Assessment and Management of Chemical Risks - American

2 Use of Toxicity Test Data in the Estimation of Risks to Human Health Downloaded by HONG KONG UNIV SCIENCE TECHLGY on January 16, 2018 | http://pubs.acs.org Publication Date: January 13, 1984 | doi: 10.1021/bk-1984-0239.ch002

NORTON NELSON Institute of Environmental Medicine, New York University Medical Center, New York, NY 10016

Historically there has been an enormous elaboration of techniques for evaluation of the toxicity of chemicals in the last thirty years. At that time chronic lifetime tests in rodents were just coming into application and tests on human subjects, prisoners and "volunteers" were not infrequent. On the other hand, there have been perhaps some retrograde changes, namely in the less frequent use of some of the larger species, such as cats, rabbits, dogs and primates. It is perhaps also true that there is now greater routinization than in earlier decades with somewhat less attention to fitting the toxicity test to the chemical and to the circumstances. The basic problems remain: biological transfer from one species to another and the need for better quantitation, greater sensitivity, and higher efficiency in cost and time. Larger test groups have brought some improvement in quantitation and sensitivity. The use of human subjects has virtually and properly disappeared with growing concern for the ethical issues involved. A heavy preoccupation with cancer as the endpoint has in some degree lessened interest in other sometimes more important endpoints. Hopefully this trend will be reversed under the new National Toxicology Program which will attempt to broaden the range of information secured. We have had many attempts to develop short term tests aimed at securing the needed information in a shorter time and less expensively. The bacterial revertant test is clearly outstanding in this regard. This still has defects which may be amenable to correction. Improvement in fields other than mutagenesis (and cancer) has been extremely uneven, and there is no counterpart "success story." An objective of the 0097-6156/84/0239-0013506.00/0 © 1984 American Chemical Society

Rodricks and Tardiff; Assessment and Management of Chemical Risks ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

Downloaded by HONG KONG UNIV SCIENCE TECHLGY on January 16, 2018 | http://pubs.acs.org Publication Date: January 13, 1984 | doi: 10.1021/bk-1984-0239.ch002

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future is to expand the range of short term tests and reduce the need for whole animal studies. Means for using a l l dose points for estimating a "pseudo" no observed effect level (NOEL) is suggested. It may well be, however, that major improvement is not to be sought in finding more rapid models for existing toxicity tests but to develop a synthesis of independent information acquired by ancillary routes. Thus, the Ames system is limited in the sense that it fails to deal, for example, with mammalian repair mechanisms. Such information might be specifically sought in separate tests. Similarly, the pharmacokinetic aspects of movement from point of entry into the body to the target tissue and target biochemical unit (DNA) could expand the utility of simple tests. In more general terms and with endpoints other than cancer, one can visualize the synthetic assembly of information from a variety of studies which could inform as to some of the biological factors that we know are involved and which cannot be derived from a single test; the tissues obtained through surgical operations and autopsies could supply the needed human tissue. It seems possible that such an approach applied to a variety of endpoints could strengthen very substantially both the quantitative and qualitative aspects of toxicological assessments and could, therefore, make quantitative risk assessment more meaningful. These and other opportunities to improve and make more efficient toxicological appraisals for risk assessment will be discussed. The l a s t 30 years have seen major changes i n the p r a c t i c e of t o x i c o l o g y , both q u a l i t a t i v e l y and q u a n t i t a t i v e l y . Quantitatively the conduct o f t o x i c o l o g i c a l p r e t e s t i n g has expanded very substant i a l l y ; there are now many contract l a b o r a t o r i e s a v a i l a b l e f o r the conduct o f such work. Q u a l i t a t i v e l y many changes have occurred over that p e r i o d o f time. Chronic l i f e t i m e t e s t i n g , e s p e c i a l l y with the cancer endpoint i n mind, was a l r e a d y e s t a b l i s h e d but was r e l a t i v e l y new as a r e g u l a r p a r t of t o x i c o l o g i c a l p r e t e s t i n g . Indeed one of the great triumphs of t o x i c o l o g i c a l t e s t i n g was the i d e n t i f i c a t i o n o f the c a r c i n o g e n i c i t y o f AAF i n 1941 by Wilson, e t a l . ( 1 ) . T h i s compound, which was o r i g i n a l l y proposed as a p e s t i c i d e , was found to be c a r c i n o g e n i c i n those FDA t e s t s , thus a b o r t i n g i t s use as a p e s t i c i d e but, a t the same time, p r o v i d i n g the experimental cancer community with one o f the most widely used research carcinogens.



