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Recognition, evaluation, and control of chemical embryotoxins in the workplace
FUNDAMENTAL
AND
APPLIED
TOXICOLOGY
5,
626-633 (1985)
Recognition, Evaluation, and Control of Chemical Embryotoxins in the Workplace’ JERRY M. SMITH AND RICHARD
D. COSTLOW
Rohm and Haas Company, 727 Norrislown Road, Spring House, Pennsylvania 19477
Recognition, Evaluation,andControlof Chemical Embryotoxins in theWorkplace.SMITH, J. M., AND COWLOW, R. D. (1985). Fundam. Appl. Toxicol. 5, 626-633. B 1985 s&q of
Recognition, evaluation, and control, that is, risk assessment and risk management, of reproductive hazards in the workplace are complex problems, both scientifically and societally, that challenge the toxicologist, the regulator, the legislator, the occupational health professional, the employer, the employee, and the yet-to-be-born. Propagation of the species is essential for survival and therefore, in Homo sapiens emotional responses can be expected. Furthermore, societal and individual expectations, existing and pending laws and regulations, and limited knowledge and resources complicate risk assessment and risk management of embryotoxins in the workplace. Therefore, coopemtion, understanding, and patience, not confrontation, are required. Application of sound toxicological principles for identification, evaluation and assessment of risks, coupled with good industrial hygiene control measures can prevent hazards and maintain safe working conditions without unnecessarily restricting the opportunity for meaningful employment of any persons. “Reproductive hazards” in its broadest definition encompasses hazards to the prospective mother, father, the unborn, and the
ultimately born child and includes effects of loss of libido, sterility, mutagenesis, teratogenesis, abortion, fetal death, perinatal death, and delayed toxicity. Therefore, it is not unreasonable for an industrial or regulatory toxicologist to be asked if a given chemical or workplace has any potential reproductive hazards, referring to all prospective targets and effects. Unfortunately, all questions cannot be addressed in a single test or a simple battery of tests, nor are all answers known for many chemicals. Most perplexing of the potential reproductive hazards are those for the unborn, that is, the embryo-fetus. The embryo-fetus can be adversely affected by mutagenic events that occurred to one of its parents or by nutritional and other conditions of its host, its mother. However, it is the conceptus’s potentially unknown presence in the workplace that presents enormous burdens upon its host, industry, society, and the regulatory agencies to provide effective control of chemicals with potential for harm to the embryofetus. Concerns arise from the fact that chemical substances can cross the placenta and cause damage to the embryo-fetus at concentrations that have no adverse effect on its host or others in the environment. Thus, the issue is not that female employees ’ Presented at the 23rd Annual Conference of the are more susceptible than male employees or societyof Toxicology,March12-16,1984,Atlanta,Ga., of females being at greater risk to adverse as part of a symposium entitledRisk Assessment for Developmental Toxicity. health effects. Women in the workplace are 0272X)590/85 $3.00 Copyright Q 1985 by the Society of Toxicology. All rigbtr of reproduction in any fom reserved.
626
CHEMICAL
EMBRYOTOXINS
only an issue because they are uniquely capable of hosting the embryo-fetus during a critical time in its development. Furthermore, it should be noted that the acceptability of a given estimated embryo-fetotoxic risk for the unborn to a given exposure is a societal and regulatory decision, while the identification, evaluation, and estimation of risk are scientific endeavors.
IN
THE
627
WORKPLACE
which may be manifest postnatal period.”
at birth or in the
PERSPECTIVE
The human reproductive system is relatively inelhcient with only 50% of all concep tions resulting in normal offspring. Approximately one-third of conceptuses are lost before pregnancy is recognized and about 15% of all recognized pregnancies end in PROPOSITION spontaneous abortion. And approximately The recognition, evaluation, and control 7.5% of all delivered infants have developof embryo-fetotoxins in the workplace are mental abnormalities that interfere with their no different from the identification, evaluasurvival or quality of life. Conversely, the tion, risk assessment, and risk management reproductive system is relatively efficient in of other potential workplace toxins. Furtherlimiting the number of offspring with major more, basic toxicological principles are ap- cytogenetic or morphological abnormalities plicable: (Hertig, 1967; McKeown and Record, 1963; Mellon and Kalzenstein, 1964; Kretchmer, (1) Risk is a function of both the intrinsic embryo-fetotoxic potential (hazard) of the 1978). Kline et al. ( 198 1) have estimated that at least 95% of all karyotypically abnormal chemical and the exposure to the chemical. conceptions are spontaneously aborted. And (2) A dose-response relationship holds for of the spontaneous abortions, at least 35% each embryo-fetotoxic response and there are karyotypically abnormal and another 30% exists a threshold exposure level (dose) for show abnormalities in morphologic develeach chemical below which no effect is to be opment. expected. There has been an awareness since the This paper provides general guidance for turn of the century that women who worked the identification, evaluation, and assessment in the lead industry had decreased fertility of embryo-fetotoxins, as well as exposure and an increased abortion rate, along with (delivered dose) evaluation and risk assess- symptoms of lead poisoning. And in 1959, ment for making risk-management decisions. the cause of Minamata disease was established The paper is neither all-inclusive nor specific, to be due to methyl mercury secondary to but recognizes the requirements for sound industrial waste discharges of mercury. And finally, in the late 1950s and early 1960s the scientific judgment for each suspect chemical. thalidomide experience heightened the public It does not address either male or female awareness of the potential of teratogenic efgonadal toxins or mutagens, but is concerned fects of chemicals. with the conceptus, from fertilization to birth. The term “embryo-fetotoxin” is used as In the practice of medicine, observations defined by Karrh et al. (1981); that is, “an have been made that abortions and abnormal development frequently accompany certain embryo-fetotoxin is defined as a chemical disease states, such as infections, including which manifests an effect upon the conceptus during any of the stages of gestation, from rubella, cytomegalic inclusion disease, herpes fertilization to birth. It may induce death, simplex, toxoplasmosis, and syphilis. Furstructural malformations, metabolic or phys- thermore, maternal metabolic imbalances such as endemic cretinism, diabetes, pheniological dysfunction, growth retardation, ylketonuria, and virilizing tumors can ador psychological and behavioral alterations
