Procedures for calculating benchmark doses for health risk assessment

Meta-metabolomic responses of river biofilms to cobalt exposure and use of dose-response model trends as an indicator of effects

The response of the meta-metabolome is rarely used to characterize the effects of contaminants on a whole community. Here, the meta-metabolomic fingerprints of biofilms were examined after 1, 3 and 7 days of exposure to five concentrations of cobalt (from background concentration to 1 × 10-5 M) in aquatic microcosms. The untargeted metabolomic data were processed using the DRomics tool to build dose-response models and to calculate benchmark-doses. This approach made it possible to use 100% of the chemical signal instead of being limited to the very few annotated metabolites (7%). These benchmark-doses were further aggregated into an empirical cumulative density function. A trend analysis of the untargeted meta-metabolomic feature dose-response curves after 7 days of exposure suggested the presence of a concentration range inducing defense responses between 1.7 × 10-9 and 2.7 × 10-6 M, and of a concentration range inducing damage responses from 2.7 × 10-6 M and above. This distinction was in good agreement with changes in the other biological parameters studied (biomass and chlorophyll content). This study demonstrated that the molecular defense and damage responses can be related to contaminant concentrations and represents a promising approach for environmental risk assessment of metals.

Benchmark dose modelling in regulatory ecotoxicology, a potential tool in pest management

For several authorities, benchmark dose (BMD) methodology has become the recommended approach by which to derive reference values for risk assessment. However, in practice, the BMD approach is not standard use in risk assessment for pesticides where the no observed adverse effect level, lowest observed adverse effect level and effective dose (ED₅₀ or ED_x ) prevail. Regression-based BMD and the benchmark dose lower confidence limit (BMDL) have several advantages, such as utilizing more information from the generated data and being less dependent on tested dose levels. However, the BMD approach requires some degree of expert knowledge for defining an appropriate risk level for estimating the BMD and using more sophisticated statistical methods to calculate BMD and BMDL. The BMD approach is one way to move away from p value-based binary decision-making towards putting the weight on effect sizes. We review the advantages and disadvantages of focusing on the BMD approach for risk assessment of pesticides. Further, we discuss potential applications in efficacy trials for pest management purposes. © 2021 Society of Chemical Industry.

Use of benchmark dose models in risk assessment for occupational handlers of eight pesticides used in pome fruit production

The benchmark dose has been frequently recommended for the creation of points of departure for regulatory dose limits, but many regulations, including pesticide risk assessment and registration in the United States, continues to rely on NOAEL methods as the OECD toxicological standard methods recommend. This study used data from studies in support of pesticide registration for eight different compounds to build dose-response models and calculate benchmark doses and confidence limits. The results were compared to the NOAEL of the same study. A probabilistic estimate of dose was compared with all points of departure to demonstrate differences in the protective ability of each different selected limit. While neither the BMD/BMDL nor the NOAEL was consistently more protective, the advantage of using the BMD in quantifying the uncertainty of the point of departure is highlighted, and the feasibility of using current OECD-guideline studies for derivation of a BMD is demonstrated in these cases.

The association between dietary cadmium exposure and renal dysfunction - the benchmark dose estimation of reference levels: the ChinaCad study

The tolerable dietary intake of cadmium was recommended at provisional tolerable monthly intake of 25 μg kg^-1 body weight. However, several studies indicated that this tolerable level should be re-evaluated for sufficient health protection. In this study, we show the reference levels of dietary cadmium intake for renal dysfunction by using a benchmark dose (BMD) approach. A total of 790 subjects (302 men and 488 women) living in control and cadmium-polluted areas were included. The dietary cadmium intake was estimated by a food survey. Blood cadmium, urinary cadmium and renal function markers (microalbuminuria, N-acetyl-β-d-glucosaminidase [NAG] and its isoform B [NAGB], β₂ -microglobulin and retinol binding protein) in urine were measured. We calculated the 95% lower confidence bounds of BMD (BMDLs) of cumulative cadmium intake. In control and two polluted areas, the median cumulative cadmium intake was 0.5, 2.1 and 11.1 g. The odds ratio of the intermediate (1.0-3.0 g), second highest (3.0-11.0 g) and the highest cumulative cadmium intake (>11.0 g) compared with the lowest cumulative cadmium intake (<1.0 g) were 2.8 (95% CI: 1.4-5.8), 8.1 (95% CI: 3.8-17.2) and 11.4 (95% CI: 6.5-26.4) for urinary NAG and 6.6 (95% CI: 3.2-13.8), 14.8 (95% CI: 6.8-32.2) and 22.5 (95% CI: 10.7-47.5) for urinary NAGB. The BMDLs of cumulative cadmium intake were 1.1-1.2 g (benchmark response [BMR] = 5%) for urinary NAG, and were 0.7-0.9 g (BMR = 5%) for urinary NAGB, and were 1.3-1.4 g (BMR = 5%) for urinary β₂ -microglobulin. The BMDLs of cumulative cadmium intake in a Chinese population were lower than the critical standard previously reported. Further evaluations are needed for sufficient health protection.

Dietary Cadmium Intake and Its Effects on Kidneys

Cadmium (Cd) is a food-chain contaminant that has high rates of soil-to-plant transference. This phenomenon makes dietary Cd intake unavoidable. Although long-term Cd intake impacts many organ systems, the kidney has long been considered to be a critical target of its toxicity. This review addresses how measurements of Cd intake levels and its effects on kidneys have traditionally been made. These measurements underpin the derivation of our current toxicity threshold limit and tolerable intake levels for Cd. The metal transporters that mediate absorption of Cd in the gastrointestinal tract are summarized together with glomerular filtration of Cd and its sequestration by the kidneys. The contribution of age differences, gender, and smoking status to Cd accumulation in lungs, liver, and kidneys are highlighted. The basis for use of urinary Cd excretion to reflect body burden is discussed together with the use of urinary N-acetyl-β-d-glucosaminidase (NAG) and β2-microglobulin (β2-MG) levels to quantify its toxicity. The associations of Cd with the development of chronic kidney disease and hypertension, reduced weight gain, and zinc reabsorption are highlighted. In addition, the review addresses how urinary Cd threshold levels have been derived from human population data and their utility as a warning sign of impending kidney malfunction.