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At that time, t e s t groups were g e n e r a l l y much smaller than they are now, and a f u l l e r awareness of the importance of group s i z e and the s t a n d a r d i z a t i o n of t e s t procedures has developed. I t i s a l s o perhaps true that there has been a major trend towards r o u t i n i z a t i o n i n t o x i c o l o g i c a l t e s t s ; i n some cases t h i s i s a step i n the wrong d i r e c t i o n , s i n c e r o u t i n i z a t i o n b r i n g s with i t two dangers: one that t e s t s i r r e l e v a n t to the chemical or to the expected use may be undertaken, and the other i s that a thoughtful s p e c i f i c adaptation of the t e s t procedures to the p a r t i c u l a r s of the circumstances may be omitted. Thus, unneeded things may be done and needed things may not be done. There i s another danger of o v e r s t a n d a r d i z a t i o n i n the sense that when a producer of a chemical i s given p r e c i s e i n s t r u c t i o n s as to what t e s t s are to be conducted, he i s to some degree r e l i e v e d of the i n t e l l e c t u a l and e t h i c a l r e s p o n s i b i l i t y f o r using the best a v a i l a b l e science and a r t to e s t a b l i s h the s a f e t y of the compound f o r the proposed uses. In t h i s sense, the one who conducts the t e s t may say he has s u p p l i e d the information requested and so has f u l f i l l e d the l e g a l commitment. The p e t i t i o n e r i s thus f r e e d of any i m p l i c a t i o n of r e s p o n s i b i l i t y f o r e x e r c i s i n g h i s own i n g e n u i t y and s c i e n t i f i c acumen i n using the best of the a v a i l a b l e science to e s t a b l i s h the s a f e t y of the agent i n question. There are, of course, other reasons f o r s t a n d a r d i z a t i o n , e s p e c i a l l y i n regard to cross-comparison and h i s t o r i c a l comparab i l i t y of data, but the dangers i n over r o u t i n i z a t i o n are ever present, should be recognized, and c o n s t a n t l y questioned by the responsible t o x i c o l o g i s t . There have been perhaps some other retrograde movements, such as a l e s s e r tendency at the present time to use l a r g e r s p e c i e s , such as r a b b i t s , c a t s , dogs, monkeys, and to depend almost e x c l u s i v e l y on rodents. Perhaps to some degree these are i n e v i t a b l e p r i c e s to pay f o r the more widespread use of t o x i c i t y prescreening t e s t s which i n i t s e l f i s of course a most s a l u t a r y t r e n d . The b a s i c problems i n the f i e l d remain, that i s , the u n c e r t a i n t i e s i n t r a n s f e r r i n g data from the t e s t species to man, the need f o r b e t t e r q u a n t i f i c a t i o n , greater s e n s i t i v i t y , and higher e f f i c i e n c y i n c o s t and time. Although the s e n s i t i v i t y of t o x i c o l o g i c a l t e s t s has improved somewhat with the trend toward l a r g e r group s i z e , the s e n s i t i v i t y thus achieved i s i n many cases f a r short of that r e l e v a n t f o r d i r e c t t r a n s f e r of the f i n d i n g s to man. In the case of cancer, f o r example, i n c i d e n c e r a t e s as a minimum a p p l i c a b l e to man of the order of 10"^, 10"^, or 10"^ are imperative; t h i s i s , of course, not even remotely achievable i n p r a c t i c a l l a b o r a t o r y experiments. In some degree t r a n s f e r a b i l i t y of data may have been impaired through the r e d u c t i o n i n the number of species g e n e r a l l y used; i n a d d i t i o n , i t must be kept i n mind that the trend toward use of h i g h l y i n b r e d s t r a i n s (although d e s i r a b l e from the p o i n t of view of u n i f o r m i t y of response) nevert h e l e s s leads towards the use of t e s t animals with h i g h l y s p e c i f i c s u s c e p t i b i l i t i e s which may l e a d to missing other endpoints were an outbred s t r a i n with more g e n e t i c d i v e r s i t y used.



