Toxicokinetics in infants and children in relation to the ADI and TDI

Dermatokinetics: Advances and Experimental Models, Focus on Skin Metabolism

Numerous dermal contact products, such as drugs or cosmetics, are applied on the skin, the first protective barrier to their entrance into the organism. These products contain various xenobiotic molecules that can penetrate the viable epidermis. Many studies have shown that keratinocyte metabolism could affect their behavior by biotransformation. While aiming for detoxification, toxic metabolites can be produced. These metabolites may react with biological macromolecules often leading to sensitization reactions. After passing through the epidermis, xenobiotics can reach the vascularized dermis and therefore be bioavailable and distributed into the entire organism. To highlight these mechanisms, dermatokinetics, based on the concept of pharmacokinetics, has been developed recently. It provides information on the action of xenobiotics that penetrate the organism through the dermal route. The purpose of this review is first to describe and synthesize the dermatokinetics mechanisms to consider when assessing the absorption of a xenobiotic through the skin. We focus on skin absorption and specifically on skin metabolism, the two main processes involved in dermatokinetics. In addition, experimental models and methods to assess dermatokinetics are described and discussed to select the most relevant method when evaluating, in a specific context, dermatokinetics parameters of a xenobiotic. We also discuss the limits of this approach as it is notably used for risk assessment in the industry where scenario studies generally focus only on one xenobiotic and do not consider interactions with the rest of the exposome. The hypothesis of adverse effects due to the combination of chemical substances in contact with individuals and not to a single molecule are being increasingly studied and embraced in the scientific community.

Skin Barrier Function in Infants: Update and Outlook

A good understanding of infant skin should provide a rationale for optimum management of the health of this integument. In this review, we discuss the skin barrier function of infants, particularly with reference to the use of diapers and baby wipes. The skin barrier of newborns continues to develop with age. Two years after birth, the barrier properties of infant skin closely resemble those of adult skin. However, several risk factors may contribute to impaired skin barrier and altered skin permeability in infants. Problems may arise from the use of diapers and baby wipes. The skin covered by a diaper is effectively an occluded environment, and thus is vulnerable to over-hydration. To date there has been no published information regarding dermal absorption of ingredients contained in baby wipes. Similarly, dermal absorption of topical ingredients in infants with underlying skin conditions has not been widely explored. Clearly, there are serious ethical concerns related to conducting skin permeation studies on infant skin. However, the increasing availability of non-invasive methods for in vivo studies is encouraging and offers new directions for studying this important patient group.

An overview of current practices for regulatory risk assessment with lessons learnt from cosmetics in the European Union

Risk assessments of various types of chemical compounds are carried out in the European Union (EU) foremost to comply with legislation and to support regulatory decision-making with respect to their safety. Historically, risk assessment has relied heavily on animal experiments. However, the EU is committed to reduce animal experimentation and has implemented several legislative changes, which have triggered a paradigm shift towards human-relevant animal-free testing in the field of toxicology, in particular for risk assessment. For some specific endpoints, such as skin corrosion and irritation, validated alternatives are available whilst for other endpoints, including repeated dose systemic toxicity, the use of animal data is still central to meet the information requirements stipulated in the different legislations. The present review aims to provide an overview of established and more recently introduced methods for hazard assessment and risk characterisation for human health, in particular in the context of the EU Cosmetics Regulation (EC No 1223/2009) as well as the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) Regulation (EC 1907/2006).

Chronic health risk comparison between China and Denmark on dietary exposure to chlorpyrifos

Chlorpyrifos is one of the most heavily used pesticides in domestic and agricultural insect prevention globally. Given the potential neurotoxicity of chlorpyrifos and its high detection rates in food and drinking water, health risks attributable to chlorpyrifos residue in Chinese drinking water and food in both China and Denmark were assessed in this study. Mixed left-censored handling models were used to deal with the non-detected values in chlorpyrifos concentrations. Results show that chronic exposure imputed to chlorpyrifos residue is much lower than the reference dose, and will thus not pose appreciable health risk to the consumer. Compared to the total exposure from chlorpyrifos in drinking water and food sources, chronic exposure from drinking water sources in China accounts for 0-4.4%. Health risk owing to chlorpyrifos in food within China is 6-7-fold higher than in Denmark, and this coincides with the fact that all application of chlorpyrifos is banned in Denmark, in contrast to China. However, the Danish consumers are still exposed from imported food items. The main health risk contributors in China are the food groups of Grains and grain-based products and Vegetable and vegetable products, while the main chronic health risk contributor in Denmark is the food group of imported fruit and fruit products.

Antibiotics in a general population: Relations with gender, body mass index (BMI) and age and their human health risks

Recently, increasing regulatory and public attention has been paid to the exposure risks of antibiotics due to their occurrence and antibiotic resistance worldwide. However, limited information on antibiotic levels in general populations is available. Forty antibiotics, including 9 sulfonamides, 5 fluoroquinolones, 4 macrolides, 4 tetracyclines, 3 chloramphenicols, 12 β-lactams and 3 others, were analyzed in 107 serum samples of normal adults collected from a hospital in Dalian, North China, between 2015 and 2016 using solid-phase extraction (SPE) coupled with HPLC-MS/MS. The results clearly showed that antibiotics were present in the serum of these adults. Specifically, 28 antibiotics were detected in the samples, with detection frequencies ranging from 0.9% to 17.8%. The total antibiotic concentrations in 26.2% of the serum samples were between the LOD and 20.0ng/mL. Importantly, the maximum concentrations of 5 antibiotics (trimethoprim, ciprofloxacin, cefaclor, lincomycin and erythromycin) were above 1000ng/mL in 3.7% of the samples. Furthermore, the detection frequencies of 5 veterinary antibiotics, 7 human antibiotics and 16 human/veterinary antibiotics in the serum samples were 23.4%, 17.8% and 29.0%, respectively. Significant differences of the veterinary antibiotics between female and male adults and of the sulfonamides between different BMI (body mass index) groups were observed (p<0.05). The concentrations of sulfonamides in elderly individuals were significantly higher (p<0.05) than those in young people. Finally, our results showed that almost all of the adults had no health risks related to exposure to antibiotics at such levels despite the high effect ratio (ER=1.74) for azithromycin in one sample. This study is the first to report the current status of antibiotics in human blood, which can help in better understanding the long-term effects of antibiotics on general populations and in identifying susceptible populations that are at high risk to antibiotic exposure.

Persistence of chlorfluazuron in cabbage under different agro-climatic conditions of India and its risk assessment

A multilocational field trial was conducted at 4 locations in India-Rajasthan, Gujarat, Madhya Pradesh, and West Bengal-to determine the persistence in cabbage of chlorfluazuron applied twice at 75 and 150 g active ingredient ha^-1 . Cabbage head samples were collected from each replicated plot on 0 (2 h after spraying), 1, 3, 5, 7, 10, and 15 d after final insecticide application, including an untreated control. Chlorfluazuron residue in cabbage and field soil was estimated by high-performance liquid chromatography using a photo diode array detector. The limit of determination and limit of quantification of the method were recorded as 0.05 and 0.10 μg g^-1 , respectively. Results revealed that chlorfluazuron dissipated linearly with progress of time, following first-order kinetics. The mean (± standard deviation) half-life value of chlorfluazuron in cabbage was found to be 7.18 ± 0.71 d, considering different locations and treatments. The residue was below the level of quantification in the harvested cabbage and soil samples. Harvesting cabbage in the experimental location, at least on day 7, after 2 applications of chlorfluazuron at the recommended dose, may not pose any ill effect for Indian adults. Environ Toxicol Chem 2017;36:3028-3033. © 2017 SETAC.