FDA-approved drugs that are spermatotoxic in animals and the utility of animal testing for human risk prediction

PURPOSE

This study reviews FDA-approved drugs that negatively impact spermatozoa in animals, as well as how these findings reflect on observations in human male gametes.

METHODS

The FDA drug warning labels included in the DailyMed database and the peer-reviewed literature in the PubMed database were searched for information to identify single-ingredient, FDA-approved prescription drugs with spermatotoxic effects.

RESULTS

A total of 235 unique, single-ingredient, FDA-approved drugs reported to be spermatotoxic in animals were identified in the drug labels. Forty-nine of these had documented negative effects on humans in either the drug label or literature, while 31 had no effect or a positive impact on human sperm. For the other 155 drugs that were spermatotoxic in animals, no human data was available.

CONCLUSION

The current animal models are not very effective for predicting human spermatotoxicity, and there is limited information available about the impact of many drugs on human spermatozoa. New approaches should be designed that more accurately reflect the findings in men, including more studies on human sperm in vitro and studies using other systems (ex vivo tissue culture, xenograft models, in silico studies, etc.). In addition, the present data is often incomplete or reported in a manner that prevents interpretation of their clinical relevance. Changes should be made to the requirements for pre-clinical testing, drug surveillance, and the warning labels of drugs to ensure that the potential risks to human fertility are clearly indicated.

A method for quantitative risk appraisal for pesticide risk assessments

Pesticide risk assessments are fraught with uncertainties that arise from the process of estimating exposure to and toxicity of chemicals. Regulatory agencies resolve those uncertainties in a health-protective (conservative) manner, typically acknowledging only inter- and intraspecies uncertainties quantitatively. Other uncertainties may be acknowledged qualitatively, but those safety factors (SF) are not enumerated. Quantitative risk appraisal may be used to enumerate the multiplicative SF generated by conservative assumptions regarding uncertainties. The magnitude of SF derived from decision points dealing with historically unquantified uncertainty may far exceed explicit SF used to gauge acceptable margins of exposure (MoE). Examination of the basis for some previously unenumerated SF may justify potential changes in regulatory practices and policies. Using past risk assessments of 3 pesticides (mevinphos, parathion, and methyl iodide) for which the California Department of Pesticide Regulation found unacceptable risk as examples, the previously unquantified SF ranged from 47 to 1 × 10⁶ for scenarios involving handlers, reentry workers, and bystanders.

Advances in Mycotoxin Research: Public Health Perspectives

Aflatoxins, ochratoxins, fumonisins, deoxynivalenol, and zearalenone are of significant public health concern as they can cause serious adverse effects in different organs including the liver, kidney, and immune system in humans. These toxic secondary metabolites are produced by filamentous fungi mainly in the genus Aspergillus, Penicillium, and Fusarium. It is challenging to control the formation of mycotoxins due to the worldwide occurrence of these fungi in food and the environment. In addition to raw agricultural commodities, mycotoxins tend to remain in finished food products as they may not be destroyed by conventional processing techniques. Hence, much of our concern is directed to chronic health effects through long-term exposure to one or multiple mycotoxins from contaminated foods. Ideally risk assessment requires a comprehensive data, including toxicological and epidemiological studies as well as surveillance and exposure assessment. Setting of regulatory limits for mycotoxins is considered necessary to protect human health from mycotoxin exposure. Although advances in analytical techniques provide basic yet critical tool in regulation as well as all aspects of scientific research, it has been acknowledged that different forms of mycotoxins such as analogs and conjugated mycotoxins may constitute a significant source of dietary exposure. Further studies should be warranted to correlate mycotoxin exposure and human health possibly via identification and validation of suitable biomarkers.

Estimation of the benchmark dose of urinary cadmium as the reference level for renal dysfunction: a large sample study in five cadmium polluted areas in China

BACKGROUND

Itai-itai disease primarily results from cadmium (Cd) exposure and is known as one of the four major pollution diseases in Japan. Cd pollution is more serious in several areas of China than in Japan. However, there is still a lack of information regarding the threshold level of Cd exposure for the adverse health effects in the general Chinese population. This study aims to evaluate the reference value of urinary Cd (UCd) for renal dysfunction in a Chinese population as the benchmark dose lower confidence limit (BMDL) based on a large sample survey.

METHODS

A total of 6103 participants who lived in five Cd polluted areas of China participated in this study. We analyzed UCd levels as a biomarker of exposure and urinary β2-microglobulin (Uβ2-MG) levels as a renal tubular effect biomarker. The BMD studies were performed using BMD software. The benchmark response (BMR) was defined as a 10% additional risk above the background.

RESULTS

There was a positive correlation between the UCd levels and the prevalence of Uβ2-MG. The BMD of UCd for Uβ2-MG was estimated for each province. The findings showed that the BMD levels were related to the participants' geographic region, which may be partially due to the large differences in Cd exposure level, ethnic group, lifestyle and diet of the sample population in these study areas. The reference level of UCd for the renal effects was further evaluated by combining the five sets of data from all 6103 subjects. The overall BMDLs of UCd for Uβ2-MG with an excess risk of 10% were 2.00 μg/g creatinine (μg/g cr) in males and 1.69 μg/g cr in females, which were significantly lower than the World Health Organization (WHO) threshold level of 5 μg/g cr for Cd-related renal effects.

CONCLUSIONS

The selection of the sample population and geographic region affected the BMDL evaluation. Based on the findings of this survey of a large sample population, the UCd BMDLs for Uβ2-MG in males with BMRs at 10% were 2.00 μg/g cr. The BMD was slightly lower in females, which indicated that females may be relatively more sensitive to Cd exposure than males.