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The use of s o - c a l l e d " n o - e f f e c t " l e v e l s i n e s t i m a t i n g " s a f e " l e v e l s f o r man from l a b o r a t o r y s t u d i e s has a l o n g t r a d i t i o n . In 1975 the author c a l l e d a t t e n t i o n to the l i m i t e d value and s t a t i s t i c a l meaninglessness of t h i s term, e s p e c i a l l y when the group s i z e i s not s p e c i f i e d ; a t that time he suggested that the term should at l e a s t i n c l u d e the q u a l i f i e r "observed," that i s , the noobserved e f f e c t l e v e l (NOEL)(2). This term i s a t l e a s t more accurate; however, i t s t i l l does not normally make f u l l use of dose response data. As p r e s e n t l y used, the NOEL i s d e f i n e d as a p o i n t between two s e q u e n t i a l data l e v e l s , one with an observed e f f e c t and one with none. Thus, i t e s s e n t i a l l y represents the use of a s i n g l e p o i n t i n the p o s i t i v e dose response data. This sometimes i n v o l v e s d i s c a r d i n g s i g n i f i c a n t a d d i t i o n a l data. I would propose that an a l t e r n a t e technique be used to develop what I would t e n t a t i v e l y c a l l a "pseudo" NOEL. This would i n v o l v e f i t t i n g a curve to the observed dose response p o i n t s . Any one of a number o f procedures could be used here (e.g., the p r o b i t o r l o g i t c u r v e ) . One would then f i n d the dose l e v e l corresponding to an a r b i t r a r i l y s e l e c t e d low i n c i d e n c e p o i n t , e.g., 1%. One percent i s an i n c i d e n c e l e v e l which could be e a s i l y overlooked i n most l a b o r a t o r y s t u d i e s i n a s i n g l e experiment u s i n g 50 animals or even i n s e v e r a l experiments. This 1% l i m i t would be regarded as a "pseudo" NOEL; i t would perhaps sometimes correspond to an a c t u a l NOEL. This technique would permit use of a l l data i n the s e l e c t i o n of t h i s s t a r t i n g point f o r whatever subsequent data treatment i s d e s i r e d . I t would b r i n g with i t such s t a t i s t i c a l parameters as confidence l e v e l s . Thus, one would r e p l a c e the present NOEL with an a r t i f i c i a l one based on an a r b i t r a r y i n c i dence l e v e l that corresponds to that i n c i d e n c e l e v e l which may or may not be d e t e c t a b l e i n normal experiments with groups of 50 animals. What one then does with a NOEL would r e q u i r e f u r t h e r consider a t i o n . One could use t h i s "pseudo" NOEL with a s a f e t y f a c t o r or one c o u l d use i t as a p o i n t f o r a l i n e a r e x t r a p o l a t i o n to "0" (or the background l e v e l ) f o r example. A somewhat s i m i l a r approach f o r a d i f f e r e n t purpose has r e c e n t l y been proposed (_3) . I t may w e l l be that a more competent s t a t i s t i c a n than the author w i l l choose other i n t e r c e p t s o r other techniques. However, the b a s i c o b j e c t i v e i s to use a l l p o s i t i v e data and i n a manner which w i l l permit the development of confidence l e v e l s . The use of human s u b j e c t s f o r t e s t purposes, i n c l u d i n g s t u d i e s on p r i s o n e r s once widely used, has h a p p i l y e s s e n t i a l l y disappeared. I t has not, however, been adequately r e p l a c e d by c a r e f u l study of i n d i v i d u a l s who have already been exposed to t o x i c agents; thus the wider use of c l i n i c a l follow-up and b i o l o g i c a l monitoring i s an urgent need i n t h i s i s s u e of t r a n s f e r of i n f o r m a t i o n from the t e s t s p e c i e s t o man. A growing emphasis on cancer as the endpoint has i n some



