The ability of predicted internal dose measures to reconcile tumor bioassay data for chloroform

Development of physiologically based toxicokinetic models for improving the human indoor exposure assessment to water contaminants: trichloroethylene and trihalomethanes

Generally, ingestion is the only route of exposure that is considered in the risk assessment of drinking water contaminants. However, it is well known that a number of these contaminants are volatile and lipophilic and therefore highly susceptible to being absorbed through other routes, mainly inhalation and dermal. The objective of this study was to develop physiologically based human toxicokinetic (PBTK) models for trihalomethanes (THM) and trichloroethylene (TCE) that will facilitate (1) the estimation of internal exposure to these chemicals for various multimedia indoor exposure scenarios, and (2) consideration of the impact of biological variability in the estimation of internal doses. Five PBTK models describing absorption through ingestion, inhalation and skin were developed for these contaminants. Their concentrations in ambient air were estimated from their respective tap water concentrations and their physicochemical characteristics. Algebraic descriptions of the physiological parameters, varying as a function of age, gender and diverse anthropometric parameters, allow the prediction of the influence of interindividual variations on absorbed dose and internal dosimetry. Simulations for various scenarios were done for a typical human (i.e., 70 kg, 1.7 m) as well as for humans of both genders varying in age from 1 to 90 years. Simulations show that ingestion contributes to less than 50% of the total absorbed dose or metabolized dose for all chemicals. This contribution to internal dosimetry, such as maximal venous blood concentrations (Cmax) and the area under the venous blood concentration time curve (AUC), decreases markedly (e.g., as low as 0.9% of Cmax for bromodichloromethane). The importance of this contribution varies mainly as a function of shower duration. Moreover, model simulations indicate that multimedia exposure is more elevated in children than adults (i.e., up to 200% of the adult internal dose). The models developed in this study allow characterization of the influence of the different routes of exposure and an improved estimation of the realistic multimedia exposure to volatile organic chemicals present in drinking water. Hence, such models will greatly improve health risk assessment for these chemicals.

Trichloroethylene cancer risk: simplified calculation of PBPK-based MCLs for cytotoxic end points

Cancer risk assessments for trichloroethylene (TCE) based on linear extrapolation from bioassay results are questionable in light of new data on TCE's likely mechanism of action involving induced cytotoxicity, for which a threshold-type dose-response model may be more appropriate. Previous studies have shown that if a genotoxic mechanism for TCE is assumed, algebraic methods can considerably simplify the use of physiologically based pharmacokinetic (PBPK) models to estimate virtually safe environmental concentrations for humans based on rodent cancer-bioassay data. We show here how such methods can be extended to the case in which TCE is assumed to induce cancer via cytotoxicity, to estimate environmentally safe concentrations based on rodent toxicity data. These methods can be substituted for the numerical methods typically used to calculate PBPK-effective doses when these are defined as peak concentrations. We selected liver and kidney as plausible target tissues, based on an analysis of rodent TCE-bioassay data and on a review of related data bearing on mechanism. Tumor patterns in rodent bioassays are shown to be consistent with our estimates of PBPK-based, effective cytotoxic doses to mice and rats used in these studies. When used with a margin of exposure of 1000, our method yielded maximum concentration levels for TCE of 16 ppb (87 micrograms/m3) for TCE in air respired 24 hr/day, 700 ppb (3.8 mg/m3) for TCE in air respired for relatively brief daily periods (e.g., 0.5 hr while showering/bathing), and 210 micrograms/liter for TCE in drinking water assuming a daily 2-liter ingestion. Cytotoxic effective doses were also estimated for occupational respiratory exposures. These estimates indicate that the current OSHA permissible exposure limit for TCE would produce metabolite concentrations that exceed an acute no observed adverse effect level for hepatotoxicity in mice. On this basis, the OSHA TCE limit is not expected to be protective.