A simple procedure for estimating pseudo risk ratios from exposure to non-carcinogenic chemical mixtures

Non-cancer risk assessment traditionally assumes a threshold of effect, below which there is a negligible risk of an adverse effect. The Agency for Toxic Substances and Disease Registry derives health-based guidance values known as Minimal Risk Levels (MRLs) as estimates of the toxicity threshold for non-carcinogens. Although the definition of an MRL, as well as EPA reference dose values (RfD and RfC), is a level that corresponds to "negligible risk," they represent daily exposure doses or concentrations, not risks. We present a new approach to calculate the risk at exposure to specific doses for chemical mixtures, the assumption in this approach is to assign de minimis risk at the MRL. The assigned risk enables the estimation of parameters in an exponential model, providing a complete dose-response curve for each compound from the chosen point of departure to zero. We estimated parameters for 27 chemicals. The value of k, which determines the shape of the dose-response curve, was moderately insensitive to the choice of the risk at the MRL. The approach presented here allows for the calculation of a risk from a single substance or the combined risk from multiple chemical exposures in a community. The methodology is applicable from point of departure data derived from quantal data, such as data from benchmark dose analyses or from data that can be transformed into probabilities, such as lowest-observed-adverse-effect level. The individual risks are used to calculate risk ratios that can facilitate comparison and cost-benefit analyses of environmental contamination control strategies.

Benchmark dose estimation of cadmium reference level for hypertension in a Chinese population

Cadmium exposure can cause high blood pressure or hypertension. Benchmark dose has been used to estimate the reference point of cadmium for kidney and bone damage. In this study, we observed the association of blood pressure and cadmium in blood (BCd) and evaluated the reference level of cadmium for hypertension using benchmark dose (BMD) approach. A total of 441 subjects were included in this study. Blood samples were collected from each individual for BCd determination. Blood pressure was measured by electronic sphygmomanometer. BMD and BMDL were calculated using BMD software corresponding to additional risk of 10%. The systolic blood pressure (SBP), diastolic blood pressure (DBP) and prevalence of hypertension increased with the increasing of BCd, especially for SBP (χ(2)=3.9, p=0.047 in men; χ(2)=4.3, p=0.037 in women). With a benchmark response of 10%, the BMDL10 for hypertension (high SBP) was 0.95μg/L and 1.02μg/L for women and men, respectively; the BMDL10 for hypertension (high DBP) was 1.8μg/L and 1.66μg/L for women and men, respectively. Our data evidenced that BCd was associated with elevation in blood pressure and hypertension, especially for women. The reference level of cadmium for hypertension with high SBP was lower than that of high DBP.

On determining the BMD from multiple outcomes in developmental toxicity studies when one outcome is intentionally missing

Public health concerns over the occurrence of developmental abnormalities that can occur as a result of prenatal exposure to drugs, chemicals, and other environmental factors has led to a number of developmental toxicity studies and the use of the benchmark dose (BMD) for risk assessment. To characterize risk from multiple sources, more recent analytic methods involve a joint modeling approach, accounting for multiple dichotomous and continuous outcomes. For some continuous outcomes, evaluating all subjects may not be feasible, and only a subset may be evaluated due to limited resources. The subset can be selected according to a prespecified probability model and the unobserved data can be viewed as intentionally missing in the sense that subset selection results in missingness that is experimentally planned. We describe a subset selection model that allows for sampling pups with malformations and healthy pups at different rates, and includes the well-known simple random sample (SRS) as a special case. We were interested in understanding how sampling rates that are selected beforehand influence the precision of the BMD. Using simulations we show how improvements over the SRS can be obtained by oversampling malformations, and how some sampling rates can yield precision that is substantially worse than the SRS. We also illustrate the potential for cost saving with oversampling. Simulations are based on a joint mixed effects model, and to account for subset selection, use of case weights to obtain valid dose-response estimates.

Estimation of benchmark dose for micronucleus occurrence in Chinese vinyl chloride-exposed workers

In this study, we estimated the possibility of using benchmark dose (BMD) to assess the dose-response relationship between vinyl chloride monomer (VCM) exposure and chromosome damage. A group of 317 workers occupationally exposed to vinyl chloride monomer and 166 normal, unexposed control in Shandong Province northern China were examined for chromosomal damage in peripheral blood lymphocytes (PBL) using the cytokinesis-blocked micronucleus (CB-MN) assay of DNA damage. The exposed group (3.47 ± 2.65)‰ showed higher micronucleus frequency than the control (1.60 ± 1.30)‰ (P < 0.01). Occupational exposure level based on micronucleus occurrence in all individuals was analyzed with benchmark dose (BMD) methods. The benchmark dose lower limit of a one-sided 95% confidence interval (BMDL) for 10% excess risk was also determined. Results showed a dose-response relationship between cumulative exposure and MN frequency, and a BMDL of 0.54 mg/m3 and 0.23 mg/m3 for males and females, respectively. Female workers were more susceptible to MN damage than male workers.

Estimation of a safe level for occupational exposure to vinyl chloride using a benchmark dose method in central China

OBJECTIVES

The aim of this study was to estimate a benchmark dose (BMD) for chromosome damage induced by vinyl chloride monomer (VCM) in VCM-exposed workers in central China and validate the published results in Shanghai.

METHODS

VCM-exposed workers who had been exposed to VCM for at least one year (n=463) and matched subjects not exposed to VCM or other toxins (n=273) were asked to participate in this study. Micronucleus (MN) frequency based on the cytokinesis-block micronucleus assay (CBMN) was used as a biomarker for chromosome damage induced by VCM exposure.

RESULTS

The MN frequency in the VCM-exposed workers was significantly higher than that in the control group, and multivariate Poisson regression suggested that gender, smoking status and VCM exposure were the significant factors influencing the risk of increased MN frequency. When subjects were further stratified according to gender and smoking status, the results showed that female VCM-exposed workers were more susceptible than the males to the risk of increased MN frequency. The MN frequency of smokers was significantly higher than that of nonsmokers in the control group. Our study also suggested that there was a strong dose-response relationship between VCM CED and the increased risk of MN frequency in the total group, males and females. The BMDL(10) was found to be 630.6, 670.2 and 273.7 mg-year for all VCM-exposed workers, males and females, respectively.