628
SMITH
AND
versely affect reproduction and development. And more recently, epidemiological studies suggest a correlation between certain lifestyle practices, (e.g. use of alcohol and tobacco, exercise, nutrition, and stress) and reproductive performance, developmental abnormalities, and abortions (Kline et al., 1981). Therefore, it should not be unexpected to observe premature termination of pregnancy and occasional developmental anomalies unrelated to the workplace or chemical, nor should it be unexpected that controllable environmental factors and chemicals might be suspect. However, of the observed teratogenie effects in humans, drugs and environmental chemicals account for only 2 to 3%, and some 65 to 70% of all developmental anomalies are of unknown causes (Karrh, 1981). Unfortunately, the embryo is most sensitive to teratogens during the period when the host, the woman, usually does not know she is pregnant, that is, during organogenesis, which is the 18th through the 60th day of gestation, with the most critical period being that of early differentiation, the 18th through the 30th day. During the period of advance differentiation (after Day 60) the susceptibility of the fetus to teratogenic agents affecting structure lessens rapidly. IDENTIFICATION OF POTENTIAL EMBRYO-FETOTOXINS The first step in controlling embryo-fetotoxins in the workplace is the identification of chemicals with intrinsic embryo-fetotoxic potential (hazard) or the determination of whether a given chemical has intrinsic embryo-fetotoxic potential. Both procedures are essential but require different resources and approaches. For a new chemical or drug, where exposure does not exist, the process is relatively simple and utilizes standard bioassay techniques. [See U.S. Environmental Protection Agency (1980, 1982), Food and Drug Administration (1966, 1970), and
COSTLOW
OECD (1983) for review of developmental bioassays] . For the workplace using existing chemicals, the process is different and may be more perplexing. If the workers are exposed to only one chemical, the determination is, again, relatively simple in that the same approach that is used for identifying the potential of a new chemical may be used. However, it is not unusual for the toxicologist or the teratologist to receive a list of chemicals used in a laboratory, process, building, plant or company, or within a country with a question such as “are there any chemicals on the list which represent a reproductive, developmental, or teratogenic hazard?’ Usually there is only one way to respond to such a problem and that is to perform some type of prioritization and to begin evaluating each chemical, one at a time, from the most likely embryo-fetotoxin to the least likely. Fortunately, if the list is not too long and if the expert is familiar with the chemicals, he/she can identify those chemicals that have or are suspected of having embryo-fetotoxic potential and those chemicals with little or no embryo-fetotoxic potential, and the evaluation and risk assessment can begin. There is no one priority scheme that is any better than another; most involve a search of the literature, evaluation of the data, assigning of a value, and the tabulation of a score (Fabro et al., 1982). A first cut of the data does need not be sophisticated but one must be careful of how the scores are used. For example, most lists of teratogens or reproductive hazards are not very sophisticated and use secondary and tertiary reference sources. They have often been misused and abused, and seldomly used properly. EVALUATION OF INTRINSIC EMBRYO-FETOTOXIC POTENTIAL (HAZARD) The intrinsic embryo-fetotoxic potential (hazard) of a chemical is dependent upon
CHEMICAL
EMBRYOTOXINS
the toxic properties of the chemical and/or its metabolites, and the delivery of the toxicant to the target receptor. Furthermore, the response may be modified by the defense mechanisms of the host and of the target embryo. While structure-activity relationships exist and provide information for developing a working hypothesis, only when a large data base exists should one attempt to generalize. Normally the hazard must first be identified for species in which tests have been conducted and under the conditions of the tests. Then and only then should one begin to speculate as to the potential for species untested. The evaluation of chemicals with intrinsic embryo-fetotoxic potential requires sound scientific data and the judgment of scientists who have appropriate training and experience. After reliable data are developed, their relevancy must be scientifically evaluated recognizing that the effect observed is dependent upon (1) the intrinsic reaction(s) of the toxin with the target receptor(s), (2) the dose of the toxin(s) delivered to the target, and (3) the response of the host and embryo-fetus to the insult. The reaction between the target receptor(s) and the delivered toxin is dependent upon their concentrations, the chemical nature of the two, and the environment in which the reaction occurs. The delivered dose is dependent upon the route of exposure, the kinetics of absorption and distribution, the dynamics and kinetics of metabolism and biotransformation, and the kinetics of excretion. And except for chemicals (xenobiotics) with high intrinsic toxic potential the systems of absorption, metabolism, distribution, and excretion normally protect the host and embryo-fetus from insults by minimizing the delivered dose of the chemical (xenobiotic). However, occasionally in the workplace and frequently under toxicity testing conditions, exposures may be so large as to overwhelm one or more of these systems, invoking secondary and tertiary pathways and mechanisms, thus altering both the quantity and structure of the delivered chemical. Therefore,
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629