An integrated approach to the safety assessment of food additives in early life

During the development of international standards by the Codex Alimentarius Commission, infant foods and their constituent ingredients are subject to rigorous risk analysis and are strictly regulated by many authorities. Various jurisdictions have approved only a limited number of additives specifically with regard to infant foods to fulfill specific technical requirements of quality. As part of the approval process, a rigorous safety assessment is essential to confirm that the use of additives does not pose any health risk for the consumer. An acceptable daily intake (ADI) may be derived from the toxicological databases. However, the ADI may not be applicable to infants because of the possible developmental sensitivities and potentially high exposure scenarios, leading to possible lower margins of safety than would often be determined for adult populations. There is interest in defining better food safety assessment approaches for pre-weaned infants aged less than 12–16 weeks. To confirm safe use in infants, we reviewed the suitability of the existing safety databases of six additives with historical uses in infant nutrition products. To determine further toxicity testing strategies, it is necessary to understand whether the chemical used in the additives is identical to endogenous physiological metabolites and/or whether immature organs of infants are targets of toxicity. Combined with an in-depth review of the existing relevant toxicological and nutritional studies, this integrated approach will facilitate decision-making. We propose a decision tree as a tool within this approach to help guide appropriate data requirements and identify data gaps. In cases of reasonable uncertainty, studies of targeted juvenile should be considered to investigate the safe use levels in food products.

Risk assessment on the use of herbal medicinal products containing pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PA) are common plantal toxins directed against insect herbivores. Unsaturated PAs are known to be hepatotoxic. Many of the PAs are in addition mutagenic and some may possibly be carcinogenic for humans. The risk of an exposure to PAs associated with their occurrence in herbal medicinal products and in foodstuff is under current discussion. The present risk assessment for herbal medicinal products containing PAs is based on a margin of safety derivation for foodstuff indicating that a life-long exposure to maximally 0.007 μg/kg bw/day is not expected to be associated with safety concerns. This approach offers a possibility to estimate the potential risk of PA-containing herbal medicinal products irrespective of the route of administration. It assumes PA levels in the final herbal medicinal product below 0.01 ppm and considers for dermal administration a 100% skin penetration of the PAs reflecting a worst-case scenario. As a result, the calculated margins of safety show a potential exposure using herbal medicinal products 70-, 45.5-, and 19.3-fold lower on a one-day base and 608-, 396-, and 168- fold lower on a one-year base for adults, children aged 12 years, and children aged 4 years, respectively, than the thresholds considered acceptable for foodstuff.

Assessment of health risks resulting from early-life exposures: Are current chemical toxicity testing protocols and risk assessment methods adequate?

Abstract Over the last couple of decades, the awareness of the potential health impacts associated with early-life exposures has increased. Global regulatory approaches to chemical risk assessment are intended to be protective for the diverse human population including all life stages. However, questions persist as to whether the current testing approaches and risk assessment methodologies are adequately protective for infants and children. Here, we review physiological and developmental differences that may result in differential sensitivity associated with early-life exposures. It is clear that sensitivity to chemical exposures during early-life can be similar, higher, or lower than that of adults, and can change quickly within a short developmental timeframe. Moreover, age-related exposure differences provide an important consideration for overall susceptibility. Differential sensitivity associated with a life stage can reflect the toxicokinetic handling of a xenobiotic exposure, the toxicodynamic response, or both. Each of these is illustrated with chemical-specific examples. The adequacy of current testing protocols, proposed new tools, and risk assessment methods for systemic noncancer endpoints are reviewed in light of the potential for differential risk to infants and young children.

Incorporating population variability and susceptible subpopulations into dosimetry for high-throughput toxicity testing

Momentum is growing worldwide to use in vitro high-throughput screening (HTS) to evaluate human health effects of chemicals. However, the integration of dosimetry into HTS assays and incorporation of population variability will be essential before its application in a risk assessment context. Previously, we employed in vitro hepatic metabolic clearance and plasma protein binding data with in vitro in vivo extrapolation (IVIVE) modeling to estimate oral equivalent doses, or daily oral chemical doses required to achieve steady-state blood concentrations (Css) equivalent to media concentrations having a defined effect in an in vitro HTS assay. In this study, hepatic clearance rates of selected ToxCast chemicals were measured in vitro for 13 cytochrome P450 and five uridine 5'-diphospho-glucuronysyltransferase isozymes using recombinantly expressed enzymes. The isozyme-specific clearance rates were then incorporated into an IVIVE model that captures known differences in isozyme expression across several life stages and ethnic populations. Comparison of the median Css for a healthy population against the median or the upper 95th percentile for more sensitive populations revealed differences of 1.3- to 4.3-fold or 3.1- to 13.1-fold, respectively. Such values may be used to derive chemical-specific human toxicokinetic adjustment factors. The IVIVE model was also used to estimate subpopulation-specific oral equivalent doses that were directly compared with subpopulation-specific exposure estimates. This study successfully combines isozyme and physiologic differences to quantitate subpopulation pharmacokinetic variability. Incorporation of these values with dosimetry and in vitro bioactivities provides a viable approach that could be employed within a high-throughput risk assessment framework.

Advancing human health risk assessment: integrating recent advisory committee recommendations

Over the last dozen years, many national and international expert groups have considered specific improvements to risk assessment. Many of their stated recommendations are mutually supportive, but others appear conflicting, at least in an initial assessment. This review identifies areas of consensus and difference and recommends a practical, biology-centric course forward, which includes: (1) incorporating a clear problem formulation at the outset of the assessment with a level of complexity that is appropriate for informing the relevant risk management decision; (2) using toxicokinetics and toxicodynamic information to develop Chemical Specific Adjustment Factors (CSAF); (3) using mode of action (MOA) information and an understanding of the relevant biology as the key, central organizing principle for the risk assessment; (4) integrating MOA information into dose-response assessments using existing guidelines for non-cancer and cancer assessments; (5) using a tiered, iterative approach developed by the World Health Organization/International Programme on Chemical Safety (WHO/IPCS) as a scientifically robust, fit-for-purpose approach for risk assessment of combined exposures (chemical mixtures); and (6) applying all of this knowledge to enable interpretation of human biomonitoring data in a risk context. While scientifically based defaults will remain important and useful when data on CSAF or MOA to refine an assessment are absent or insufficient, assessments should always strive to use these data. The use of available 21st century knowledge of biological processes, clinical findings, chemical interactions, and dose-response at the molecular, cellular, organ and organism levels will minimize the need for extrapolation and reliance on default approaches.