CONCLUSIONS

These results invite further scrutiny of the current VCM occupational exposure limits and warrant further study of the risk of VCM genotoxicity and carcinogenicity.

A signal-to-noise crossover dose as the point of departure for health risk assessment

BACKGROUND

The U.S. National Toxicology Program (NTP) cancer bioassay database provides an opportunity to compare both existing and new approaches to determining points of departure (PoDs) for establishing reference doses (RfDs).

OBJECTIVES

The aims of this study were a) to investigate the risk associated with the traditional PoD used in human health risk assessment [the no observed adverse effect level (NOAEL)]; b) to present a new approach based on the signal-to-noise crossover dose (SNCD); and c) to compare the SNCD and SNCD-based RfD with PoDs and RfDs based on the NOAEL and benchmark dose (BMD) approaches.

METHODS

The complete NTP database was used as the basis for these analyses, which were performed using the Hill model. We determined NOAELs and estimated corresponding extra risks. Lower 95% confidence bounds on the BMD (BMDLs) corresponding to extra risks of 1%, 5%, and 10% (BMDL01, BMDL05, and BMDL10, respectively) were also estimated. We introduce the SNCD as a new PoD, defined as the dose where the additional risk is equal to the "background noise" (the difference between the upper and lower bounds of the two-sided 90% confidence interval on absolute risk) or a specified fraction thereof.

RESULTS

The median risk at the NOAEL was approximately 10%, and the default uncertainty factor (UF = 100) was considered most applicable to the BMDL10. Therefore, we chose a target risk of 1/1,000 (0.1/100) to derive an SNCD-based RfD by linear extrapolation. At the median, this approach provided the same RfD as the BMDL10 divided by the default UF.

CONCLUSIONS

Under a standard BMD approach, the BMDL10 is considered to be the most appropriate PoD. The SNCD approach, which is based on the lowest dose at which the signal can be reliably detected, warrants further development as a PoD for human health risk assessment.

Genotoxicity in vinyl chloride-exposed workers and its implication for occupational exposure limit

BACKGROUND

Vinyl chloride monomer (VCM) is a colorless gas under room temperature and has been mostly used to produce polyvinyl chloride (PVC) since the 1970s. It is classified by the International Agency of Research on Cancer (IARC) as a known human carcinogen (Group 1). In this study, genetic damage in VCM workers was evaluated in relation to their occupational cumulative exposure to VCM.

METHODS

Cytokinesis-block micronucleus assay was conducted in 229 VCM workers and 138 controls to detect chromosome damage in peripheral blood lymphocytes. The cumulative exposure dose (CED) of VCM was calculated based on the job type and duration of each worker and the workplace VCM concentration. Dose-response relationships between VCM CED and micronucleus frequency or chromosomal damage were evaluated, and benchmark doses (BMDs) estimated.

RESULTS

Dose-response relationships between VCM CED and chromosomal damage were obtained. The 95% lower confidence bound of BMD of VCM CED was 2.86 mg/m(3) -year for both genders combined, leading to an estimated exposure limit of 0.072 mg/m(3) assuming a work life of 40 years.

CONCLUSIONS

VCM exposure may induce chromosomal damage at occupational exposure levels below the Chinese national occupational health standard. Further research is needed to better understand micronuclei as biomarker of VCM genotoxicity. Better dose-response assessment and BMD estimation are desirable in order to improve the quantification of occupational exposure limits for VCM with respect to non-cancer risk.

In vitro neurotoxicity data in human risk assessment of polybrominated diphenyl ethers (PBDEs): overview and perspectives

Polybrominated diphenyl ethers (PBDEs) are flame retardants routinely detected in samples of cord blood and breast milk. Concerns have been raised with regard to the toxicity of both pre- and postnatal exposures towards the developing nervous system. Although there is an increasing body of literature on the disruption of brain cell functions by certain PBDE congeners in vitro, some challenges have yet to be tackled to enable the translation of in vitro findings into their in vivo counterparts. In this paper, we review findings on the PBDE neurotoxicity in human cells and discuss the research gaps to be addressed. Moreover, we propose a scheme for the incorporation of in vitro data in human risk assessment, namely through (i) the determination of in vitro cell benchmark levels; (ii) the consideration of uncertainties in establishing equivalency between the in vitro and the in vivo tissue benchmark levels (e.g., chronic vs. acute exposure, interactions with other chemicals); and (iii) relating tissue benchmark levels to surrogate levels of internal exposure. Alongside the assessment of brain dosimetry following exposure to PBDEs, in vitro neurotoxicity data provide a unique opportunity to evaluate the risks of prenatal and early life exposures on children neurodevelopment.

Association of lifetime cadmium intake or drinking Jinzu River water with the occurrence of renal tubular dysfunction

This study was performed to clarify whether lifetime cadmium (Cd) intake (LCd) calculated from consumption of Cd-polluted rice and other foods, or drinking and/or cooking with Jinzu River water is associated with the occurrence of renal tubular dysfunction and to estimate the threshold level of LCd using a benchmark dose procedure. From participants in the 1967 and 1968 health examinations, the 2607 subjects (1430 men and 1177 women) who were aged 50 years or more, and who had either resided in the current hamlet since birth or moved there from a nonpolluted area and resided for a total of 30 years or longer were chosen as the target population. Based on a multiple logistic regression analysis performed for abnormal urinary findings (proteinuria, glucosuria, or proteinuria with glucosuria) using LCd as the explanatory variable with or without consideration of use of the river water for drinking and/or cooking, it was inferred that both factors of LCd and use of the river water influenced the occurrence of renal tubular dysfunction, with LCd appearing to exert a greater impact compared to drinking/cooking with the river water. Moreover, the threshold values of LCd for proteinuria with glucosuria were estimated to be 2.91 g in men and 1.79 g in women, without adjustment for use of the river water. After adjustment for its use, the values of proteinuria were calculated to be 0.93 g in women. When assuming user/nonuser status of the river water, the threshold level was calculated as 0.75/0.93 g in women. The difference of 0.18 g was interpreted as representing the influence of drinking/cooking with the river water.