the test species must be exposed under conditions where the chemical delivered to the target is representative of what is expected to occur in the workplace. That is, exaggerated exposure may invoke different and compounding responses from those expected to occur at realistic exposures. The possible responses of the host and conceptus are numerous. They may be biochemical, physiological, immunological, pharmacological, or pathological, affecting the embryo-fetus directly or indirectly. Tissue and biochemical repair may occur. And as previously indicated, if the exposures are in excess of those which are expected to occur under conditions of use and misuse, erroneous results may be expected. Thus, to evaluate the effects of a chemical in a test system the investigator must be aware of the conditions of exposure and delivered dose in the workplace and examine the test species under similar exposure conditions. While it is commonplace to conduct teratogenic and other developmental studies at doses that produce maternal toxicity to demonstrate the absence of adverse effects upon the conceptus, the investigator and evaluator must be careful not to over interpret adverse effects observed at or near these overwhelming exposures. Furthermore, it is critical to the interpretation and evaluation of the results to gain some understanding and insight of the mode of action of the chemical. The “mode of action” does not mean the mechanism of action at the level of chemical reactivity. It is used in this context to determine if the effect is direct or indirect and through which system the toxicant is exerting its effects. For example, if the chemical is acting through an alteration of the host’s hormonal balance, nutritional state, normal functions, etc., a toxicological evaluation of these parameters may provide a more appropriate effect for establishing workplace exposure limits. Statistical evaluations of teratogenic, developmental, and reproductive studies are complicated and will not be discussed in
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detail. The investigator must be aware of the general issues and consult a statistician as appropriate. For example, there is the question of whether the offspring or the litter is the unit of analysis; the issue of nonindependence of response within a litter; and the relationship of litter responses to factors other than dose, that is, covariates. Consideration must be given to parametric versus nonparametric approaches, discrete versus continuous outcomes, weighted analysis, two-stage analyses, analysis of covariance, building heredity into the analysis, trend tests and pairwise comparisons, and that there are many ties at extreme values, i.e., 0 and loo%, respectively (Hogan and Hoel, 1982; Nelson and Holson, 1978). Once the pharmacokinetics, metabolism, and mode of action are understood, appropriate dose-response relationships can be estimated such as threshold exposure levels and slope of the dose-response curve, as well as determination of significant effects. With an understanding of the above parameters and responses, the evaluating scientist develops confidence in the effect and response to be expected within the tested species. To develop confidence in extrapolation beyond the tested species, comparative testing and bridge building becomes important noting differences between species and species responses. If results are replicated within and between species and a common mode of action is identified and commonality exists between the species tested and humans, extrapolation to humans and the workplace becomes relatively easy (Fraser, 1977; Hogan and Hoel, 1982).
EVALUATION OF HUMAN EXPOSURE Risk, which is the probability of occurring, is dependent upon not intrinsic embryo-fetotoxic potential of a chemical, but also upon the
an event only the (hazard) potential
COSTLOW
exposure and the delivered dose. And, while the intrinsic property of a chemical cannot be controlled, exposures in the workplace are controllable. Therefore, understanding the degree and nature of the potential human exposure permits some measure of control of risk. Essential to an understanding of the degree and nature of exposure are the physical chemical properties of the chemical, the conditions of the workplace, the activities of the work, the characteristics of exposure, and the population at risk. The physical chemical properties, such as vapor pressure, melting point, boiling point, and solubility, and conditions of the work determine if the chemical which has intrinsic embryo-fetotoxic potential will be a solid, liquid, or gas and whether aerosol, vapor, dust, or splash exposures might occur; that is, will the exposure be respiratory, percutaneous, or oral? Likewise the type of work and conditions of the workplace affect the concentrations, volume, frequency, and duration of exposure and whether the exposure is continuous or intermittent. And because the population at risk is the conceptus (embryo-fetus), characteristics of the worker population, that is, the potential host, must be evaluated. These include the age, sex, and health of the workers. ASSESSMENT
OF RISK
After a chemical has been identified as having intrinsic embryo-fetotoxin potential, after the experimental data, including epidemiological, have been evaluated, and after potential exposure conditions have been identified and evaluated, then the expert or a panel of experts can make an assessment of existing and potential risk (Schardein, 1983). As previously stated, the assessment of risk is a scientific endeavor and must be based on sound relevant scientific data and principles. Best estimates or, most likely,
CHEMICAL
EMBRYOTOXINS
estimates of risk must be stated with limits of confidence expressed. Conservatism, margins of safety, uncertainty factors, etc., are risk-management tools not scientific riskassessment calculations. There are three critical issues or factors in risk assessment, whether it is of a chemical with general toxic potential or one with embryo-fetotoxic potential. First, is the assessment of the data within the context of the “experimental conditions.” Second, is the extrapolation of data within the tested species from the observable to the nonobservable range. Third, is the extrapolation of risk estimate from the tested to the untested or untestable species. Unfortunately, the extrapolation both within and between species is frequently combined and complicated with risk management, societal, or policy overtones. The process of risk assessment can be performed more simply, and more easily critiqued, and of greater utility for risk-management decision if evaluated separately from consideration of risk acceptability. Observations of effects in most embryofetal toxicity protocols are limited approximately to a plus or minus two standard deviations different from controls for continuous data, a 10% effect for discrete data or a doubling of the observation over worst case individual histarical control data. Simulated studies and the few large studies (Nelson and Holson, 1978) conducted suggest that with “good” dose-response data in the observed range, extrapolation to a “no observed effect level” (NOEL) or a 1% effect level may be more consistent than actual observed NOELs. And for most responses, the lower 95% confidence limits of the estimated NOEL will include the threshold levels for the population represented by the test population. Any extrapolation beyond these limits exceeds today’s scientific knowledge and understanding and has little, if any, scientific credibility. The best hope for extending or extrapolation of results lies in an understanding and an appreciation of the mode of action and controlling factors of delivered dose.