Extreme sensitivity and the practical implications of risk assessment thresholds

Traditional risk-assessment theory assumes the existence of a threshold for non-cancer health effects. However, a recent trend in environmental regulation rejects this assumption in favor of non-threshold linearity for these endpoints. This trend is driven largely by two related concepts: (1) a theoretical assumption of wide-ranging human sensitivity, and (2) inability to detect thresholds in epidemiologic models. Wide-ranging sensitivity assumes a subpopulation with extreme background vulnerability, so that even trivial environmental exposures are hazardous to someone somewhere. We use examples from the real world of clinical medicine to show that this theoretical assumption is inconsistent with the biology of mammalian systems and the realities of patient care. Using examples from particulate-matter air-pollution research, we further show that failure to reject linearity is usually driven by statistical rather than biological considerations, and that nonlinear/threshold models often have a similar or better fit than their linear counterparts. This evidence suggests the existence of practical, real-world thresholds for most chemical exposures.

Early lifestage exposure and potential developmental susceptibility to tetrachloroethylene

BACKGROUND

Tetrachloroethylene, also known as perchloroethylene or "perc", is a highly volatile and lipophilic solvent widely used in dry cleaning, textile processing, and metal-cleaning operations. The limited epidemiological and toxicological data available for exposure to perc during developmental lifestages, as well as the evidence for critical windows of exposure, highlight early life as a period of potential susceptibility.

METHODS

A literature search was performed to identify all peer-reviewed epidemiological and toxicologial studies examining outcomes from early lifestage exposure to perc, and reviewed by developmental stage for both exposure and outcome.

RESULTS

Exposure scenarios to perc unique to early lifestages include transplacental and breast milk intake, along with inhalation, ingestion, or dermal exposure. Toxicokinetics factors that may influence early lifestage susceptibility to perc, along with existing physiologically based pharmacokinetic (PBPK) models, are described. Adverse outcomes examined include: reproductive outcomes examined prior to conception including reduced fertility, adverse effects on sperm, or altered reproductive hormones; prenatal outcomes examined after exposure prior to conception or prenatally including fetal death, birth defects, and decreased birth weight; postnatal outcomes examined after exposure prior to conception, prenatally, or during childhood including neurotoxicity, immunotoxicity, cancer, hepatotoxicity, congential anomalies and mortality; and adult schizophrenia examined after exposure prior to conception.

CONCLUSIONS

The limited evidence on early lifestage exposure to perc does not provide sufficient evidence of this sensitive period as being more or less important than exposure at a later lifestage, such as during adulthood. However, there are a number of adverse health effects observed uniquely in early lifestages, and increased sensitivity to visual system deficits is suggested in children. Other outcomes observed in adults may not have been adequately assessed in children to directly compare sensitivity.

The influence of genetic polymorphisms on population variability in six xenobiotic-metabolizing enzymes

This review provides variability statistics for polymorphic enzymes that are involved in the metabolism of xenobiotics. Six enzymes were evaluated: cytochrome P-450 (CYP) 2D6, CYP2E1, aldehyde dehydrogenase-2 (ALDH2), paraoxonase (PON1), glutathione transferases (GSTM1, GSTT1, and GSTP1), and N-acetyltransferases (NAT1 and NAT2). The polymorphisms were characterized with respect to (1) number and type of variants, (2) effects of polymorphisms on enzyme function, and (3) frequency of genotypes within specified human populations. This information was incorporated into Monte Carlo simulations to predict the population distribution and describe interindividual variability in enzyme activity. The results were assessed in terms of (1) role of these enzymes in toxicant activation and clearance, (2) molecular epidemiology evidence of health risk, and (3) comparing enzyme variability to that commonly assumed for pharmacokinetics. Overall, the Monte Carlo simulations indicated a large degree of interindividual variability in enzyme function, in some cases characterized by multimodal distributions. This study illustrates that polymorphic metabolizing systems are potentially important sources of pharmacokinetic variability, but there are a number of other factors including blood flow to liver and compensating pathways for clearance that affect how a specific polymorphism will alter internal dose and toxicity. This is best evaluated with the aid of physiologically based pharmacokinetic (PBPK) modeling. The population distribution of enzyme activity presented in this series of articles serves as inputs to such PBPK modeling analyses.

Mycotoxins and child health: the need for health risk assessment

The occurrences of mycotoxins as food contaminants in different localities particularly in developing countries and the inevitable exposure of populations and children to these toxins with probable adverse outcomes need be scientifically and systematically assessed. Health risk assessment developed in the 1980s is separate from risk management, both with risk communication form the risk analysis framework adopted by the World Health Organization. The process contributes increasingly to policy development, public health decision making, the establishment of mycotoxin regulations and research planning. However, the exercise of the risk assessment structured approach is not simple and is faced up to lack of data, capable infrastructure facilities and need for trained personnel and resources. Furthermore, adopted methodologies need be developed focusing on child characteristics and health concerns.

Potential new approaches for children's inhalation risk assessment

The U.S. Environmental Protection Agency (EPA) practice of risk assessment is moving toward more thoroughly considering children's unique susceptibilities and exposure potential. Childhood is assessed as a sequence of life stages that reflects the fact that as humans develop, windows of susceptibility may appear that lead to enhanced sensitivity to exposure of environmental agents, while changes in behavior and physiology may increase exposure and dose. The U.S. EPA developed guidance in the past few years that addresses some aspects of increased susceptibility and exposure and dose. However, when it comes to considering inhalation exposure, dose, and risk, current U.S. EPA practice does not explicitly address children. The purpose here is to begin studying the adequacy of practice for children's health and to explore possible next steps in developing new methods to more accurately assess life-stage-specific differences. The existing guidelines and policies used to address potentially unique susceptibilities of children for inhaled environmental chemicals were considered, as well as what may be learned from examples of approaches that have been applied by state agencies (such as the California Environmental Protection Agency) or in the literature, to incorporate potentially unique susceptibilities and exposures to children. Finally, there is a discussion of possible approaches for considering inhalation exposure and susceptibility in U.S. EPA risk assessments.

Assessment factors--applications in health risk assessment of chemicals

We review the scientific basis for default assessment factors used in risk assessment of nongenotoxic chemicals including the use of chemical- and pathways specific assessment factors, and extrapolation approaches relevant to species differences, age and gender. One main conclusion is that the conventionally used default factor of 100 does not cover all inter-species and inter-individual differences. We suggest that a species-specific default factor based on allometric scaling should be used for inter-species extrapolation (basal metabolic rate). Regarding toxicodynamic and remaining toxicokinetic differences we suggest that a percentile from a probabilistic distribution is chosen to derive the assessment factor. Based on the scarce information concerning the human-to-human variability it is more difficult to suggest a specific assessment factor. However, extra emphasis should be put on sensitive populations such as neonates and genetically sensitive subgroups, and also fetuses and children which may be particularly vulnerable during development and maturation. Factors that also need to be allowed for are possible gender differences in sensitivity, deficiencies in the databases, nature of the effect, duration of exposure, and route-to-route extrapolation. Since assessment factors are used to compensate for lack of knowledge we feel that it is prudent to adopt a "conservative" approach, erring on the side of protectiveness.

