 5 minutes, or the nme it takes to smoke the one cigarette. It is important to realize that risks appear to be very different when expressed in different ways (19). One example of dus can be seen if we consider the cancer risk to those persons exposed to radionuclides after die Chernobyl disaster. According to the Soviets (29), the 24,000 persons between 3 and 15 kilometers from the plant, but excluding the town of Pripyat, recaved and are expected to receive 1.05 million man-rems total integrated dose, or about 44 rems average. Even if we assume a linear dose-response relation, with 8000 man-rems pet cancer, the risk may be expressed in different ways. Dividing 1.05 million man-rems by 8000 gives 131 cancers expected in the lifetimes of that population. This is larger than, and for some people more alarming than, the 31 people within the power plant itself who died within 60 days of acute radiation sickness combined with bums. Dividing the 131 again by the approximately 5000 cancer deaths expected from other causes, the accident causal "only'' a 2.6% increase in cancer. This seems small compared to the 30% of cancers attributable to cigarette smoking. The difference is even more striking if we consider the 75 million people in Byelorussia and die Ukraine who received, and will receive, 29 million man-rems over their lifetimes. On the linear doseresponse relation this leads to 3500 "extra cancels,'' surely a large number for one accident. But dividing by the 15 million cancers expected in this population leads to an "insignificant" increase of 0.0047%. Ofcourse, none ofthe methods ofexpressing the nsk can be considered "right" in an absolute sense. Indeed, it is our belief that a full understanding ofthe risk involves expressing it in as many different ways as possible. Cost of Reducing a Risk Another interesting and instructive way of comparing risks is by comparing die amount people have paid in the past to reduce them. It might be thought that people would try to adjust their activities until the amount spent is roughly die same. Cohen (21) has shown that the amounts spent vary by a factor of more than a million. He shows that it would be possible even for an American to save fives in Indonesia by aiding in immunization at $100 per life saved. Society is willing to spend more on environmental protection to prevent cancer (over $1 million per life) than on cures (about $50,000 per life with the high value of $200,000 for kidney dialysis raising some objections). This ratio is in rough accord with the maxim "an ounce of protccnon is better than a pound of cure." People are willing to spend still more on radiation protection at nuclear power plants and Table 3. Comparison of two very toxic chemicals, aflatonn B1 (22) and dionn (23), CDC, Centers for Disease Control Measure Afiatoun BL Dionn Acute retoatv Carcinogenic potency to people [(kg * day)/mg] Carcinogenic potency to rats ((kg davVmgl Mutagenic Certainty of information on human caronogematy Activity (tmuaror or promoter) PotubUity of threshold dose response Source Common knowledge FDA action level in peanuts (ppb) CDC level of concern in toil (ppb) High -500 -5000 Yes High lnmarof Low Natural Little known 20 Equal Unknown -5000 No Low Promoter (*) High Artificial Agent Orange 1 on waste disposal. Economists and others often argue that efficiency depends on adjusting society until the amounts spent to save lives m different situations are equalized. It seems to us that society docs not i work that way. People are aware ofthe order of magnitude of these differences, and approve of them. Nonetheless, we believe that | providing this information to a decision-maker is essentia] for an ! informed decision. REFERENCES AND NOTES 1 L 8 Lave, Sana 236, 291 (1987) 2 R_ Wilson, E A. C Crouch, L, Zasc, m Rat Qtumttnmi mad Ramlmarr Poitrt (Cold Spring Haibor laboratory Pros, Cold Spring Harbor, NY, 198S). Banbury Repost, vol 19, pp. 133-147 3 N C Rasmussen a mi, "Reactor safety study--an assessment of accident nsks in - U.S commercial nuclear power plans' (WASH 1400, NUREC 75/014, US Nuclear Regulatory Commission, Washington. DC, 197$) See also D Obent, i Soma 236, 296 (1987) I 4 R- DoS and R. Pern,/. Nad Omarhat. 66,1191 (1984) 5 E L Anderson a tL, JtatAnaf 3,277(1983) 6. E. A.C Crouch and R. Wilson, J. TunaL bnwrm Hahk 5,1095 (1979) 7. E J Calabrese. Prmafla mfAnmol Eanpsfrnsir (Wilev, New York. 1983) 8 R Feta, mAmimminrmfRatfitmLmo-LinlExpaar afmdtmnmm mad donah, A D Woodhcad, C J SheUabarger, V Pond, A. Hollaender, Eds (Plenum. New York, 198S), pp 3-16. 9 E N Ames. R Msgs*. L. S Gold, Sana 236. 271 (1987) 10 "Conool of mhalomethanes in drinking water,' proposed nik. Fid Ram 43. I 5756 (1968) Sec also the advanced nonce [iM 41,28991 (1976)] and the Anal rule jiW 44.68624 (1979)] 11 M Mcadson and K Russai. in Orwmr if Hamm* Cmnttr, H H Hon. ) D Watson, } A Wuurrn, Eds (Cold Spring Harbor Lsboraron Cold Sprmr Harbor, NY, 1977), p 1473 12 S Parodi, M Tanmgher, P Boero, L- Sinn,Mum Ha 93, I (1982) 13 L Zeuc, R. Wilson, E A C Crouch, Rut Anal 4, 187(1984) 14 Sasetny madHaiti, ajtewfrefnbe 5y Gnsensl (PH579-S0066, Public Health Sennet, Washington, DC, 1979) 15 R_ A. Squire, Sana 214,877 (1981) i 16 'Sscdunn arid ns salts,'proposed rule and hearing,Ted Rtgat 42.19996 11977) 17 R. Wilson and W J loncs, Fjarmy, Laiotr' mad ua Emnremmtnl fAcademic Prrsi New York, 1974), table 9-6 Other errata may be readily calculated from data in the reporo of the United Nations iacnn& committee on die effects of atomic radiation pSources and effects of tomzing radiaoon' (United Nations. Sms York. 1977)) and the report of the Committee on die Biological Effects of Ionizing Radianons ("The effects on populations of exposure to low leseh of ionizing radiamns* (Nnsoml Academy Press, Washington, DC 1980)) 18 M Russell and M Gruber, Sana 236, 286 (1987) 19 A.Tvctskv and D Krfmcnun,aM 211,4S3 (1981) See also P Sknie.iM 236. 280 (1987) 20. USSR. State Gommmcc fee the Uohzamn of Atomic Energy, "The accident n the Chernobyl Nuclear Power Plant and it* consequences,' working document for die Phil Accident Review Meeting, 2S-29 August 1986, International Atomic Energy Agency, Vienna 21 B L Cohen, Haiti, Pby, 38, 33 (1980) 22. H R. Roberts, "The regulatory outlook far nut produets,' paper presented ir the Annual Convention of the Peanut Butter Manufacturers and Nut Salters Associa tion, West Palm Beach, FL November 1977. 23 R_ D Kimbrough. H Falk, P Stehr, G Fries, / Tonal Enrol* Haiti 14, 47 24 Las' work on nsk assessment has been supported by donations bom ClauoL Inc, the Dow Chemical Company, the Cabot Corporation, die General Electric Foundanon, and the Monsanto Corporation *70 CBY 1912373 science, vol 236 RSV0034179