The French approach to deriving toxicity reference values: an example using reprotoxic effects

Following the French health and environment action plan, the French Agency for Environmental and Occupational Health and Safety set up a workgroup to standardise a method of deriving toxicity reference values (TRVs). Over the last few decades, there has been increasing concern about the effect of exposure to chemicals on reproductive function, leading the group to take an interest in reprotoxic effects. This article presents the recommendations of the workgroup regarding specific reprotoxic effects. Abnormal development of foetuses and infants, together with impairment of reproduction were considered to be critical effects. Where critical windows of exposure were concerned, quantitative analysis suggested the need for several types of toxicity reference value, as a function of exposure duration: reprotoxic effects may result from acute or chronic exposure at any time of life, whilst developmental effects may occur after exposure during the pregnancy or during the lactation period. The choice of a critical study is based on epidemiological or toxicological quality criteria. The working group recommends the use of the benchmark dose approach in estimating the critical dose. Finally, the working group considered the application of uncertainty factors typically used to take into account the variability between animal and human, between different individuals, and the availability of the data.

Proposal for a revised Reference Concentration (RfC) for manganese based on recent epidemiological studies

In 1993, based on observations of subclinical neurological effects in workers, the United States Environmental Protection Agency (US EPA) published a Reference Concentration (RfC) of 0.05 microg/m(3) for manganese (Mn). The geometric mean exposure concentration, 150 microg/m(3) respirable Mn, was considered the lowest observable adverse effect level (LOAEL), and uncertainty factors (UFs) were applied to account for sensitive populations, database limitations, a LOAEL, subchronic exposure, and potential differences in toxicity of different forms of Mn. Based on a review of more recent literature, we propose two alternate Mn RfCs. Of 12 more recent occupational studies of eight cohorts with chronic exposure durations, examining subclinical neurobehavioral effects, predominantly on the motor system, three were considered appropriate for development of an RfC. All three studies yielded no observable adverse effect levels (NOAELs) of approximately 60 microg/m(3) respirable Mn. Converting the occupational NOAEL to a human equivalent concentration (HEC) of 21microg/m(3) (for continuous exposure) and applying a UF of 10 to account for intraspecies variability yielded an RfC of 2microg/m(3). We also derived a similar RfC (7 microg/m(3)) using an Mn benchmark dose (BMD) as the point of departure. Overall confidence in both RfCs is medium.

Estimation of benchmark rice cadmium doses as threshold values for abnormal urinary findings with adjustment for consumption of Jinzu River water

We performed this study to determine whether both eating cadmium (Cd)-polluted rice and drinking and/or cooking with Jinzu River water are associated with renal tubular dysfunction. A multiple logistic regression analysis of retrospective data indicated that both factors may contribute to this condition. Estimated threshold values of rice Cd concentration in men were 0.13-0.27 ppm and 0.09-0.18 ppm in women, without adjustment for use of Jinzu River water. The additional influence of drinking and/or cooking with Jinzu River water was estimated to be about 0.008 ppm.

Interspecies extrapolation in environmental exposure standard setting: A Bayesian synthesis approach

Currently the extrapolation of evidence from studies of non-human species to the setting of environmental exposure standards for humans includes the imposition of a variety of uncertainty factors reflecting unknown aspects of the procedure, including the relevance of evidence from one species to impacts in another. This paper develops and explores more flexible modelling of aspects of this extrapolation, using models proposed by DuMouchel [DuMouchel, W.H., Harris, J.E., 1983. Bayes methods for combining the results of cancer studies in humans and other species (with comment). J. Am. Statist. Assoc. 78, 293-308.] The approaches are based on Bayesian meta-analysis methods involving explicit modelling of relevance in the prior distributions, estimated using Markov chain Monte Carlo (MCMC) methods. The methods are applied to evidence relating chlorinated by-products exposure to adverse reproductive health effects. The relative merits of various approaches are discussed, and developments and next steps are outlined.

Assessing and managing risks arising from exposure to endocrine-active chemicals

Managing risks to human health and the environment produced by endocrine-active chemicals (EAC) is dependent on sound principles of risk assessment and risk management, which need to be adapted to address the uncertainties in the state of the science of EAC. Quantifying EAC hazard identification, mechanisms of action, and dose-response curves is complicated by a range of chemical structure/toxicology classes, receptors and receptor subtypes, and nonlinear dose-response curves with low-dose effects. Advances in risk science including toxicogenomics and quantitative structure-activity relationships (QSAR) along with a return to the biological process of hormesis are proposed to complement existing risk assessment strategies, including that of the Endocrine Disruptor Screening and Testing Advisory Committee (EDSTAC 1998). EAC represents a policy issue that has captured the public's fears and concerns about environmental health. This overview describes the process of EAC risk assessment and risk management in the context of traditional risk management frameworks, with emphasis on the National Research Council Framework (1983), taking into consideration the strategies for EAC management in Canada, the United States, and the European Union.

Statistical power considerations show the endocrine disruptor low-dose issue in a new light

BACKGROUND

The endocrine disruptor field has been vexed by difficulties in reproducing various claims of effects at unusually low doses. In previous analyses, variations in control responses from experiment to experiment and problems with observing effects in positive controls have been identified as possible explanations of the resulting impasse.

OBJECTIVE

In this article, we argue that both of these viewpoints fail to take sufficient account of the problems that exist in estimating low effects and low-effect doses. We have carried out post hoc power analyses on selected published data to illustrate that claims of low-dose effects (or their absence) are often compromised by insufficient statistical power of the chosen experimental design.