Pediatric susceptibility to 18 industrial chemicals: A comparative analysis of newborn with young animals

We comprehensively re-analyzed the toxicity data for 18 industrial chemicals from repeated oral exposures in newborn and young rats, which were previously published. Two new toxicity endpoints specific to this comparative analysis were identified, the first, the presumed no observed adverse effect level (pNOAEL) was estimated based on results of both main and dose-finding studies, and the second, the presumed unequivocally toxic level (pUETL) was defined as a clear toxic dose giving similar severity in both newborn and young rats. Based on the analyses of both pNOAEL and pUETL ratios between the different ages, newborn rats demonstrated greater susceptibility (at most 8-fold) to nearly two thirds of these 18 chemicals (mostly phenolic substances), and less or nearly equal sensitivity to the other chemicals. Exceptionally one chemical only showed toxicity in newborn rats. In addition, Benchmark Dose Lower Bound (BMDL) estimates were calculated as an alternative endpoint. Most BMDLs were comparable to their corresponding pNOAELs and the overall correlation coefficient was 0.904. We discussed how our results can be incorporated into chemical risk assessment approaches to protect pediatric health from direct oral exposure to chemicals.

Risk assessment of triclosan [Irgasan] in human breast milk

Triclosan is an established bacteriostatic compound widely used in topical and dental preparations. Its pharmacokinetics and toxicology have been extensively studied in humans and animals. It is known to be absorbed from the gastrointestinal tract and across the skin. A recent report noted its occurrence in human breast milk and this has now been further investigated. Sixty two unselected samples of human milk from Breast Milk Banks in California and Texas have been analysed for triclosan; the concentration ranged from 0 to 2100 microg/kg lipid. A risk assessment of triclosan in human milk has been made, based on a conservative calculation of exposure of neonates and experimental toxicity test results. The broad set of reproduction toxicity tests of triclosan includes a 2-generation study in the rat, in which there was considerable exposure of dams and pups to triclosan throughout fetal development and up to sexual maturity in the F2 generation, and a further study in which pups of dosed dams were followed to weaning. They established an oral NOAEL for pups of 50 mg/kg/d. The maximum exposure of babies via breast milk calculated using very conservative additive assumptions is approximately 7.4 microg/kg/d. The 'Margin of Exposure' between the NOAEL and that calculated in breast fed babies is approximately 6760-fold. It is concluded that there is no evidence to indicate that the presence of a miniscule amount of triclosan in breast milk presents a risk to babies.

Human milk biomonitoring data: interpretation and risk assessment issues

Biomonitoring data can, under certain conditions, be used to describe potential risks to human health (for example, blood lead levels used to determine children's neurodevelopmental risk). At present, there are very few chemical exposures at low levels for which sufficient data exist to state with confidence the link between levels of environmental chemicals in a person's body and his or her risk of adverse health effects. Human milk biomonitoring presents additional complications. Human milk can be used to obtain information on both the levels of environmental chemicals in the mother and her infant's exposure to an environmental chemical. However, in terms of the health of the mother, there are little to no extant data that can be used to link levels of most environmental chemicals in human milk to a particular health outcome in the mother. This is because, traditionally, risks are estimated based on dose, rather than on levels of environmental chemicals in the body, and the relationship between dose and human tissue levels is complex. On the other hand, for the infant, some information on dose is available because the infant is exposed to environmental chemicals in milk as a "dose" from which risk estimates can be derived. However, the traditional risk assessment approach is not designed to consider the benefits to the infant associated with breastfeeding and is complicated by the relatively short-term exposures to the infant from breastfeeding. A further complexity derives from the addition of in utero exposures, which complicates interpretation of epidemiological research on health outcomes of breastfeeding infants. Thus, the concept of "risk assessment" as it applies to human milk biomonitoring is not straightforward, and methodologies for undertaking this type of assessment have not yet been fully developed. This article describes the deliberations of the panel convened for the Technical Workshop on Human Milk Surveillance and Biomonitoring for Environmental Chemicals in the United States, held at the Hershey Medical Center, Pennsylvania State College of Medicine, on several issues related to risk assessment and human milk biomonitoring. Discussion of these topics and the thoughts and conclusions of the panel are described in this article.

Assessing susceptibility from early-life exposure to carcinogens

Cancer risk assessment methods currently assume that children and adults are equally susceptible to exposure to chemicals. We reviewed available scientific literature to determine whether this was scientifically supported. We identified more than 50 chemicals causing cancer after perinatal exposure. Human data are extremely limited, with radiation exposures showing increased early susceptibility at some tumor sites. Twenty-seven rodent studies for 18 chemicals had sufficient data after postnatal and adult exposures to quantitatively estimate potential increased susceptibility from early-life exposure, calculated as the ratio of juvenile to adult cancer potencies for three study types: acute dosing, repeated dosing, and lifetime dosing. Twelve of the chemicals act through a mutagenic mode of action. For these, the geometric mean ratio was 11 for lifetime exposures and 8.7 for repeat exposures, with a ratio of 10 for these studies combined. The geometric mean ratio for acute studies is 1.5, which was influenced by tissue-specific results [geometric mean ratios for kidney, leukemia, liver, lymph, mammary, nerve, reticular tissue, thymic lymphoma, and uterus/vagina > 1 (range, 1.6-8.1); forestomach, harderian gland, ovaries, and thyroid < 1 (range, 0.033-0.45)]. Chemicals causing cancer through other modes of action indicate some increased susceptibility from postnatal exposure (geometric mean ratio is 3.4 for lifetime exposure, 2.2 for repeat exposure). Early exposures to compounds with endocrine activity sometimes produce different tumors after exposures at different ages. These analyses suggest increased susceptibility to cancer from early-life exposure, particularly for chemicals acting through a mutagenic mode of action.

Human variability in xenobiotic metabolism and pathway-related uncertainty factors for chemical risk assessment: a review

This review provides an account of recent developments arising from a database that defined human variability in phase I metabolism (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, hydrolysis, alcohol dehydrogenase), phase II metabolism (N-acetyltransferases, glucuronidation, glycine conjugation, sulphation) and renal excretion. This database was used to derive pathway-related uncertainty factors for chemical risk assessment that allow for human variability in toxicokinetics. Probe substrates for each pathway of elimination were selected on the basis that oral absorption was >95% and that the metabolic route was the primary route of elimination of the compound (60-100% of a dose). Intravenous data were used for compounds for which absorption was variable. Human variability in kinetics was quantified for each compound from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups of the population using parameters relating to chronic exposure (metabolic and total clearances, area under the plasma concentration-time curve (AUC)) and acute exposure (Cmax) (data not presented here). The pathway-related uncertainty factors were calculated to cover 95%, 97.5% and 99% of the population of healthy adults and of each subgroup. Pathway-related uncertainty factors allow metabolism data to be incorporated into the derivation of health-based guidance values. They constitute an intermediate approach between the general kinetic default factors (3.16) and a chemical-specific adjustment factor. Applications of pathway-related uncertainty factors for chemical risk assessment and future refinements of the approach are discussed. A knowledge-based framework to predict human variability in kinetics for xenobiotics showing a threshold dose below which toxic effects are not observed, is proposed to move away from default assumptions.

