Children's exposure assessment: a review of factors influencing Children's exposure, and the data available to characterize and assess that exposure.

We review the factors influencing children's exposure to environmental contaminants and the data available to characterize and assess that exposure. Children's activity pattern data requirements are demonstrated in the context of the algorithms used to estimate exposure by inhalation, dermal contact, and ingestion. Currently, data on children's exposures and activities are insufficient to adequately assess multimedia exposures to environmental contaminants. As a result, regulators use a series of default assumptions and exposure factors when conducting exposure assessments. Data to reduce uncertainty in the assumptions and exposure estimates are needed to ensure chemicals are regulated appropriately to protect children's health. To improve the database, advancement in the following general areas of research is required: identification of appropriate age/developmental benchmarks for categorizing children in exposure assessment; development and improvement of methods for monitoring children's exposures and activities; collection of activity pattern data for children (especially young children) required to assess exposure by all routes; collection of data on concentrations of environmental contaminants, biomarkers, and transfer coefficients that can be used as inputs to aggregate exposure models.

A modeling framework for estimating children's residential exposure and dose to chlorpyrifos via dermal residue contact and nondietary ingestion.

To help address the Food Quality Protection Act of 1996, a physically based probabilistic model has been developed to quantify and analyze dermal and nondietary ingestion exposure and dose to pesticides. The Residential Stochastic Human Exposure and Dose Simulation Model for Pesticides (Residential-SHEDS) simulates the exposures and doses of children contacting residues on surfaces in treated residences and on turf in treated residential yards. The simulations combine sequential time-location-activity information from children's diaries with microlevel videotaped activity data, probability distributions of measured surface residues and exposure factors, and pharmacokinetic rate constants. Model outputs include individual profiles and population statistics for daily dermal loading, mass in the blood compartment, ingested residue via nondietary objects, and mass of eliminated metabolite, as well as contributions from various routes, pathways, and media. To illustrate the capabilities of the model framework, we applied Residential-SHEDS to estimate children's residential exposure and dose to chlorpyrifos for 12 exposure scenarios: 2 age groups (0-4 and 5-9 years); 2 indoor pesticide application methods (broadcast and crack and crevice); and 3 postindoor application time periods (< 1, 1-7, and 8-30 days). Independent residential turf applications (liquid or granular) were included in each of these scenarios. Despite the current data limitations and model assumptions, the case study predicts exposure and dose estimates that compare well to measurements in the published literature, and provides insights to the relative importance of exposure scenarios and pathways.

Ozone uptake in the intact human respiratory tract: relationship between inhaled dose and actual dose.

Inhaled concentration (C), minute volume (MV), and exposure duration (T) are factors that may affect the uptake of ozone (O(3)) within the respiratory tract. Ten healthy adult nonsmokers participated in four sessions, inhaling 0.2 or 0.4 ppm O(3) through an oral mask while exercising continuously to elicit a MV of 20 l/min for 60 min or 40 l/min for 30 min. In each session, fractional absorption (FA) was determined on a breath-by-breath basis as the ratio of O(3) uptake to the inhaled O(3) dose. The mean +/- SD value of FA for all breaths was 0.86 +/- 0.06. Although C, MV, and T all had statistically significant effects on FA (P < 0.0001, P = 0.004, and P = 0.026, respectively), the magnitudes of these effects were small compared with intersubject variability. For an average subject, a 0. 05 change in FA would require that C change by 1.3 ppm, MV change by 46 l/min, or T change by 1.7 h. It is concluded that inhaled dose is a reasonable surrogate for the actual dose delivered to a particular subject during O(3) exposures of <2 h, but it is not a reasonable surrogate when comparisons are made between individuals.

Use of a pharmacokinetic model to assess chlorpyrifos exposure and dose in children, based on urinary biomarker measurements.

Chlorpyrifos is a common agricultural insecticide and has been used residentially in the United States until the year 2000 when this use was restricted by the U.S. Environmental Protection Agency (U.S. EPA). A chlorpyrifos metabolite, 3,5,6-trichloro-2-pyridinol (TCPy) has been found in urine samples collected during exposure field studies. In this work, we use urinary biomarker data and the inverse solution of a simple pharmacokinetic (PK) model for chlorpyrifos to estimate the magnitude and timing of doses. Three urine samples were collected on separate days from each of 15 children (ages 3-12) who were participants in the Minnesota Children's Pesticide Exposure Study (MNCPES). The total volume of urine was noted and samples analyzed for TCPY: The urinary data was used along with constraints imposed on dose timing, based on responses of the individuals to pesticide-use surveys. We predicted the time and magnitude of multiple "event" exposures characterized by short-term, relatively high doses superimposed over a continuous background exposure. The average dose of chlorpyrifos predicted by the model was 1.61 microg/kg per reported event. Average background dose rate for these children that reported exposure events was 0.0062 microg/kg/h, or 0.15 microg/kg/day. In addition to predicting the total dose of chlorpyrifos received by an individual from urinary biomarker measurements, the model can then be run in a forward manner once the exposure regime is determined. This will allow the prediction of the total amount of TCPy eliminated in the urine over any time period of interest.

Improvement of a respiratory ozone analyzer.