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degree preempted i n t e r e s t from other e q u a l l y important endpoints. I t now appears that t h i s trend may be stemmed and perhaps reversed with the development of the new N a t i o n a l Toxicology Program which w i l l s y s t e m a t i c a l l y work towards the development of t e s t s aimed at r e v e a l i n g e f f e c t s other than cancer, such as on the v a r i o u s organ systems and b e h a v i o r a l responses. The d r i v e towards s e c u r i n g i n f o r m a t i o n l e s s expensively and i n a s h o r t e r time has met with outstanding success i n cancer t e s t i n g where the b a c t e r i a l r e v e r t a n t t e s t s , such as the Ames Test, have proven to be a very u s e f u l screen f o r mutagenic agents and, thus, f o r c e r t a i n kinds of chemical carcinogens. S i m i l a r l y , c e l l transformation s t u d i e s and t e s t s f o r DNA damage have been developed which can strengthen the relevance of t e s t s of t h i s s o r t f o r p o t e n t i a l c a r c i n o g e n i c i t y and mutagenicity. In other f i e l d s of t o x i c o l o g y , there has been s u b s t a n t i a l l y l e s s success. A number of attempts have been made i n the f i e l d of t e r a t o l o g y , and these have some u t i l i t y ; but by and l a r g e attempts to develop short term t e s t s using i s o l a t e d c e l l or enzyme systems have not been h i g h l y fruitful. Again, although some usage of organ f u n c t i o n t e s t s i s underway ( p a r t i c u l a r l y lung, l i v e r and k i d n e y ) , these have not been s y s t e m a t i c a l l y explored with the view toward adapting them e f f i c i e n t l y and meaningfully to l a b o r a t o r y animal s t u d i e s . What i s r e q u i r e d here i s a systematic attempt to streamline such t e s t s and to improve them i n respect to s e n s i t i v i t y , r e p e a t a b i l i t y and i n f o r m a t i v e n e s s . This i s an area i n which some degree of s t a n d a r d i z a t i o n would be h i g h l y u s e f u l and represents a f i e l d f o r systematic study. Of course, the need i s not merely to shorten the time of t e s t i n g and save money, but to improve t h e i r u t i l i t y . An i n t e r e s t i n g formal treatment of r i s k e s t i m a t i o n has r e c e n t l y been put forward by Nordberg and Strangert ( 4 ) . Conceptually i t deals with compartmental movement, metabolism and the d e f i n i t i o n of the c r i t i c a l organ, c r i t i c a l e f f e c t s and c r i t i c a l c o n c e n t r a t i o n . I t a l s o d e f i n e s a new concept, the "damage" f u n c t i o n , " r e l a t i n g to the c r i t i c a l i n j u r y . There i s a s t r a t e g y which has been used only to a l i m i t e d extent which merits f u l l s c a l e e x p l o r a t i o n as a route to b e t t e r i n f o r m a t i o n , b e t t e r q u a n t i t a t i o n and greater relevance to humans. I r e f e r to the o r d e r l y assembly of i n f o r m a t i o n from d i f f e r e n t t e s t s i n t o a coherent approach to an attempt to r e l i a b l y r e l a t e l a b o r a t o r y data q u a l i t a t i v e l y and q u a n t i t a t i v e l y to human h e a l t h e f f e c t s . Figure 1 i l l u s t r a t e s some of the routes and mechanisms which determine the end e f f e c t of a t o x i c chemical on a mammalian organism, be i t man or a rodent. T h i s i s intended to i l l u s t r a t e those steps which each of us, of course, are very f a m i l i a r with, namely entry i n t o the body v i a i n h a l a t i o n , v i a s k i n p e n e t r a t i o n , v i a o r a l i n g e s t i o n , the extent of a b s o r p t i o n , a l t e r a t i o n during or a f t e r a b s o r p t i o n , through enzymatic or chemical processes ( t o x i c a t i o n - a c t i v a t i o n , d e t o x i c a t i o n - i n a c t i v a t i o n ) , the t r a n s p o r t through the organism ( r a t e s depending upon compartment i n t e r f a c e s , whether the process i s a c t i v e or p a s s i v e ) , the a t t a c k on the end




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c r i t i c a l c e l l or biochemical u n i t , and f i n a l l y the r e p a i r mechanisms (or, i f f u n c t i o n i s a l t e r e d , the r e s t o r a t i v e f u n c t i o n a l c a p a c i t y ) . Even t h i s complicated sequence of i s s u e s i s , of


