Michael A. Jayjock

A PROPOSAL FOR IMPROVING THE ROLE OF EXPOSURE MODELING IN RISK ASSESSMENT

This paper explores the adequacy of using currently available exposure assessment models for indoor air inhalation exposures in the context of the uncertainty that exists in both the dose-response assessment and the exposure assessment. A tiered system is proposed for implementing exposure assessments. Each tier involves additional research, but also would lead to reductions in uncertainty. The authors discuss a possible comprehensive research program that will permit the building of well-validated models for indoor air inhalation exposures among classes of chemicals, building types, and exposure scenarios. The authors believe that until such a research program is undertaken, modeling of exposures (and therefore risks) using the current, unvalidated modeling approaches provides little more than worst-case estimates that are useful primarily for making screening decisions about risks.

A rationale and framework for establishing the quality of human exposure assessments

Exposure assessments, performed as input to an evaluation of potential human health risk, are an element of risk assessment. Risk assessment results feed governmental and corporate risk management decisions, which seek to balance the estimated potential human health risks with other factors. Risk managers presume that the risk information provided to them is scientifically valid and accurate. Government agencies have begun to apply a system known as good laboratory practices (GLP) to ensure adequate data quality on animal studies, which are often the first step in the health-effects evaluation of risk assessment. This paper explores a rationale and framework for establishing the quality of human exposure assessments and proposes a set of good exposure assessment practices (GEAP). The components of the proposed GEAP include the writing of a study protocol before conducting the study, consideration of available resources, specification of an exposure model, a study design (including sampling and analytical methods and data analysis), quality assurance, archiving, communications, and a statement of overall uncertainty in exposure estimates. The GEAP concept is offered as a starting point for developing a consensus among the community of exposure assessors regarding a minimum standard for good practices. If a consensus on GEAP can be reached and applied, exposure assessments would have improved scientific bases, interpretability, and utility.

Uncertainty Analysis in the Estimation of Exposure

This article addresses the ubiquitous nature of uncertainty in industrial hygiene-related risk assessment and defines two basic types of uncertainty: natural variability and a basic lack of knowledge. A relatively simple physical-chemical modeling example is provided as an illustration in which uncertainty and sensitivity are described using two methods, a conventional technique and a readily available and user-friendly computer simulation software. The need for and value of monitoring data for validation and as a “reality check” is emphasized.

Back Pressure Modeling of Indoor Air Concentrations From Volatilizing Sources

Indoor air pollution models estimate the concentration and potential exposure (concentration · time) to persons from sources of airborne contamination. They typically describe the independent variables of contaminant generation and control that determine and predict exposure. An important source of airborne contamination is volatilization. Volatilization is driven by the difference between the equilibrium or saturation partial pressure of a compound and the partial pressure present in the receiving air volume. Given a relatively small volatilizing source the resulting concentration in room air will most likely be a small fraction of its equilibrium or saturation partial pressure. Under these conditions a relatively large gradient is maintained and the volatilization rate is maximized. The model is simplified since any decrease in the gradient can be considered insignificant and thus ignored in the estimation. For large volatilizing sources (e.g., rugs, painted walls, spills, etc.), however, the concentrat...

Assessment of inhalation exposure potential from vapors in the workplace.

Quantitative knowledge of human exposure to a chemical represents half of the required input for the health risk assessment process by which occupational health professionals seek to integrate exposure and inherent hazard in clinically meaningful terms. This work reviews and presents explicit methodologies and approaches to the estimation of inhalation exposure to chemicals. Special attention is given to estimating vapor exposures and the critical elements of an indoor air pollution model that assumes conservation of contaminant mass in a specified box of workroom air.

The Daubert Standard as Applied to Exposure Assessment Modeling Using the Two-Zone (NF/FF) Model Estimation of Indoor Air Breathing Zone Concentration as an Example

T he Daubert standard governs the admissibility of expert witness testimony during all U.S. federal legal proceedings and over half of state proceedings. It allows for the legal challenge of any expert witness testimony. In essence, the expert scientific witness has the burden of proof relative to the validity and acceptability of his or her scientific conclusions. An example of a potential Daubert challenge for exposure assessors or industrial hygienists exists in the use of first principle physical-chemical models of human exposure. We present the relevant details of Daubert in the context of an industrial hygienist providing testimony based on these types of models. As an example, we present the Daubert criteria to address a potential challenge for the Near Field/Far Field (two-zone) indoor air model of breathing zone concentration.