CONCLUSIONS

We demonstrate that low-dose estimates such as the no observed adverse effect levels derived from statistical hypothesis-testing procedures are dependent on the specific experimental conditions used for testing. Thus, below the statistical detection limit of the experiment, the presence of effects can neither be proven nor ruled out. Common practice is to attempt to establish "doses without effect." However, low-dose estimations in the endocrine-disruptor field could be improved if decisions regarding the toxicologic effect size of relevance formed the starting point of testing procedures. Statistical power considerations could then reveal the resources necessary to demonstrate effect magnitudes of concern.

Statistics for risk assessment of chemical carcinogens

Risk assessment is a scientific process of evaluation of potential health risks of chemical exposures to humans from available information. It involves analysis of the relationship between exposure and health related outcomes to derive an allowable exposure level. Because of lack of human exposure data, the major source of information for studying potential health effects of chemicals on humans is generally obtained from animal dose response experiments. Animal data are often evaluated in two aspects via statistical analysis: qualitative testing and quantitative estimation. The qualitative testing is to determine if the chemical causes an adverse health effect, i.e., if there is a statistically significant difference between treated and control animals. Quantitative estimation involves fitting a dose-response model to derive an allowable exposure level for humans. This paper reviews statistical principles and procedures for qualitative and quantitative approaches to human risk assessment.

Estimation of benchmark dose for renal dysfunction in a cadmium non-polluted area in Japan

Previously, the association between urinary cadmium (Cd) concentration and indicators of renal dysfunction, including beta(2)-microglobulin (beta(2)-MG), total protein and N-acetyl-beta-D-glucosaminidase (NAG) were investigated in 1270 inhabitants > or = 50 years of age (547 men, 723 women) in a Cd non-polluted area in Japan and showed that a dose-response relationship existed between renal effects and Cd exposure in the general environment without any known Cd pollution. However, the threshold levels of urinary Cd could not be estimated at that time. In the present study, the threshold levels of urinary Cd were estimated as the benchmark dose low (BMDL) using the benchmark dose (BMD) approach. Urinary Cd excretion was divided into 6-7 categories, and an abnormality rate was calculated for each. Cut-off values for urinary substances were defined as corresponding to the 84% upper limit values, which were calculated from 2034 persons who had been living in the non-polluted areas and did not smoke. Then the BMD and BMDL were calculated using a log-logistic model. The values of BMD and BMDL for all urinary substances could be calculated. The BMDL for the 84% cut-off value of beta(2)-MG, setting an abnormal value at 5%, was 2.0 microg g(-1) creatinine (cr) in men and 1.6 microg g(-1) cr in women. In conclusion, the present study demonstrated that the threshold level of urinary Cd could be estimated in people living in the general environment without any known Cd-pollution in Japan, and the value was inferred to be almost the same as that in Belgium and Sweden.

Benchmark dose approach for renal dysfunction in workers exposed to lead

Benchmark dose (BMD) and the lower confidence limit on the benchmark dose (BMDL) of blood lead were estimated to explore the biologic exposure limits for renal dysfunction caused by lead. One hundred thirty-five workers from one storage battery plant were selected as lead exposure group while 143 mechanics as the control. The relationship between the blood lead concentration and the urinary excretion of total protein (TP), beta2-microglobulin (beta2-MG), and N-acetyl-beta-D-glucosaminidase (NAG) was studied. The quantal linear logistic regression model (BMDS Version 1.3.1) was used to calculate BMD and BMDL of blood lead. The results showed that the levels of NAG, beta2-MG, and TP in lead-exposed workers were higher than those of control group and elevated along with rising length of employment. The levels of three indices for renal dysfunction increased with the elevated blood lead. The BMD and BMDL of blood lead for renal dysfunction were from 299.4 to 588.7 microg/L and from 253.4 to 402.3 microg/L, respectively. The BMDL of blood lead was ranged from high to low as TP, beta2-MG, and NAG. It is suggested that the urinary NAG activity could be a sensitive and early biomarker of renal tubular dysfunction induced by lead. When assessing renal function in workers occupationally exposed to lead, a blood lead level of 250 microg/L could serve as a warning signal.

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.

Bootstrap estimation of benchmark doses and confidence limits with clustered quantal data

The benchmark dose (BMD) is an exposure level that would induce a small risk increase (BMR level) above the background. The BMD approach to deriving a reference dose for risk assessment of noncancer effects is advantageous in that the estimate of BMD is not restricted to experimental doses and utilizes most available dose-response information. To quantify statistical uncertainty of a BMD estimate, we often calculate and report its lower confidence limit (i.e., BMDL), and may even consider it as a more conservative alternative to BMD itself. Computation of BMDL may involve normal confidence limits to BMD in conjunction with the delta method. Therefore, factors, such as small sample size and nonlinearity in model parameters, can affect the performance of the delta method BMDL, and alternative methods are useful. In this article, we propose a bootstrap method to estimate BMDL utilizing a scheme that consists of a resampling of residuals after model fitting and a one-step formula for parameter estimation. We illustrate the method with clustered binary data from developmental toxicity experiments. Our analysis shows that with moderately elevated dose-response data, the distribution of BMD estimator tends to be left-skewed and bootstrap BMDL s are smaller than the delta method BMDL s on average, hence quantifying risk more conservatively. Statistically, the bootstrap BMDL quantifies the uncertainty of the true BMD more honestly than the delta method BMDL as its coverage probability is closer to the nominal level than that of delta method BMDL. We find that BMD and BMDL estimates are generally insensitive to model choices provided that the models fit the data comparably well near the region of BMD. Our analysis also suggests that, in the presence of a significant and moderately strong dose-response relationship, the developmental toxicity experiments under the standard protocol support dose-response assessment at 5% BMR for BMD and 95% confidence level for BMDL.