course,

a

s i m p l i f i c a t i o n ; nevertheless,

i t does r e p r e s e n t

a

p a t t e r n which i n v a r y i n g degrees determines the outcome. The s t r a t e g y suggested here i s to f i r s t define the c r i t i c a l processes or c r i t i c a l organ. This could be done where p o s s i b l e on s t r u c t u r a l grounds, by analogy w i t h other chemicals, or best from l a b o r a t o r y assays of appropriate l e n g t h and complexity. The next step would be to undertake p a r a l l e l l a b o r a t o r y s t u d i e s of animal and human ( s u r g i c a l , autopsy) t i s s u e s to e s t a b l i s h the q u a l i t a t i v e and q u a n t i t a t i v e r e l a t i o n s h i p between the t e s t s p e c i e s ( s ) and humans. Having defined the c r i t i c a l organ, c e l l or biochemical u n i t , the o b j e c t i v e would then be to define the r e l a t i o n s h i p between the entry dose (e.g., i n h a l e d , ingested, etc.) and the t a r g e t receptor dose through these a n c i l l a r y s t u d i e s . This r e l a t i o n s h i p obviously i n v o l v e s the pharmacokinetic and metabolic patterns to which the chemical i s s u b j e c t . Next to be taken i n t o account are the nature of i n j u r y to the t a r g e t system, the r e p a i r (or f u n c t i o n a l adaptive response) and the r e v e r s i b i l i t y of the e f f e c t s . Such an approach would i n v o l v e , according to need, study of i s o l a t e d systems (human t i s s u e s as w e l l as animal t i s s u e s ) , pharmacokinetic s t u d i e s (on l a b o r a t o r y animals), and the examinat i o n of r e p a i r mechanisms. Examination of metabolic a c t i v a t i o n or i n a c t i v a t i o n w i l l i n v o l v e organ systems, i s o l a t e d enzyme or c e l l systems i n c l u d i n g c e l l c u l t u r e s , as r e q u i r e d . The s t r a t e g y i s then to attempt to i d e n t i f y the p a r t i c u l a r l y c r i t i c a l stages between exposure and e f f e c t and to focus study on these i n a comparative manner. The attempt would then be to synthesize or assemble these components i n t o a q u a n t i t a t i v e and q u a l i t a t i v e chain l i n k i n g the l a b o r a t o r y s t u d i e s to man. Such an approach i s o u t l i n e d i n s k e l e t o n manner i n Figure 2 , which i n a very much s i m p l i f i e d manner suggests an o r g a n i z i n g scheme f o r animal to man e x t r a p o l a t i o n of chemical carcinogens. I wish to acknowledge my indebtedness to my colleague, Professor Bernard A l t s h u l e r , f o r t h i s schema. As you w i l l note, i t b r i e f l y o u t l i n e s the s e v e r a l stages of entry, a c t i v a t i o n , i n a c t i v a t i o n , movement to the t a r g e t biochemical u n i t (DNA), on to the s e v e r a l r e p a i r mechanisms, i n i t i a t i o n , e a r l y c e l l transformation, c e l l progression and growth ( f r e q u e n t l y through a benign s t a g e ) , f i n a l l y to u n c o n t r o l l e d growth and a malignant tumor. The s t r a t e g y proposed here would very much depend on the use of human t i s s u e s from a c c i d e n t cases, from s u r g i c a l operations, and such sources; the o b j e c t i v e i s a q u a l i t a t i v e and q u a n t i t a t i v e comparison of human t i s s u e with the t i s s u e s of the s p e c i e s ( s ) s t u d i e d i n the l a b o r a t o r y . At t h i s time our b i o l o g i c a l knowledge of the a c t i o n of chemical carcinogens makes the a p p l i c a t i o n of t h i s s t r a t e g y to cancer p a r t i c u l a r l y appealing. Even so, i t has not yet been a p p l i e d i n a systematic manner. There have been a s e r i e s of



ENTRY DOSE Passive and active transport


Enzymatic and non-enzymatic chemical alterations activation, inactivation, and elimination DNA DOSE TARGET RECEPTOR DOSE Excision repair Post-replication misrepair INITIATION EARLY CELL TRANSFORMATION Cell progression through unknown processes Clonal growth Cell death Benign

Malignant

MALIGNANT TUMOR Experimental

Species (Strain, Sex)

Organ

Animal Experiments

Man

Skin

Cell Culture: Mutagens, transformation

Mouse

Liver

Rat

Bladder

Hamster

Lung

Organ Culture: Inflammation, other lesions

Possibly others CARCINOGENS Direct Acting: BCME, BPL, DMCC, EPI Indirect Acting: FANF, Nitrosomines, AAF, Aromatic Hydrocarbons Promoters Cocarcinogens Methotrexate Figure 2 .

Animal-to-man e x t r a p o l a t i o n : o r g a n i z i n g scheme.