Incorporating pharmacokinetic differences between children and adults in assessing children's risks to environmental toxicants

Children's risks from environmental toxicant exposure can be affected by pharmacokinetic factors that affect the internal dose of parent chemical or active metabolite. There are numerous physiologic differences between neonates and adults that affect pharmacokinetics including size of lipid, and tissue compartments, organ blood flows, protein binding capacity, and immature function of renal and hepatic systems. These factors combine to decrease the clearance of many therapeutic drugs, which can also be expected to occur with environmental toxicants in neonates. The net effect may be greater or lesser internal dose of active toxicant depending upon how the agent is distributed, metabolized, and eliminated. Child/adult pharmacokinetic differences decrease with increasing postnatal age, but these factors should still be considered in any children's age group, birth through adolescence, for which there is toxicant exposure. Physiologically based pharmacokinetic (PBPK) models can simulate the absorption, distribution, metabolism, and excretion of xenobiotics in both children and adults, allowing for a direct comparison of internal dose and risk across age groups. This review provides special focus on the development of hepatic cytochrome P-450 enzymes (CYPs) in early life and how this information, along with many factors unique to children, can be applied to PBPK models for this receptor population. This review describes a case study involving the development of neonatal PBPK models for the CYP1A2 substrates caffeine and theophylline. These models were calibrated with pharmacokinetic data in neonates and used to help understand key metabolic differences between neonates and adults across these two drugs.

Role of biokinetics in risk assessment of drugs and chemicals in children

Whether children incur different risks from xenobiotics than adults will depend on the exposure, biokinetics, and dynamics of compound. In this paper, current knowledge on developmental physiology and possible effects on biokinetics are evaluated and the role of biokinetics in risk assessment both for drugs and chemicals is discussed. It is concluded that most dramatic age-related physiological changes that may affect biokinetics occur in the first 6-12 months of age. The difference in internal exposure between children and adults can generally be predicted from already known developmental physiological differences. However, for risk assessment it will also be necessary to determine whether internal exposure is within the drug's therapeutic window or if it will exceed the NOAEL of a chemical. Furthermore, the effects of internal exposure of potentially harmful compounds on developing organ systems is of utmost importance. However, knowledge on this aspect is very limited. Risk assessment in children could be improved by: (1) application of pediatric PBPK-models in order to gain insight into internal exposure in children, (2) studies in juvenile animals for studying effects on developing systems, and (3) extrapolation of knowledge on the relationship between internal exposure and dynamics for drugs to other chemicals.

Physiologically based pharmacokinetic (PBPK) modeling of caffeine and theophylline in neonates and adults: implications for assessing children's risks from environmental agents

Children's risks can differ from those in adults for numerous reasons, one being differences in the pharmacokinetic handling of chemicals. Immature metabolism and a variety of other factors in neonates can affect chemical disposition and clearance. These factors can be incorporated into physiologically based pharmacokinetic (PBPK) models that simulate the fate of environmental toxicants in both children and adults. PBPK models are most informative when supported by empirical data, but typically pediatric pharmacokinetic data for toxicants are not available. In contrast, pharmacokinetic data in children are readily available for therapeutic drugs. The current analysis utilizes data for caffeine and theophylline, closely related xanthines that are both cytochrome P-450 (CYP) 1A2 substrates, in developing PBPK models for neonates and adults. Model development involved scale-up of in vitro metabolic parameters to whole liver and adjusting metabolic function for the ontological pattern of CYP1A2 and other CYPs. Model runs were able to simulate the large differences in half-life and clearance between neonates and adults. Further, the models were able to reproduce the faster metabolic clearance of theophylline relative to caffeine in neonates. This differential between xanthines was found to be due primarily to an extra metabolic pathway available to theophylline, back-methylation to caffeine, that is not available to caffeine itself. This pathway is not observed in adults exemplifying the importance of secondary or novel routes of metabolism in the immature liver. Greater CYP2E1 metabolism of theophylline relative to caffeine in neonates also occurs. Neonatal PBPK models developed for these drugs may be adapted to other CYP1A2 substrates (e.g., arylamine toxicants). A stepwise approach for modeling environmental toxicants in children is proposed.

Incorporating children's toxicokinetics into a risk framework

Children's responses to environmental toxicants will be affected by the way in which their systems absorb, distribute, metabolize, and excrete chemicals. These toxicokinetic factors vary during development, from in utero where maternal and placental processes play a large role, to the neonate in which emerging metabolism and clearance pathways are key determinants. Toxicokinetic differences between neonates and adults lead to the potential for internal dosimetry differences and increased or decreased risk, depending on the mechanisms for toxicity and clearance of a given chemical. This article raises a number of questions that need to be addressed when conducting a toxicokinetic analysis of in utero or childhood exposures. These questions are organized into a proposed framework for conducting the assessment that involves problem formulation (identification of early life stage toxicokinetic factors and chemical-specific factors that may raise questions/concerns for children); data analysis (development of analytic approach, construction of child/adult or child/animal dosimetry comparisons); and risk characterization (evaluation of how children's toxicokinetic analysis can be used to decrease uncertainties in the risk assessment). The proposed approach provides a range of analytical options, from qualitative to quantitative, for assessing children's dosimetry. Further, it provides background information on a variety of toxicokinetic factors that can vary as a function of developmental stage. For example, the ontology of metabolizing systems is described via reference to pediatric studies involving therapeutic drugs and evidence from in vitro enzyme studies. This type of resource information is intended to help the assessor begin to address the issues raised in this paper.

A framework for assessing risks to children from exposure to environmental agents

In recent years there has been an increasing focus in environmental risk assessment on children as a potentially susceptible population. There also has been growing recognition of the need for a systematic approach for organizing, evaluating, and incorporating the available data on children's susceptibilities in risk assessments. In this article we present a conceptual framework for assessing risks to children from environmental exposures. The proposed framework builds on the problem formulation-->analysis-->risk characterization paradigm, identifying at each phase the questions and issues of particular importance for characterizing risks to the developing organism (from conception through organ maturation). The framework is presented and discussed from the complementary perspectives of toxicokinetics and toxicodynamics.