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Finally, and the most difficult, is the extrapolation from tested species to humans. First, the evaluation must consider all toxicological data on the chemical in question and data on chemicals which may be structurally or toxicologically similar, for bridges to humans that may exist. Second, does the effect appear to be universal; that is, does it occur in more than one species and are the effects embryologically or toxicologically similar between species. Third, do the embryo-fetotoxic effects consistently occur at exposure levels substantially below exposures which produce other nonteratogenic toxic effects. Fourth, which of the test species examined is the most appropriate model for the class of chemical evaluated and was the experimental exposure route applicable to the workplace. Fifth, what are the characteristics of the dose-response curve and what are the threshold and the no observable effect levels. And finally as with extrapolation between species, what is known about the mode of action and the controlling factors of delivered dose. With these data in hand, the scientific expert will be able to provide those responsible with making judgments of acceptable exposure levels with two action exposure levels. The first, or immediate action level, should be that exposure where the toxicologist/teratologist believes detectable effects would occur. The second, or nonaction level, should be that exposure where the toxicologist/teratologist believes no effects would occur. The difference between these exposure levels is an expression of the scientist’s confidence. The scientist must state the data and rationale for the selection of these two exposure levels. ESTIMATION OF ACCEPTABLE EXPOSURE LEVELS With the two action levels at hand-that is, the “immediate-action” level and “nonaction” level-those responsible for risk
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AND COSTLOW
management can make judgments of “acceptable risk” and actions to be taken to control exposures. The acceptable risk level must reflect not only the data provided by the scientist but societal value judgments, governmental and regulatory policy (public policy), and company management policy, as well as the relative risk, cost, and benefits derived from the use or nonuse of the chemical. There are no rules or special formula for determining acceptable risk, but the rule of the “action of a prudent man” provides some guidance. The scientist who understands best the uncertainties of scientific estimation of risk should not be excluded from the process of establishing acceptable exposure levels, but he or she must continually be aware of what are scientific facts, scientific working hypotheses, and societal value judgments. SUGGESTED
ACTIONS
Once it has been determined that a chemical with potential for embryo-fetotoxicity may exist in the workplace and exposure levels have been established, the following actions should be considered. (I) Employees who may be affected should be informed of the possible consequences of exposure to such substances and appropriate safe handling procedures established and communicated. (2) Engineering controls should be used to the extent practical to reduce and maintain exposure at acceptable levels. Engineering controls should be augmented by administrative controls. (3) Whenever engineering and administrative controls are not practical for maintaining exposures at or below acceptable levels, the use of personal protective equipment should be required. Employees who are required to use such equipment should be adequately trained in their proper use. (4) Where there is potential for exposure
to an embryo-fetotoxin for which an acceptable exposure level cannot be set due to inadequate data, women of reproductive potential should be excluded from the work area. (5) Where engineering and administrative controls, augmented as appropriate by personal protective equipment, are determined to be inadequate to ensure acceptable levels of exposure to an embryo-fetotoxic compound, women of reproductive potential should be excluded from the work area. REFERENCES Food and Drug Administration (1966). Guidelines for Reproduction and Teratology of Drugs. Bureau of Drugs. Food and Drug Administration (1970). Advisory Committee on Protocols for Safety Evaluation: Panel on Reproduction Report on Reproduction Studies in the Safety Evaluation of Food Additives and Pesticide Residues. Toxicol. Appl. Pharmacol. 16, 264-296. FABRO, S., SCHULL, G., AND BROWN, N. A. (1982). The relative teratogenic index and teratogenic potency: proposed components of developmental toxicity risk assessment. Teratogen. Carcinogen. Mutagen. 2, 6176.
FRASER,F. C. (1977). Relation of animal studies to the problem in man. In Handbook of Teratology (J. G. Wilson and F. C. Fraser, eds.), Vol. 1, pp. 75-96. Plenum, New York. HERTIG, A. T. (1967). The overall problem in man. In Comparative Aspects of Reproductive Failure (L. Benirschke, ed.), pp. 1I-40. Springer-Verlag, New York. HOGAN, M. D., AND HOEL, D. G. (1982). Extrapolation to man. In Principles and Methods of Toxicology (A. W. Hayes, ed.), pp. 71 l-73 1, Raven Press, New York. KARRH, B. W., CARMODY, T. W., CLYNE, R. M., GOULD, K. G., PORTELA-CUBRIA, G., SMITH, J. M., AND FREIFELD,M. (1981). Guidance for the evaluation, risk assessment and control of chemical embryo-fetotoxins. J. Occup. Med. 23, 397-402. KLINE, J., LEVIN, B., STEIN, Z., SUSSER, M., AND WARBUTON, D. (1981). Epidemiologic detection of low dose effects on the developing fetus. Environ. Health Perspect. 42, 119- 126. KRETCHMER, N. (1978). Perspectives in teratologic research. Teratology 17, 203-2 12. MCKEOWN, T., AND RECORD, R. C. (1963). Mafformation in a population observed for five years after birth. In Ciba Foundation Symposium on Congenital Mal-
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EMBRYOTOXINS
formations (G. E. W. Welstenholme and C. M. O’Conner, eds.). Little Brown, Boston. MELLON, G. W., AND KAUENSTEIN, M. (1964). Increased incidence of malformations-chance or change? J. Amer. Med. Assoc. 187, 5X-573. NELSON, C. J., AND HOL.SON, J. F. (1978). Statistical analysis of teratogenic data: problems and advancements. J. Environ. Pathol. Toxicol. 2, 187-199. Organization for Economic Cooperation and Develop ment (OECD) (1983). Test Guidelines Nos. 414, 415. and 416. Available from the OECD Publications and Information Center, Suite 1207, 1750 Pennsylvania Ave., N.W., Washington DC. 20006. SCHARDEIN, 3. L. (1983). Teratogenic risk assessment.
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In Issues and Reviews in Teratology (H. Kalter, ed.). Plenum, New York. U.S. Environmental Protection Agency (1980). Assessment of Risk to Human Reproduction and to Development of Human Conceptus from Exposure to Environmental Substances. Available from the National Technical Information Service, U.S. Department of Commerce, No. 82-007897. U.S. Environmental Protection Agency (1982). Health effects test guidelines, Chap II. Specific Organ/Tissue Toxicity-Teratogenicity. Office of Toxic Substances. Available from the National Technical Information Service, U.S. Department of Commerce, No. PB82232984.