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i s o l a t e d s t u d i e s which use components f o r t h i s purpose, such as the work of Gehring and colleagues ( 5 , 6 ) on the examination of the pharmacokinetics of v i n y l c h l o r i d e , t h e i r importance f o r dose-response r e l a t i o n s h i p s of v i n y l c h l o r i d e , and t h e i r i m p l i c a t i o n s f o r man; these were a l s o explored by Anderson, Hoel and Kaplan ( 7 ) . Studies by Autrup, et a l . (8), examine comparative patterns of t i s s u e metabolism of polynuclear aromatic compounds. D i r e c t p a r a l l e l c r o s s - s p e c i e s s t u d i e s of r e p a i r mechanisms of damaged DNA r e l e v a n t to t h i s s t r a t e g y w i l l , of course, a l s o be needed. This i s obviously a c r i t i c a l i s s u e i n moving from such s i m p l i f i e d systems as b a c t e r i a l revertant t e s t s to mammalian systems where r e p a i r mechanisms are of v i t a l importance and are very d i f f e r e n t i n b a c t e r i a l than i n mammalian systems. The r e l a t i v e l y o r d e r l y i s s u e of e x t r a p o l a t i o n of cancer from l a b o r a t o r y to man, although very complex, i s nevertheless probably c l o s e r than other non-cancer endpoints to p r o v i d i n g underlying concepts upon which to develop t h i s " s y n t h e t i c " r i s k assessment; the f u l l development of t h i s s t r a t e g y to other endpoints may be w e l l i n the f u t u r e . Nevertheless, I b e l i e v e that i t i s only through such attempts, c a r e f u l l y and s e l e c t i v e l y a p p l i e d , that we w i l l move beyond the present long term e l a b o r a t e , expensive and poorly informative t o x i c o l o g i c a l s t u d i e s toward an approach that may be more r e l i a b l e , more q u a n t i t a t i v e and more r e l e v a n t to man, perhaps i n some cases shorter i n time and even perhaps l e s s expensive. Neither of the l a s t two o b j e c t i v e s should, however, be of o v e r r i d i n g consequence. Q u a n t i t a t i v e r i s k assessment depends on data that are r e l i a b l e , s e n s i t i v e and q u a n t i t a t i v e . I t may w e l l be that the numerical e x t r a p o l a t i o n from the current small s c a l e (but manageable) l a b o r a t o r y t e s t s can be s u b s t a n t i a l l y improved and moved downward to the e f f e c t s of lower dose l e v e l s through the shrewd use of these i s o l a t e d c e l l and biochemical t e s t systems where the i n t e r p l a y of i n a c t i v a t i o n , a c t i v a t i o n and target molec u l e i n j u r y can be s t u d i e d at concentrations w e l l below those p o s s i b l e where one i s l o o k i n g at endpoints i n r e l a t i v e l y small groups of whole animals. Although I have d e a l t i n broad g e n e r a l i t i e s and no doubt have s i m p l i f i e d many issues and underestimated the s c i e n t i f i c d i f f i c u l t i e s , nevertheless the promise of such s t r a t e g i e s i s so great that s u b s t a n t i a l endeavors i n s e l e c t e d areas should be undertaken now without f u r t h e r delay. Unquestionably e r r o r s w i l l be made and f a l s e s t a r t s w i l l ensue, but t h i s i s i n e v i t a b l e i n d e a l i n g with a f i e l d of t h i s degree of complexity.


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ASSESSMENT AND MANAGEMENT OF CHEMICAL RISKS

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Wilson, R.H.; De Eds, F.; Cox, A.J. Jr. Cancer Res. 1941, 1, 595-608.


2.

3. 4. 5.

Nelson, N., Chairman, Committee for the Working Conference on Principles of Protocols for Evaluating Chemicals in the Environment; National Academy of Sciences: Washington, D.C., 1975, 454 pp. Albert, R.E., personal communication. Nordberg, G.F.; Strangert, P. "Risk Estimation Models Derived from Metabolic and Damage Parameter Variation in a Population," 1982, to be published. Gehring, P.J.; Watanabe, P.G.; Blau, G.E. Ann. N.Y. Acad. Sci.

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7. 8.

1979, 329, 137-52.

Ramsey, J.C.; Gehring, P.J. in Health Risk Analysis, Proceedings of the Third Life Sciences Symposium, Gatlinburg, TN, 27-30 October 1980; Richmond, C.R.; Walsh, P.J.; Copenhaver, E.D., Eds.; Chapter 1 7 . Anderson, M.W.; Hoel, D.G.; Kaplan, N.L. Toxicol. Appl. Pharmacol. 1980, 5 5 , 154-61. Autrup, H.; Wefald, F.C.; Jeffrey, A.M.; Tate, H.; Schwartz, R.D.; Trump, B.F.; Harris, C.C. Int. J. Cancer 1980, 2 5 , 293-300.

RECEIVED

July 5, 1983


Assessment and Management of Chemical Risks - American

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