Assessing cancer risks from short-term exposures in children

For the vast majority of chemicals that have cancer potency estimates on IRIS, the underlying database is deficient with respect to early-life exposures. This data gap has prevented derivation of cancer potency factors that are relevant to this time period, and so assessments may not fully address children's risks. This article provides a review of juvenile animal bioassay data in comparison to adult animal data for a broad array of carcinogens. This comparison indicates that short-term exposures in early life are likely to yield a greater tumor response than short-term exposures in adults, but similar tumor response when compared to long-term exposures in adults. This evidence is brought into a risk assessment context by proposing an approach that: (1) does not prorate children's exposures over the entire life span or mix them with exposures that occur at other ages; (2) applies the cancer slope factor from adult animal or human epidemiology studies to the children's exposure dose to calculate the cancer risk associated with the early-life period; and (3) adds the cancer risk for young children to that for older children/adults to yield a total lifetime cancer risk. The proposed approach allows for the unique exposure and pharmacokinetic factors associated with young children to be fully weighted in the cancer risk assessment. It is very similar to the approach currently used by U.S. EPA for vinyl chloride. The current analysis finds that the database of early life and adult cancer bioassays supports extension of this approach from vinyl chloride to other carcinogens of diverse mode of action. This approach should be enhanced by early-life data specific to the particular carcinogen under analysis whenever possible.

Differences in pharmacokinetics between children and adults--II. Children's variability in drug elimination half-lives and in some parameters needed for physiologically-based pharmacokinetic modeling

In earlier work we assembled a database of classical pharmacokinetic parameters (e.g., elimination half-lives; volumes of distribution) in children and adults. These data were then analyzed to define mean differences between adults and children of various age groups. In this article, we first analyze the variability in half-life observations where individual data exist. The major findings are as follows. The age groups defined in the earlier analysis of arithmetic mean data (0-1 week premature; 0-1 week full term; 1 week to 2 months; 2-6 months; 6 months to 2 years; 2-12 years; and 12-18 years) are reasonable for depicting child/adult pharmacokinetic differences, but data for some of the earliest age groups are highly variable. The fraction of individual children's half-lives observed to exceed the adult mean half-life by more than the 3.2-fold uncertainty factor commonly attributed to interindividual pharmacokinetic variability is 27% (16/59) for the 0-1 week age group, and 19% (5/26) in the 1 week to 2 month age group, compared to 0/87 for all the other age groups combined between 2 months and 18 years. Children within specific age groups appear to differ from adults with respect to the amount of variability and the form of the distribution of half-lives across the population. The data indicate departure from simple unimodal distributions, particularly in the 1 week to 2 month age group, suggesting that key developmental steps affecting drug removal tend to occur in that period. Finally, in preparation for age-dependent physiologically-based pharmacokinetic modeling, nationally representative NHANES III data are analyzed for distributions of body size and fat content. The data from about age 3 to age 10 reveal important departures from simple unimodal distributional forms-in the direction suggesting a subpopulation of children that are markedly heavier than those in the major mode. For risk assessment modeling, this means that analysts will need to consider "mixed" distributions (e.g., two or more normal or log-normal modes) in which the proportions of children falling within the major versus highweight/fat modes in the mixture changes as a function of age. Biologically, the most natural interpretation of this is that these subpopulations represent children who have or have not yet received particular signals for change in growth pattern. These apparently distinct subpopulations would be expected to exhibit different disposition of xenobiotics, particularly those that are highly lipophilic and poorly metabolized.

Population distribution of aldehyde dehydrogenase-2 genetic polymorphism: implications for risk assessment

The role of genetic polymorphisms in modulating xenobiotic metabolism and susceptibility to cancer and other health effects has been suggested in numerous studies. However, risk assessments have generally not used this information to characterize population variability or adjust risks for susceptible subgroups. This paper focuses upon the aldehyde dehydrogenase-2 (ALDH2) system because it exemplifies the pivotal role genetic polymorphisms can play in determining enzyme function and susceptibility. Allelic variants in ALDH2 cause decreased ability to clear acetaldehyde and other aldehyde substrates, with homozygous variants (ALDH2*2/2) having no activity and heterozygotes (ALDH2*1/2) having intermediate activity relative to the predominant wild type (ALDH2*1/1). These polymorphisms are associated with increased buildup of acetaldehyde following ethanol ingestion and increased immediate symptoms (flushing syndrome) and long-term cancer risks. We have used Monte Carlo simulation to characterize the population distribution of ALDH2 allelic variants and inter-individual variability in aldehyde internal dose. The nonfunctional allele is rare in most populations, but is common in Asians such that 40% are heterozygotes and 5% are homozygote variants. The ratio of the 95th or 99th percentiles of the Asian population compared to the median of the U.S. population is 14- to 26-fold, a variability factor that is larger than the default pharmacokinetic uncertainty factor (3.2-fold) commonly used in risk assessment. Approaches are described for using ALDH2 population distributions in physiologically based pharmacokinetic-Monte Carlo refinements of risk assessments for xenobiotics which are metabolized to aldehyde intermediates (e.g., ethanol, toluene, ethylene glycol monomethyl ether).

Ontogeny of hepatic and renal systemic clearance pathways in infants: part I

Dramatic developmental changes in the physiological and biochemical processes that govern drug pharmacokinetics and pharmacodynamics occur during the first year of life. These changes may have significant consequences for the way infants respond to and deal with drugs. The ontogenesis of systemic clearance mechanisms is probably the most critical determinant of a pharmacological response in the developing infant. In recent years, advances in molecular techniques and an increased availability of fetal and infant tissues have afforded enhanced insight into the ontogeny of clearance mechanisms. Information from these studies is reviewed to highlight the dynamic and complex nature of developmental changes in clearance mechanisms in infants during the first year of life. Hepatic and renal elimination mechanisms constitute the two principal clearance pathways of the developing infant. Drug metabolising enzyme activity is primarily responsible for the hepatic clearance of many drugs. In general, when compared with adult activity levels normalised to amount of hepatic microsomal protein, hepatic cytochrome P450-mediated metabolism and the phase II reactions of glucuronidation, glutathione conjugation and acetylation are deficient in the neonate, but sulfate conjugation is an efficient pathway at birth. Parturition triggers the dramatic development of drug metabolising enzymes, and each enzyme demonstrates an independent rate and pattern of maturation. Marked interindividual variability is associated with their developmental expression, making the ontogenesis of hepatic metabolism a highly variable process. By the first year of life, most enzymes have matured to adult activity levels. When compared with adult values, renal clearance mechanisms are compromised at birth. Dramatic increases in renal function occur in the ensuing postpartum period, and by 6 months of age glomerular filtration rate normalised to bodyweight has approached adult values. Maturation of renal tubular functions exhibits a more protracted time course of development, resulting in a glomerulotubular imbalance. This imbalance exists until adult renal tubule function values are approached by 1 year of age. The ontogeny of hepatic biliary and renal tubular transport processes and their impact on the elimination of drugs remain largely unknown. The summary of the current understanding of the ontogeny of individual pathways of hepatic and renal elimination presented in this review should serve as a basis for the continued accruement of age-specific information concerning the ontogeny of clearance mechanisms in infants. Such information can only help to improve the pharmacotherapeutic management of paediatric patients.