AND
APPLIED
TOXICOLOGY
5,
626-633 (1985)
Recognition, Evaluation, and Control of Chemical Embryotoxins in the Workplace’ JERRY M. SMITH AND RICHARD
D. COSTLOW
Rohm and Haas Company, 727 Norrislown Road, Spring House, Pennsylvania 19477
Recognition, Evaluation,andControlof Chemical Embryotoxins in theWorkplace.SMITH, J. M., AND COWLOW, R. D. (1985). Fundam. Appl. Toxicol. 5, 626-633. B 1985 s&q of
Recognition, evaluation, and control, that is, risk assessment and risk management, of reproductive hazards in the workplace are complex problems, both scientifically and societally, that challenge the toxicologist, the regulator, the legislator, the occupational health professional, the employer, the employee, and the yet-to-be-born. Propagation of the species is essential for survival and therefore, in Homo sapiens emotional responses can be expected. Furthermore, societal and individual expectations, existing and pending laws and regulations, and limited knowledge and resources complicate risk assessment and risk management of embryotoxins in the workplace. Therefore, coopemtion, understanding, and patience, not confrontation, are required. Application of sound toxicological principles for identification, evaluation and assessment of risks, coupled with good industrial hygiene control measures can prevent hazards and maintain safe working conditions without unnecessarily restricting the opportunity for meaningful employment of any persons. “Reproductive hazards” in its broadest definition encompasses hazards to the prospective mother, father, the unborn, and the
ultimately born child and includes effects of loss of libido, sterility, mutagenesis, teratogenesis, abortion, fetal death, perinatal death, and delayed toxicity. Therefore, it is not unreasonable for an industrial or regulatory toxicologist to be asked if a given chemical or workplace has any potential reproductive hazards, referring to all prospective targets and effects. Unfortunately, all questions cannot be addressed in a single test or a simple battery of tests, nor are all answers known for many chemicals. Most perplexing of the potential reproductive hazards are those for the unborn, that is, the embryo-fetus. The embryo-fetus can be adversely affected by mutagenic events that occurred to one of its parents or by nutritional and other conditions of its host, its mother. However, it is the conceptus’s potentially unknown presence in the workplace that presents enormous burdens upon its host, industry, society, and the regulatory agencies to provide effective control of chemicals with potential for harm to the embryofetus. Concerns arise from the fact that chemical substances can cross the placenta and cause damage to the embryo-fetus at concentrations that have no adverse effect on its host or others in the environment. Thus, the issue is not that female employees ’ Presented at the 23rd Annual Conference of the are more susceptible than male employees or societyof Toxicology,March12-16,1984,Atlanta,Ga., of females being at greater risk to adverse as part of a symposium entitledRisk Assessment for Developmental Toxicity. health effects. Women in the workplace are 0272X)590/85 $3.00 Copyright Q 1985 by the Society of Toxicology. All rigbtr of reproduction in any fom reserved.
626
CHEMICAL
EMBRYOTOXINS
only an issue because they are uniquely capable of hosting the embryo-fetus during a critical time in its development. Furthermore, it should be noted that the acceptability of a given estimated embryo-fetotoxic risk for the unborn to a given exposure is a societal and regulatory decision, while the identification, evaluation, and estimation of risk are scientific endeavors.
IN
THE
627
WORKPLACE
which may be manifest postnatal period.”
at birth or in the
PERSPECTIVE
The human reproductive system is relatively inelhcient with only 50% of all concep tions resulting in normal offspring. Approximately one-third of conceptuses are lost before pregnancy is recognized and about 15% of all recognized pregnancies end in PROPOSITION spontaneous abortion. And approximately The recognition, evaluation, and control 7.5% of all delivered infants have developof embryo-fetotoxins in the workplace are mental abnormalities that interfere with their no different from the identification, evaluasurvival or quality of life. Conversely, the tion, risk assessment, and risk management reproductive system is relatively efficient in of other potential workplace toxins. Furtherlimiting the number of offspring with major more, basic toxicological principles are ap- cytogenetic or morphological abnormalities plicable: (Hertig, 1967; McKeown and Record, 1963; Mellon and Kalzenstein, 1964; Kretchmer, (1) Risk is a function of both the intrinsic embryo-fetotoxic potential (hazard) of the 1978). Kline et al. ( 198 1) have estimated that at least 95% of all karyotypically abnormal chemical and the exposure to the chemical. conceptions are spontaneously aborted. And (2) A dose-response relationship holds for of the spontaneous abortions, at least 35% each embryo-fetotoxic response and there are karyotypically abnormal and another 30% exists a threshold exposure level (dose) for show abnormalities in morphologic develeach chemical below which no effect is to be opment. expected. There has been an awareness since the This paper provides general guidance for turn of the century that women who worked the identification, evaluation, and assessment in the lead industry had decreased fertility of embryo-fetotoxins, as well as exposure and an increased abortion rate, along with (delivered dose) evaluation and risk assess- symptoms of lead poisoning. And in 1959, ment for making risk-management decisions. the cause of Minamata disease was established The paper is neither all-inclusive nor specific, to be due to methyl mercury secondary to but recognizes the requirements for sound industrial waste discharges of mercury. And finally, in the late 1950s and early 1960s the scientific judgment for each suspect chemical. thalidomide experience heightened the public It does not address either male or female awareness of the potential of teratogenic efgonadal toxins or mutagens, but is concerned fects of chemicals. with the conceptus, from fertilization to birth. The term “embryo-fetotoxin” is used as In the practice of medicine, observations defined by Karrh et al. (1981); that is, “an have been made that abortions and abnormal development frequently accompany certain embryo-fetotoxin is defined as a chemical disease states, such as infections, including which manifests an effect upon the conceptus during any of the stages of gestation, from rubella, cytomegalic inclusion disease, herpes fertilization to birth. It may induce death, simplex, toxoplasmosis, and syphilis. Furstructural malformations, metabolic or phys- thermore, maternal metabolic imbalances such as endemic cretinism, diabetes, pheniological dysfunction, growth retardation, ylketonuria, and virilizing tumors can ador psychological and behavioral alterations