The breath-to-breath measurement of total respiratory ozone (O3) uptake requires monitoring O3 concentration at the airway opening with an instrument that responds rapidly relative to the breathing frequency. Our original chemiluminescent analyzer, using 2-methyl-2-butene as the reactant gas, had a 10% to 90% step-response time of 110 msec and a minimal detectable concentration of 0.018 parts per million (ppm) O3 (Ben-Jebria et al. 1990). This instrument was suitable for respiratory O3 monitoring during quiet breathing and light exercise. For this study, we constructed a more self-contained analyzer with a faster response time using ethylene as the reactant gas. When the analyzer was operated at a reaction chamber pressure of 350 torr, an ethylene-to-sample flow ratio of 4:1, and a sampling flow of 0.6 liters per minute (Lpm), it had a 10% to 90% step-response time of 70 msec and a minimal detectable concentration of 0.006 ppm. These specifications make respiratory O3 monitoring possible during moderate-to-heavy exercise. In addition, the nonlinear calibration and the carbon dioxide (CO2) interference exhibited by the original analyzer were eliminated. In breath-to-breath measurements in two healthy men, the fractional uptake of O3 during one minute of quiet breathing was comparable to the results obtained by using a slowly responding commercial analyzer with a quasi-steady material balance method (Wiester et al. 1996). In fact, fractional uptake was about 0.8 regardless of O3 exposure concentration (0.11 to 0.43 ppm) or ventilation rate (4 to 41 Lpm/m2).

Longitudinal distribution of ozone absorption in the lung: effects of nitrogen dioxide, sulfur dioxide, and ozone exposures.

Investigators used an ozone bolus inhalation method to study the effects of continuous exposure to ozone, nitrogen dioxide, and sulfur dioxide on ozone absorption in the conducting airways of human lungs. Healthy, young nonsmokers (6 males, 6 females) were exposed on separate days for 2 h to air containing 0.36 ppm nitrogen dioxide, 0.75 ppm nitrogen dioxide, 0.36 ppm sulfur dioxide, or 0.36 ppm ozone. Every 30 min, the subject interrupted exposure for approximately 5 min, during which he or she orally inhaled five ozone boluses-each in a separate breath. Investigators targeted penetration of the boluses distal to the lips in the 70-130-ml range, which corresponded to the lower conducting airways. The authors computed the change in absorption resulting from exposure (delta lambda) by comparing the amount of each ozone bolus that was absorbed with a corresponding value obtained prior to exposure. Results indicated that ozone exposure caused delta lambda to decrease relative to air exposure (p < .01), whereas both nitrogen dioxide and sulfur dioxide exposures caused an increase in delta lambda that was not significantly different from air exposure. This resulted, at least in part, to an artifact caused by preexposure to ozone boluses. The authors concluded that exposure of the lower conducting airways to nitrogen dioxide or sulfur dioxide increased their capacity to absorb ozone because more of the biochemical substrates that are normally oxidized by ozone were made available. During continuous ozone exposure, this excess of substrate is depleted and the absorption of ozone boluses decreases.

Longitudinal distribution of ozone absorption in the lung: effect of continuous inhalation exposure.

The effect of continuous exposure to ozone on the absorption of ozone in the conducting airways of human lungs was investigated with a bolus-response method. Eleven healthy nonsmoking college students (8 males, 3 females) were exposed at rest for 2 h on 3 separate days to air containing 0 ppm, 0.12 ppm, and 0.36 ppm ozone. A personal inhalation chamber equipped with a head-only clear plastic dome was used for exposure. Every 30 min a subject removed the dome and orally inhaled a series of five ozone-air boluses, each in a separate breath. Penetration of the boluses distal to the lips was targeted in the range of 70-120 ml (corresponding to the central conducting airways). By integrating the inhaled and exhaled-ozone concentration curves, we obtained the absorbed fraction (lambda) and the dispersion (sigma2) of the ozone bolus for each test breath. In addition, the subtraction of baseline measurements made just before exposure enabled us to determine the changes in absorbed fraction (deltalambda) and in dispersion (deltasigma2) that resulted from exposure alone. Absorbed fraction decreased, but sigma2 increased during O3 exposure, and the differences in deltalambda and in deltasigma2 between breathing air and exposure to either 0.12 ppm or 0.36 ppm O3 were significant. We concluded that exposure of the conducting airways to O3 reduced their capacity to absorb O3, possibly by the depletion of biochemical substrates that are normally oxidized by O3.

A respiratory ozone analyzer optimized for high resolution and swift dynamic response during exercise conditions.

The breath-to-breath determination of total respiratory ozone (O3) uptake requires the monitoring of O3 concentration at the airway opening with an instrument that responds rapidly relative to the frequency of respiration. Originally, the authors developed an analyzer that used the homogeneous chemiluminescent reaction of O3 with 2-methyl-2-butene, but it was suitable only for monitoring O3 during quiet breathing and light exercise (Ben-Jebria and Ultman, Rev Sci Instrum 1989; 60:3004-11, and Ben-Jebria et al., Rev Sci Instrum 1990; 61:3435-39). The improvement of performance characteristics of the aforementioned analyzer enabled the authors to use the newly constructed and self-contained instrument, which used ethylene as the reactant gas, for respiratory O3 monitoring during moderate-to-heavy exercise. Operating at a reaction chamber pressure of 350 torr, an ethylene/sample flow ratio of 4:1, and a sampling flow of 0.6 lpm, the authors achieved an optimum analyzer performance (i.e., 10-90% step-response of 70 msec and a minimum resolution of 0.006 ppm O3). Furthermore, the new instrument did not exhibit the nonlinear calibration and the CO2 interference suffered by the original analyzer. To demonstrate the quality of the new O3 analyzer in a respiratory application (i.e., total O3 uptake), the authors measured a series of single breaths on two subjects who breathed 0.11 and 0.43 ppm O3-in-air mixtures for 15 min during rest, and during moderate and heavy exercise.