Human health significance of organochlorine and mercury contaminants in Japanese whale meat

The concentrations of total mercury, polychlorinated biphenyls (PCBs), and organochlorine pesticides (SigmaDDT, dieldrin, hexachlorobenzene [HCB], and SigmaHCH) were determined in 61 whale meat products (bacon, blubber, red meat, liver, intestine, and tongue) purchased from retail outlets across Japan. Mean (range) concentrations of contaminants in all samples were: total mercury 4.17 (0.01-204); SigmaPCB 1.14 (0-8.94); SigmaDDT 0.98 (0-7.46); dieldrin 0.07 (0-0.35); HCB 0.06 (0-0.22); and SigmaHCH 0.07 (0-0.19) micro g/g (wet weight). The data were used to calculate estimated daily intakes (EDIs) of contaminants at two hypothetical levels of whale meat consumption. These EDIs were compared with FAO/WHO "tolerable daily intake" (TDI) values for each chemical. EDIs calculated for higher levels of whale meat consumption were in some cases exceptionally high and for many products exceeded FAO/ WHO-TDIs for total mercury, PCBs, and dieldrin, with exceedance factor values (EDI/TDI) for total mercury, PCBs, and dieldrin reaching maxima of 175, 5.36, and 2.1, respectively. For sensitive consumers and those with high-level consumption (e.g., whaling communities), exposure to mercury and to a lesser extent PCBs from certain whale blubber and bacon and striped dolphin liver products could lead to chronic health effects. The Japanese community should therefore exercise a precautionary approach to the consumption of such foods in excess, particularly by high-risk members of the population.

Differences between children and adults: implications for risk assessment at California EPA

The California legislature enacted a law requiring the California Environmental Protection Agency (Cal/EPA) Office of Environmental Health Hazard Assessment (OEHHA) to evaluate whether our risk assessment methodologies are adequately protective of infants and children. In addition both OEHHA and the California Air Resources Board must examine whether the Ambient Air Quality Standards set for criteria air pollutants and the health values developed for air toxics are adequately protective of infants and children. We have initiated a program to look at potential differences in response to toxicants between children and adults. We are evaluating this issue from the perspective of exposure differences as well as toxicokinetic and toxicodynamic differences between children and adults. Data on specific chemicals are rather limited. As a result, we will be pooling information to determine whether there are generic differences between children and adults that may be applicable to risk assessment in general or to risk assessment of specific classes of compounds. This paper discusses the rationale for approaching the issue of determining whether our risk assessment methods are adequate for infants and children and includes a discussion of some of the available information on both qualitative and quantitative differences in response to toxicants between children and adults or immature and mature laboratory animals. We provide examples of differences between children and adults in absorption, metabolism, and excretion of toxicants as well as qualitative differences in toxic response.

Differential sensitivity of children and adults to chemical toxicity. II. Risk and regulation

Animals can be useful predictors of chemical hazards to humans. Growth and development are compressed into a shorter period in animals, which makes interpretation of animal testing inherently more difficult. However, similar events occur in both humans and laboratory animals and testing that covers the full period of animal development can reasonably be considered an appropriate surrogate for human development. Some have proposed an additional 10-fold factor for the extra protection of children when estimating safe exposures. Use of such an additional factor, as required by the Food Quality Protection Act (FQPA), is meant to address the same issues covered by the EPA's database uncertainty factor, UF(D), and additional issues related to exposure uncertainty. Thus, when UF(D) has already been deployed, the EPA modifies its use of the FQPA factor. Based on our analysis, we agree with the EPA. Drawing conclusions about the adequacy of UF(H), the uncertainty factor used to account for intrahuman variability, in terms of its ability to protect children on the basis of the modest data available is challenging. However, virtually all studies available suggest that a high percentage of the population, including children, is protected by using a 10-fold uncertainty factor for human variability or by using a 3.16-fold factor each for toxicokinetic and toxicodynamic variability. Based on specific comparisons for newborns, infants, children, adults, and those with severe disease, the population protected is between 60 and 100%, with the studies in larger populations that include sensitive individuals suggesting that the value is closer to 100%.

Mathematical modelling and quantitative methods

The present review reports on the mathematical methods and statistical techniques presently available for hazard characterisation. The state of the art of mathematical modelling and quantitative methods used currently for regulatory decision-making in Europe and additional potential methods for risk assessment of chemicals in food and diet are described. Existing practices of JECFA, FDA, EPA, etc., are examined for their similarities and differences. A framework is established for the development of new and improved quantitative methodologies. Areas for refinement, improvement and increase of efficiency of each method are identified in a gap analysis. Based on this critical evaluation, needs for future research are defined. It is concluded from our work that mathematical modelling of the dose-response relationship would improve the risk assessment process. An adequate characterisation of the dose-response relationship by mathematical modelling clearly requires the use of a sufficient number of dose groups to achieve a range of different response levels. This need not necessarily lead to an increase in the total number of animals in the study if an appropriate design is used. Chemical-specific data relating to the mode or mechanism of action and/or the toxicokinetics of the chemical should be used for dose-response characterisation whenever possible. It is concluded that a single method of hazard characterisation would not be suitable for all kinds of risk assessments, and that a range of different approaches is necessary so that the method used is the most appropriate for the data available and for the risk characterisation issue. Future refinements to dose-response characterisation should incorporate more clearly the extent of uncertainty and variability in the resulting output.

Uncertainty factors for chemical risk assessment. human variability in the pharmacokinetics of CYP1A2 probe substrates

A 100-fold uncertainty factor is used to derive acceptable daily intakes for compounds causing thresholded toxicity. The 10-fold factor for human variability can be further subdivided into two factors of 10(0.5) (3.16) to allow for toxicokinetics and toxicodynamics. The validity of the human kinetic subfactor has been analysed in relation to CYP1A2 metabolism using published in vivo pharmacokinetic parameters selected to reflect chronic exposure (metabolic and total clearances and area under the plasma concentration-time curve: CLm, CL and AUC) and acute exposure (the peak plasma concentration, C(max)). The variability in CYP1A2 activity in healthy adults, based on data after oral and intravenous dosage (CLm, CL and AUC), ranged from 34 to 42%. The variability in C(max) was 21%. The default kinetic factor of 3.16 would cover at least 99% of the healthy adult population, assuming that the data were log-normally distributed, but would give lower protection for some subgroups (pregnant women at term, healthy elderly, patients with liver disease), and was inadequate for neonates. This analysis of in vivo kinetic data for CYP1A2 substrates illustrates the importance of quantifying human variability in specific metabolic pathways, and of identifying potentially susceptible subgroups of the human population, in order to determine the scientific validity of uncertainty factors.

Children's health, susceptibility, and regulatory approaches to reducing risks from chemical carcinogens

Risk-based regulation of chemical exposures from the environment generally relies on assumptions about the extent of people's susceptibility to chemically induced diseases. Those assumptions are intended to be health-protective; that is, they err on the side of overstating susceptibility. Recent concern about children's special susceptibilities has led to proposals that would make risk-based regulations one-tenth more stringent, unless data are available to refute the assumption that children are more susceptible than adults. In this paper we highlight some of the questions that should be addressed in the context of risk assessment to determine whether such increased stringency would accomplish the desired result of improving children's health. In particular, characterizing benefits of greater stringency requires more information about dose-response relationships than is currently available. Lowering regulatory levels has attendant costs but may not achieve benefits, for example, if the previous level were already below an actual or practical threshold. Without an ability to understand the potential benefit (or lack thereof) of the additional stringency, an appropriate consideration of benefits and costs is not possible.