628
SMITH
AND
versely affect reproduction and development. And more recently, epidemiological studies suggest a correlation between certain lifestyle practices, (e.g. use of alcohol and tobacco, exercise, nutrition, and stress) and reproductive performance, developmental abnormalities, and abortions (Kline et al., 1981). Therefore, it should not be unexpected to observe premature termination of pregnancy and occasional developmental anomalies unrelated to the workplace or chemical, nor should it be unexpected that controllable environmental factors and chemicals might be suspect. However, of the observed teratogenie effects in humans, drugs and environmental chemicals account for only 2 to 3%, and some 65 to 70% of all developmental anomalies are of unknown causes (Karrh, 1981). Unfortunately, the embryo is most sensitive to teratogens during the period when the host, the woman, usually does not know she is pregnant, that is, during organogenesis, which is the 18th through the 60th day of gestation, with the most critical period being that of early differentiation, the 18th through the 30th day. During the period of advance differentiation (after Day 60) the susceptibility of the fetus to teratogenic agents affecting structure lessens rapidly. IDENTIFICATION OF POTENTIAL EMBRYO-FETOTOXINS The first step in controlling embryo-fetotoxins in the workplace is the identification of chemicals with intrinsic embryo-fetotoxic potential (hazard) or the determination of whether a given chemical has intrinsic embryo-fetotoxic potential. Both procedures are essential but require different resources and approaches. For a new chemical or drug, where exposure does not exist, the process is relatively simple and utilizes standard bioassay techniques. [See U.S. Environmental Protection Agency (1980, 1982), Food and Drug Administration (1966, 1970), and
COSTLOW
OECD (1983) for review of developmental bioassays] . For the workplace using existing chemicals, the process is different and may be more perplexing. If the workers are exposed to only one chemical, the determination is, again, relatively simple in that the same approach that is used for identifying the potential of a new chemical may be used. However, it is not unusual for the toxicologist or the teratologist to receive a list of chemicals used in a laboratory, process, building, plant or company, or within a country with a question such as “are there any chemicals on the list which represent a reproductive, developmental, or teratogenic hazard?’ Usually there is only one way to respond to such a problem and that is to perform some type of prioritization and to begin evaluating each chemical, one at a time, from the most likely embryo-fetotoxin to the least likely. Fortunately, if the list is not too long and if the expert is familiar with the chemicals, he/she can identify those chemicals that have or are suspected of having embryo-fetotoxic potential and those chemicals with little or no embryo-fetotoxic potential, and the evaluation and risk assessment can begin. There is no one priority scheme that is any better than another; most involve a search of the literature, evaluation of the data, assigning of a value, and the tabulation of a score (Fabro et al., 1982). A first cut of the data does need not be sophisticated but one must be careful of how the scores are used. For example, most lists of teratogens or reproductive hazards are not very sophisticated and use secondary and tertiary reference sources. They have often been misused and abused, and seldomly used properly. EVALUATION OF INTRINSIC EMBRYO-FETOTOXIC POTENTIAL (HAZARD) The intrinsic embryo-fetotoxic potential (hazard) of a chemical is dependent upon
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the toxic properties of the chemical and/or its metabolites, and the delivery of the toxicant to the target receptor. Furthermore, the response may be modified by the defense mechanisms of the host and of the target embryo. While structure-activity relationships exist and provide information for developing a working hypothesis, only when a large data base exists should one attempt to generalize. Normally the hazard must first be identified for species in which tests have been conducted and under the conditions of the tests. Then and only then should one begin to speculate as to the potential for species untested. The evaluation of chemicals with intrinsic embryo-fetotoxic potential requires sound scientific data and the judgment of scientists who have appropriate training and experience. After reliable data are developed, their relevancy must be scientifically evaluated recognizing that the effect observed is dependent upon (1) the intrinsic reaction(s) of the toxin with the target receptor(s), (2) the dose of the toxin(s) delivered to the target, and (3) the response of the host and embryo-fetus to the insult. The reaction between the target receptor(s) and the delivered toxin is dependent upon their concentrations, the chemical nature of the two, and the environment in which the reaction occurs. The delivered dose is dependent upon the route of exposure, the kinetics of absorption and distribution, the dynamics and kinetics of metabolism and biotransformation, and the kinetics of excretion. And except for chemicals (xenobiotics) with high intrinsic toxic potential the systems of absorption, metabolism, distribution, and excretion normally protect the host and embryo-fetus from insults by minimizing the delivered dose of the chemical (xenobiotic). However, occasionally in the workplace and frequently under toxicity testing conditions, exposures may be so large as to overwhelm one or more of these systems, invoking secondary and tertiary pathways and mechanisms, thus altering both the quantity and structure of the delivered chemical. Therefore,