Copper and human health: biochemistry, genetics, and strategies for modeling dose-response relationships

Copper (Cu) and its alloys are used extensively in domestic and industrial applications. Cu is also an essential element in mammalian nutrition. Since both copper deficiency and copper excess produce adverse health effects, the dose-response curve is U-shaped, although the precise form has not yet been well characterized. Many animal and human studies were conducted on copper to provide a rich database from which data suitable for modeling the dose-response relationship for copper may be extracted. Possible dose-response modeling strategies are considered in this review, including those based on the benchmark dose and categorical regression. The usefulness of biologically based dose-response modeling techniques in understanding copper toxicity was difficult to assess at this time since the mechanisms underlying copper-induced toxicity have yet to be fully elucidated. A dose-response modeling strategy for copper toxicity was proposed associated with both deficiency and excess. This modeling strategy was applied to multiple studies of copper-induced toxicity, standardized with respect to severity of adverse health outcomes and selected on the basis of criteria reflecting the quality and relevance of individual studies. The use of a comprehensive database on copper-induced toxicity is essential for dose-response modeling since there is insufficient information in any single study to adequately characterize copper dose-response relationships. The dose-response modeling strategy envisioned here is designed to determine whether the existing toxicity data for copper excess or deficiency may be effectively utilized in defining the limits of the homeostatic range in humans and other species. By considering alternative techniques for determining a point of departure and low-dose extrapolation (including categorical regression, the benchmark dose, and identification of observed no-effect levels) this strategy will identify which techniques are most suitable for this purpose. This analysis also serves to identify areas in which additional data are needed to better define the characteristics of dose-response relationships for copper-induced toxicity in relation to excess or deficiency.

SIMULTANEOUS CONFIDENCE BOUNDS FOR LOW-DOSE RISK ASSESSMENT WITH NONQUANTAL DATA

We study the use of simultaneous confidence bounds for making low-dose inferences in quantitative risk analysis. Confidence limits are constructed for outcomes measured on a continuous scale, assuming a simple linear model for the observed response. From the simultaneous confidence bounds, simultaneous lower limits on the benchmark dose associated with a particular risk are also constructed.

A benchmark dose analysis for sodium monofluoroacetate (1080) using dichotomous toxicity data

The use of a benchmark dose (BMD) as an alternative to a no-observed-adverse-effect-level (NOAEL) approach was investigated as a means to improve current risk assessment values of sodium monofluoroacetate (1080). The feasibility of implementing the two approaches was investigated for three critical toxicological end points, namely cardiomyopathy, testicular toxicity and teratogenic effects identified from the few available critical studies. The BMD provides better representation of the dose-response relationship, offering an advantage over the current NOAEL approach. The calculated BMDs and lower-bound confidence limits (BMDLs) for the three end points were estimated using the Weibull, probit and quantal linear models for each end point. All models passed the chi2 test statistics (p > or = 0.1) for all three toxicity endpoints tested. A benchmark response (BMR) of 10% (extra risk) was chosen and the Akaike's information criterion (AIC) was used in selecting the appropriate model. The BMDL estimates derived were found to be generally slightly higher but comparable to the NOAEL for those same endpoints. The BMD(10) and BMDL(10) for cardiomyopathy and testicular effects were 0.21 mgkg(-1) bw and 0.10 mgkg(-1) bw, respectively. These values are proposed for use in the eventual determination of the tolerable daily intake (TDI) for 1080.

The use of myocardial and testicular end points as a basis for estimating a proposed tolerable daily intake for sodium monofluoroacetate (1080)

This paper presents the development of a tolerable daily intake (TDI) for sodium monofluoroacetate (1080) using the quantal myocardial and testicular toxicity end points derived from the traditional NOAEL and newer benchmark dose (BMD) methods. 1080 is a highly toxic vertebrate pesticide that has been proven to be effective in controlling possums and other pests. By convention, the TDIs are derived using the traditional no-observed-adverse-effect-level (NOAEL) and applying appropriate default uncertainty factors (UF). In addition to the default UF, a statistically derived UF was also employed in deriving the TDI. The TDIs derived from the NOAEL and BMD approach, 0.075 and 0.10 mg/kg bw/day, respectively, were compared. The resulting TDI estimates using the BMDL, a statistical lower confidence bound on the BMD, were generally consistently slightly higher than those derived using the NOAEL approach. Based on the best fit of modelled dose-response data, a TDI of 0.03 micro g/kg bw/day is proposed for human health risk assessment of 1080.

Estimation of benchmark doses for urinary cadmium based on beta2-microglobulin excretion in cadmium-polluted regions of the Kakehashi River basin, Japan

A benchmark dose low (BMDL) is used as a replacement for the no observed adverse effect level. The threshold levels of urinary cadmium (Cd) as BMDL were estimated using data from the Kakehashi River basin. The target population (>or=50 years) comprised 3178 and 294 participants inhabiting Cd-polluted and non-polluted areas, respectively. Cut-off values for beta2-MG-uria were defined as the 84 and 95% upper limit values calculated from control subjects, and 1000 microg/l or microg/g cr of beta2-MG. Using these cut-off values, the BMDL at which the excess risk is 0.05 was determined to be 2.9 - 4.0 microg/g cr (males) and 1.5 - 3.6 microg/g cr (females). The present study demonstrated that a BMD approach is useful to estimate the threshold level of urinary Cd in Cd-exposed subjects and people living in general environment without any known Cd-pollution since a BMD approach does not need abnormality rates of urinary findings in the controls.

Estimation of benchmark dose as the threshold levels of urinary cadmium, based on excretion of total protein, beta2-microglobulin, and N-acetyl-beta-D-glucosaminidase in cadmium nonpolluted regions in Japan

Previously, we investigated the association between urinary cadmium (Cd) concentration and indicators of renal dysfunction, including total protein, beta2-microglobulin (beta2-MG), and N-acetyl-beta-D-glucosaminidase (NAG). In 2778 inhabitants 50 years of age (1114 men, 1664 women) in three different Cd nonpolluted areas in Japan, we showed that a dose-response relationship existed between renal effects and Cd exposure in the general environment without any known Cd pollution. However, we could not estimate the threshold levels of urinary Cd at that time. In the present study, we estimated the threshold levels of urinary Cd as the benchmark dose low (BMDL) using the benchmark dose (BMD) approach. Urinary Cd excretion was divided into 10 categories, and an abnormality rate was calculated for each. Cut-off values for urinary substances were defined as corresponding to the 84% and 95% upper limit values of the target population who have not smoked. Then we calculated the BMD and BMDL using a log-logistic model. The values of BMD and BMDL for all urinary substances could be calculated. The BMDL for the 84% cut-off value of beta2-MG, setting an abnormal value at 5%, was 2.4 microg/g creatinine (cr) in men and 3.3 microg/g cr in women. In conclusion, the present study demonstrated that the threshold level of urinary Cd could be estimated in people living in the general environment without any known Cd-pollution in Japan, and the value was inferred to be almost the same as that in Belgium, Sweden, and China.