The use of safety or uncertainty factors in the setting of acute reference doses

A 100-fold safety or uncertainty factor has been used for about 40 years to derive safe daily intakes for humans based on animal studies; the 100-fold factor comprises separate 10-fold factors to allow for species differences and inter-individual variability. Each factor has to allow for toxicokinetic and toxicodynamic differences. Sub-dividing the 10-fold factors into kinetic and dynamic defaults, which when multiplied give a product of 10, offers a number of advantages. The main rationale for this sub-division is so that chemical-specific data can be introduced to replace one or more of the default sub-factors, hence contributing to a chemical-related overall factor. However, sub-division of the 10-fold factors has allowed analysis of the appropriateness of the overall 10-fold defaults, and analysis of special situations, such as infants and children. The establishment of an acute reference dose based on animal studies has to allow for both species differences and inter-individual variability; comparison with the factors used for chronic effects suggests that modification of the usual defaults may be appropriate under certain specific circumstances, but that the usual default of 100 remains appropriate for most cases.

An analysis of the need for an additional uncertainty factor for infants and children

Uncertainty factors have been used for over 40 years to establish safe intakes of threshold toxicants. Tenfold factors are used to allow for species differences and for human variability, with extra factors for database inadequacies. The proposal to introduce an additional 10-fold factor for pesticides when exposure of infants and children is anticipated implies either age-related differences between species or differences within humans which exceed those present in adults. Alternatively, the extra factor could be related to deficiencies of current testing methods or concerns over irreversibility in developing organ systems. Available data do not provide a scientific rationale for the extra factor due to inadequacy of inter- and intraspecies uncertainty factors. Justification for the factor therefore must relate to the adequacy and sensitivity of current methods or concern about irreversible effects in the developing organism.

The role of developmental neurotoxicology studies in risk assessment

A number of questions have been raised about the use of the US Environmental Protection Agency's Developmental Neurotoxicity Testing Guideline (DNTG) in the hazard identification of chemicals. The applicability and sensitivity of animal tests in the DNTG relative to human developmental neurotoxicity have recently been questioned. In a workshop held in 1989, participants compared the effects of several known developmental neurotoxicants in humans and animal models and concluded that the DNTG would have detected known human developmental neurotoxicants. They also concluded that although procedural differences may differ in the testing of humans and animals, the neurobiologic functions (ie, autonomic, sensory, motor, and cognitive) affected by chemical exposure were similar. In cases where the DNTG has been compared with other measures of reproductive and developmental toxicity, the DNTG has been relatively sensitive and specific. To date, DNTGs have been required 12 times, for 9 pesticides and 3 solvents. The sensitivity of the measures in the DNTG relative to other measures of developmental and adult toxicity supports the continued use of the DNTG in risk assessment.

Addressing human variability in risk assessment--the robustness of the intraspecies uncertainty factor

Addressing human variability and sensitive subpopulations is one of the challenges of risk assessment and is an important aspect of the Food Quality Protection Act, the law passed in 1996 that regulates food use pesticides in the United States. The intraspecies uncertainty factor is intended to address differences in susceptibility within the human population. This paper examines the history and scientific basis for the intraspecies uncertainty factor. Our best source of knowledge about human variability in the response to chemicals comes from clinical trials of pharmaceuticals. This large body of data allows both qualitative and quantitative characterization of variability in pharmacokinetic and pharmacodynamic parameters in the general population and in subgroups such as children. The preponderance of evidence in the areas of pharmacodynamics and pharmacokinetics supports the routine use of an intraspecies uncertainty factor in the range of 1-10 as being protective of greater than 99% of the human population. The intraspecies uncertainty factor is highly protective of various subpopulations, including infants and children.

Duration of intake above the ADI/TDI in relation to toxicodynamics and toxicokinetics

The duration of intake necessary for the production of a toxic response depends on the mechanism of toxicity and the accumulation of the chemical to reach a toxic body load. There is a paucity of data on the cellular processes and changes associated with non-cancer effects detected in subchronic and chronic studies, and which are usually the basis for calculation of health-based exposure limits, such as the ADI. Data on the time course for the critical mechanistic process are normally unknown at the cellular level. Consequently, the duration of an excess intake in humans must be compared with the time course for the generation of the overall toxic effects in animals, which will include both toxicodynamic and toxicokinetic components. The extent of accumulation of a chemical during chronic intake is directly proportional to the half-life. The duration of intake at dosages above the ADI, necessary to produce body loads greater than those resulting from intake at the ADI, depends on the magnitude of the excess intake, the elimination half-life of the chemical, and the initial intake and body load. The effect on the body load of a short period of intake above the ADI is inversely proportional to the half-life of the chemical.

Incidence and severity in relation to magnitude of intake above the ADI or TDI: use of critical effect data

Noncancer effects are considered to show thresholds, such that no effect would be produced when the intake is below the threshold. Application of a 100-fold uncertainty factor to the no-observed-adverse-effect level (NOAEL) for the critical effect in animal studies provides an estimate of an intake for sensitive humans (the acceptable daily intake or ADI) without significant adverse health effects. The risks of intakes by humans above the ADI theoretically move the most sensitive subjects from negligible risk to possible risk. An increase in intake above the ADI would move the population distribution of internal dose toward the dose-response curve for sensitive subjects. The proportion of a population affected depends on the magnitude of the excess intake, the relationship of the NOAEL and the biological threshold in animals, and the coefficient of variation for the kinetic parameters (e.g., clearance and bioavailability) which determine the internal dose in humans. The severity of any effect in sensitive and high-intake individuals depends on the magnitude of the excess intake, the nature of the critical effect, and the slope of the dose-response relationship.

Toxicity testing and derivation of the ADI

The ADI is an estimate of the amount of a chemical that can be ingested daily over a lifetime without appreciable health risk. It is derived from No-Observed-Adverse-Effect-Levels (NOAELs) determined in a battery of toxicity tests in animals and augmented by human data where available. The toxicity tests are intended (with a few exceptions) to embrace all the circumstances of human exposure to dietary chemicals. Hence these tests include chronic studies, sometimes with prior exposure in utero, and reproduction tests covering the reproductive phase including effects on the parental animals and the offspring. The tests should also cover the rapid growth phase from weaning to maturing. The ADI is calculated from the lowest NOAEL in the most sensitive test and the most sensitive species, unless other data indicate otherwise, and if the reproductive, neonatal or rapid growth phases indicate particular periods of sensitivity this should drive the numerical derivation of the ADI. In calculating the ADI, the NOAEL is divided by appropriate arbitrary or data-derived safety or uncertainty factors. Care in selecting the pivotal test, the NOAEL and the safety factors should ensure that the ADI does apply to children (or other age groups). However, because of the higher food intake of children on a body weight basis, specific risk management measures may be needed to ensure that the ADI is not exceeded.