Monte Carlo Uncertainty Analysis of a Diffusion Model for the Assessment of Halogen Gas Exposure During Dosing of Brominators

Monte Carlo simulation was incorporated into a diffusion-based exposure assessment model for the estimation of worker exposure to halogen gases during dosing of 500-lb sacks of a bromine-based biocide (BCDMH) into brominators. Indoor and outdoor dosing scenarios were modeled for small and large brominators. The diffusion model used describes a concentration gradient of halogen as a function of distance and time from the source instead of ascribing worst-case single point value estimates to the variables used in the diffusion model. Monte Carlo simulation was used to describe a distribution of values for each appropriate model variable. Using a personal computer and Monte Carlo simulation software, 10,000 iterations of the diffusion model were performed for four different dosing scenarios using random and independent samples from the distributions entered. The corresponding output distributions of predicted exposures were then calculated and displayed graphically for each scenario. The results of the Monte Carlo simulation predict that outdoor dosing of either small or large brominators with BCDMH is highly unlikely to result in an exceedance of the working occupational exposure limit for total halogen. In most ambient wind speed conditions, diffusion prevents appreciable airborne exposure to workers in the immediate vicinity of the brominator. Although relatively uncommon, dosing of brominators indoors in the assumed absence of local exhaust ventilation may generate airborne concentrations of total halogen that exceed the working short-term occupational exposure limit. Although very limited and inconclusive, field trial monitoring of BCDMH transfer operations indoors resulted in halogen concentrations well within the distribution of concentrations predicted by the Monte Carlo simulation of the diffusion model.

Exposure Estimation and Interpretation of Occupational Risk: Enhanced Information for the Occupational Risk Manager

The fundamental goal of this article is to describe, define, and analyze the components of the risk characterization process for occupational exposures. Current methods are described for the probabilistic characterization of exposure, including newer techniques that have increasing applications for assessing data from occupational exposure scenarios. In addition, since the probability of health effects reflects variability in the exposure estimate as well as the dose-response curve—the integrated considerations of variability surrounding both components of the risk characterization provide greater information to the occupational hygienist. Probabilistic tools provide a more informed view of exposure as compared to use of discrete point estimates for these inputs to the risk characterization process. Active use of such tools for exposure and risk assessment will lead to a scientifically supported worker health protection program. Understanding the bases for an occupational risk assessment, focusing on important sources of variability and uncertainty enables characterizing occupational risk in terms of a probability, rather than a binary decision of acceptable risk or unacceptable risk. A critical review of existing methods highlights several conclusions: (1) exposure estimates and the dose-response are impacted by both variability and uncertainty and a well-developed risk characterization reflects and communicates this consideration; (2) occupational risk is probabilistic in nature and most accurately considered as a distribution, not a point estimate; and (3) occupational hygienists have a variety of tools available to incorporate concepts of risk characterization into occupational health and practice.

Development and Evaluation of a Source/Sink Model of Indoor Air Concentrations from Isothiazolone-Treated Wood Used Indoors

Airborne exposure models were developed to account for volatilizing sources and adsorbing surface sinks of isothiazolone biocide volatilized from treated wood into glass chambers and real-world indoor environments. The initial effort described the time-course fate of biocide that becomes airborne and deposits onto the glass surfaces of small ventilated test chambers containing the wood. After the biocide-treated wood was put into a clean-ventilated chamber, the airborne level of biocide built slowly as the biocide initially volatilized and deposited onto the chamber walls. Subsequent re-emission from the glass walls to the air added to the airborne concentration until equilibrium was established between adsorption onto and desorption from the sink, when the airborne concentration leveled off. Adsorption and apparent degradation of the active ingredient on the chamber walls resulted in equilibrium airborne concentrations about a third of those predicted without these mechanisms. Glass and stainless steel t...

How Protective Are Respirator Assigned Protection Factors: An Uncertainty Analysis

This investigation evaluated the risk of overexposure for a selected assigned protection factor by performing Monte Carlo simulations. A model was constructed to assess respirator performance by calculating the concentration inside the respirator. Estimates of the factors that affect respirator performance were described as distributions. The distributions used a worst case estimate for concentration in the workplace, the worst case for respirator performance (the fifth percentile person), and the worst case for exhalation valve leakage. A Monte Carlo analysis then provided estimates of the percentage of time that concentration inside the respirator exceeded the occupational exposure limit (OEL). For a half-facepiece respirator with an APF of 10, the calculations indicated a low risk of being exposed above an OEL, with mean exposures being controlled well below an OEL.