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the test species must be exposed under conditions where the chemical delivered to the target is representative of what is expected to occur in the workplace. That is, exaggerated exposure may invoke different and compounding responses from those expected to occur at realistic exposures. The possible responses of the host and conceptus are numerous. They may be biochemical, physiological, immunological, pharmacological, or pathological, affecting the embryo-fetus directly or indirectly. Tissue and biochemical repair may occur. And as previously indicated, if the exposures are in excess of those which are expected to occur under conditions of use and misuse, erroneous results may be expected. Thus, to evaluate the effects of a chemical in a test system the investigator must be aware of the conditions of exposure and delivered dose in the workplace and examine the test species under similar exposure conditions. While it is commonplace to conduct teratogenic and other developmental studies at doses that produce maternal toxicity to demonstrate the absence of adverse effects upon the conceptus, the investigator and evaluator must be careful not to over interpret adverse effects observed at or near these overwhelming exposures. Furthermore, it is critical to the interpretation and evaluation of the results to gain some understanding and insight of the mode of action of the chemical. The “mode of action” does not mean the mechanism of action at the level of chemical reactivity. It is used in this context to determine if the effect is direct or indirect and through which system the toxicant is exerting its effects. For example, if the chemical is acting through an alteration of the host’s hormonal balance, nutritional state, normal functions, etc., a toxicological evaluation of these parameters may provide a more appropriate effect for establishing workplace exposure limits. Statistical evaluations of teratogenic, developmental, and reproductive studies are complicated and will not be discussed in
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detail. The investigator must be aware of the general issues and consult a statistician as appropriate. For example, there is the question of whether the offspring or the litter is the unit of analysis; the issue of nonindependence of response within a litter; and the relationship of litter responses to factors other than dose, that is, covariates. Consideration must be given to parametric versus nonparametric approaches, discrete versus continuous outcomes, weighted analysis, two-stage analyses, analysis of covariance, building heredity into the analysis, trend tests and pairwise comparisons, and that there are many ties at extreme values, i.e., 0 and loo%, respectively (Hogan and Hoel, 1982; Nelson and Holson, 1978). Once the pharmacokinetics, metabolism, and mode of action are understood, appropriate dose-response relationships can be estimated such as threshold exposure levels and slope of the dose-response curve, as well as determination of significant effects. With an understanding of the above parameters and responses, the evaluating scientist develops confidence in the effect and response to be expected within the tested species. To develop confidence in extrapolation beyond the tested species, comparative testing and bridge building becomes important noting differences between species and species responses. If results are replicated within and between species and a common mode of action is identified and commonality exists between the species tested and humans, extrapolation to humans and the workplace becomes relatively easy (Fraser, 1977; Hogan and Hoel, 1982).
EVALUATION OF HUMAN EXPOSURE Risk, which is the probability of occurring, is dependent upon not intrinsic embryo-fetotoxic potential of a chemical, but also upon the
an event only the (hazard) potential
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exposure and the delivered dose. And, while the intrinsic property of a chemical cannot be controlled, exposures in the workplace are controllable. Therefore, understanding the degree and nature of the potential human exposure permits some measure of control of risk. Essential to an understanding of the degree and nature of exposure are the physical chemical properties of the chemical, the conditions of the workplace, the activities of the work, the characteristics of exposure, and the population at risk. The physical chemical properties, such as vapor pressure, melting point, boiling point, and solubility, and conditions of the work determine if the chemical which has intrinsic embryo-fetotoxic potential will be a solid, liquid, or gas and whether aerosol, vapor, dust, or splash exposures might occur; that is, will the exposure be respiratory, percutaneous, or oral? Likewise the type of work and conditions of the workplace affect the concentrations, volume, frequency, and duration of exposure and whether the exposure is continuous or intermittent. And because the population at risk is the conceptus (embryo-fetus), characteristics of the worker population, that is, the potential host, must be evaluated. These include the age, sex, and health of the workers. ASSESSMENT
OF RISK
After a chemical has been identified as having intrinsic embryo-fetotoxin potential, after the experimental data, including epidemiological, have been evaluated, and after potential exposure conditions have been identified and evaluated, then the expert or a panel of experts can make an assessment of existing and potential risk (Schardein, 1983). As previously stated, the assessment of risk is a scientific endeavor and must be based on sound relevant scientific data and principles. Best estimates or, most likely,
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estimates of risk must be stated with limits of confidence expressed. Conservatism, margins of safety, uncertainty factors, etc., are risk-management tools not scientific riskassessment calculations. There are three critical issues or factors in risk assessment, whether it is of a chemical with general toxic potential or one with embryo-fetotoxic potential. First, is the assessment of the data within the context of the “experimental conditions.” Second, is the extrapolation of data within the tested species from the observable to the nonobservable range. Third, is the extrapolation of risk estimate from the tested to the untested or untestable species. Unfortunately, the extrapolation both within and between species is frequently combined and complicated with risk management, societal, or policy overtones. The process of risk assessment can be performed more simply, and more easily critiqued, and of greater utility for risk-management decision if evaluated separately from consideration of risk acceptability. Observations of effects in most embryofetal toxicity protocols are limited approximately to a plus or minus two standard deviations different from controls for continuous data, a 10% effect for discrete data or a doubling of the observation over worst case individual histarical control data. Simulated studies and the few large studies (Nelson and Holson, 1978) conducted suggest that with “good” dose-response data in the observed range, extrapolation to a “no observed effect level” (NOEL) or a 1% effect level may be more consistent than actual observed NOELs. And for most responses, the lower 95% confidence limits of the estimated NOEL will include the threshold levels for the population represented by the test population. Any extrapolation beyond these limits exceeds today’s scientific knowledge and understanding and has little, if any, scientific credibility. The best hope for extending or extrapolation of results lies in an understanding and an appreciation of the mode of action and controlling factors of delivered dose.