Benchmark concentrations for methyl mercury obtained from the 9-year follow-up of the Seychelles Child Development Study

Methyl mercury (MeHg) is highly toxic to the developing nervous system. Human exposure is mainly from fish consumption since small amounts are present in all fish. Findings of developmental neurotoxicity following high-level prenatal exposure to MeHg raised the question of whether children whose mothers consumed fish contaminated with background levels during pregnancy are at an increased risk of impaired neurological function. Benchmark doses determined from studies in New Zealand, and the Faroese and Seychelles Islands indicate that a level of 4-25 parts per million (ppm) measured in maternal hair may carry a risk to the infant. However, there are numerous sources of uncertainty that could affect the derivation of benchmark doses, and it is crucial to continue to investigate the most appropriate derivation of safe consumption levels. Earlier, we published the findings from benchmark analyses applied to the data collected on the Seychelles main cohort at the 66-month follow-up period. Here, we expand on the main cohort analyses by determining the benchmark doses (BMD) of MeHg level in maternal hair based on 643 Seychellois children for whom 26 different neurobehavioral endpoints were measured at 9 years of age. Dose-response models applied to these continuous endpoints incorporated a variety of covariates and included the k-power model, the Weibull model, and the logistic model. The average 95% lower confidence limit of the BMD (BMDL) across all 26 endpoints varied from 20.1 ppm (range=17.2-22.5) for the logistic model to 20.4 ppm (range=17.9-23.0) for the k-power model. These estimates are somewhat lower than those obtained after 66 months of follow-up. The Seychelles Child Development Study continues to provide a firm scientific basis for the derivation of safe levels of MeHg consumption.

Identification of a Critical Dose Level for Risk Assessment: Developments in Benchmark Dose Analysis of Continuous Endpoints

The benchmark dose (BMD) method has been recommended to replace the no-observed-adverse-effect-level (NOAEL) approach in health risk assessment of chemical substances. In the present article, developments in BMD analysis from continuous experimental data are proposed. The suggested approach defines the BMD as the dose at which the slope of the S-shaped dose-response relationship changes the most in the low-dose region. This dose resides in a region where the sensitivity to chemical exposure may start to change noticeably. It is shown that the response (defined as a percent change relative to the magnitude, or size, of response) corresponding to the dose where the slope changes the most depends on the geometrical shape of the dose-response curve; the response becomes lower as the curve becomes more asymmetrical and threshold-like in the low-dose region. Given a symmetrical case, described by the Hill function, the response associated with the critical dose level becomes 21% (defined as a percent change relative to the magnitude, or size, of response). According to a limiting case of asymmetry and threshold-like characteristics, reflected by a Gompertz curve, the response corresponding to the dose of interest becomes as low as 7.3% (defined as a percent change relative to the magnitude, or size, of response). Use of a response in the range of 5-10% when estimating the BMD conservatively accounts for uncertainties associated with the proposed strategy, and may be appropriate in a risk assessment point of view. The present investigation also indicated that a BMD defined according to the suggested procedure may be estimated more precisely relative to BMDs defined under other approaches for continuous data.

The role of the benchmark dose in a regulatory context

The use of no observed adverse effect levels (NOAELs) as a way of interpreting toxicology studies carries a number of problems, and the benchmark dose (BMD), or its lower confidence limit have been proposed as potential replacements. In practice, the theoretical advantages of the BMD approach are often outweighed by the practical disadvantages posed in a regulatory context. Attempts to seek consensus for the routine use of BMD methodology tend to involve diluting its potential advantages as much as they address the disadvantages, resulting in a relatively complex interpolation tool that delivers little more than the NOAEL. It is time to recognise that the BMD will never entirely replace the NOAEL. The two methods can have complementary roles. The NOAEL is well suited as a routine simple summary of effects in toxicology studies, whilst the BMD can be a higher tier approach for the interpretation of the most critical studies in a regulatory data package.

Acute inhalative exposure assessment: Derivation of guideline levels with special regard to sensitive subpopulations and time scaling

Risk assessment for acute airborne exposure to volatile organic compounds (VOCs), including exposure to chemical warfare agents, requires consideration of local and systemic effects at high concentrations. The operating procedure developed by the US Acute Exposure Guideline Level (AEGL) committee has gained special attention, in part because of the international collaboration in the project. The procedure defines three levels (AEGL-1: discomfort; AEGL-2: irreversible or other serious, long-lasting adverse effects; AEGL-3: life-threatening effects or death) for different exposure times (10 and 30 min, and 1, 4 and 8 h). In this article, the methodology for deriving AEGL values is reported. Extending the areas covered by the existing AEGL methodology, sensitive subpopulations are dealt with in more detail. Sensitive persons are expected to suffer from stronger effects when exposed to a given external concentration. Using a kinetic model with the sample substance dichloromethane (DCM), the higher internal exposure of children is quantified and compared to a healthy, young adult. The difference is shown to depend on age, on dose, and on duration of exposure. Furthermore, several ways are presented to derive AEGL values for exposure times which differ from the exposure duration in animal studies ('time scaling'). In comparison to the conventional procedure, the alternative approaches are based on mechanistic models of the toxicodynamic effect. Use of these models results in AEGL values which are biologically justified.