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Finally, and the most difficult, is the extrapolation from tested species to humans. First, the evaluation must consider all toxicological data on the chemical in question and data on chemicals which may be structurally or toxicologically similar, for bridges to humans that may exist. Second, does the effect appear to be universal; that is, does it occur in more than one species and are the effects embryologically or toxicologically similar between species. Third, do the embryo-fetotoxic effects consistently occur at exposure levels substantially below exposures which produce other nonteratogenic toxic effects. Fourth, which of the test species examined is the most appropriate model for the class of chemical evaluated and was the experimental exposure route applicable to the workplace. Fifth, what are the characteristics of the dose-response curve and what are the threshold and the no observable effect levels. And finally as with extrapolation between species, what is known about the mode of action and the controlling factors of delivered dose. With these data in hand, the scientific expert will be able to provide those responsible with making judgments of acceptable exposure levels with two action exposure levels. The first, or immediate action level, should be that exposure where the toxicologist/teratologist believes detectable effects would occur. The second, or nonaction level, should be that exposure where the toxicologist/teratologist believes no effects would occur. The difference between these exposure levels is an expression of the scientist’s confidence. The scientist must state the data and rationale for the selection of these two exposure levels. ESTIMATION OF ACCEPTABLE EXPOSURE LEVELS With the two action levels at hand-that is, the “immediate-action” level and “nonaction” level-those responsible for risk
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management can make judgments of “acceptable risk” and actions to be taken to control exposures. The acceptable risk level must reflect not only the data provided by the scientist but societal value judgments, governmental and regulatory policy (public policy), and company management policy, as well as the relative risk, cost, and benefits derived from the use or nonuse of the chemical. There are no rules or special formula for determining acceptable risk, but the rule of the “action of a prudent man” provides some guidance. The scientist who understands best the uncertainties of scientific estimation of risk should not be excluded from the process of establishing acceptable exposure levels, but he or she must continually be aware of what are scientific facts, scientific working hypotheses, and societal value judgments. SUGGESTED
ACTIONS
Once it has been determined that a chemical with potential for embryo-fetotoxicity may exist in the workplace and exposure levels have been established, the following actions should be considered. (I) Employees who may be affected should be informed of the possible consequences of exposure to such substances and appropriate safe handling procedures established and communicated. (2) Engineering controls should be used to the extent practical to reduce and maintain exposure at acceptable levels. Engineering controls should be augmented by administrative controls. (3) Whenever engineering and administrative controls are not practical for maintaining exposures at or below acceptable levels, the use of personal protective equipment should be required. Employees who are required to use such equipment should be adequately trained in their proper use. (4) Where there is potential for exposure
to an embryo-fetotoxin for which an acceptable exposure level cannot be set due to inadequate data, women of reproductive potential should be excluded from the work area. (5) Where engineering and administrative controls, augmented as appropriate by personal protective equipment, are determined to be inadequate to ensure acceptable levels of exposure to an embryo-fetotoxic compound, women of reproductive potential should be excluded from the work area. REFERENCES Food and Drug Administration (1966). Guidelines for Reproduction and Teratology of Drugs. Bureau of Drugs. Food and Drug Administration (1970). Advisory Committee on Protocols for Safety Evaluation: Panel on Reproduction Report on Reproduction Studies in the Safety Evaluation of Food Additives and Pesticide Residues. Toxicol. Appl. Pharmacol. 16, 264-296. FABRO, S., SCHULL, G., AND BROWN, N. A. (1982). The relative teratogenic index and teratogenic potency: proposed components of developmental toxicity risk assessment. Teratogen. Carcinogen. Mutagen. 2, 6176.
FRASER,F. C. (1977). Relation of animal studies to the problem in man. In Handbook of Teratology (J. G. Wilson and F. C. Fraser, eds.), Vol. 1, pp. 75-96. Plenum, New York. HERTIG, A. T. (1967). The overall problem in man. In Comparative Aspects of Reproductive Failure (L. Benirschke, ed.), pp. 1I-40. Springer-Verlag, New York. HOGAN, M. D., AND HOEL, D. G. (1982). Extrapolation to man. In Principles and Methods of Toxicology (A. W. Hayes, ed.), pp. 71 l-73 1, Raven Press, New York. KARRH, B. W., CARMODY, T. W., CLYNE, R. M., GOULD, K. G., PORTELA-CUBRIA, G., SMITH, J. M., AND FREIFELD,M. (1981). Guidance for the evaluation, risk assessment and control of chemical embryo-fetotoxins. J. Occup. Med. 23, 397-402. KLINE, J., LEVIN, B., STEIN, Z., SUSSER, M., AND WARBUTON, D. (1981). Epidemiologic detection of low dose effects on the developing fetus. Environ. Health Perspect. 42, 119- 126. KRETCHMER, N. (1978). Perspectives in teratologic research. Teratology 17, 203-2 12. MCKEOWN, T., AND RECORD, R. C. (1963). Mafformation in a population observed for five years after birth. In Ciba Foundation Symposium on Congenital Mal-
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formations (G. E. W. Welstenholme and C. M. O’Conner, eds.). Little Brown, Boston. MELLON, G. W., AND KAUENSTEIN, M. (1964). Increased incidence of malformations-chance or change? J. Amer. Med. Assoc. 187, 5X-573. NELSON, C. J., AND HOL.SON, J. F. (1978). Statistical analysis of teratogenic data: problems and advancements. J. Environ. Pathol. Toxicol. 2, 187-199. Organization for Economic Cooperation and Develop ment (OECD) (1983). Test Guidelines Nos. 414, 415. and 416. Available from the OECD Publications and Information Center, Suite 1207, 1750 Pennsylvania Ave., N.W., Washington DC. 20006. SCHARDEIN, 3. L. (1983). Teratogenic risk assessment.
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In Issues and Reviews in Teratology (H. Kalter, ed.). Plenum, New York. U.S. Environmental Protection Agency (1980). Assessment of Risk to Human Reproduction and to Development of Human Conceptus from Exposure to Environmental Substances. Available from the National Technical Information Service, U.S. Department of Commerce, No. 82-007897. U.S. Environmental Protection Agency (1982). Health effects test guidelines, Chap II. Specific Organ/Tissue Toxicity-Teratogenicity. Office of Toxic Substances. Available from the National Technical Information Service, U.S. Department of Commerce, No. PB82232984.