Exposure Assessment Strategies Research Articles

Pollution atmosphérique et risques professionnels

Occupational exposure monitoring is one step of risk assessment methodology which is extensivelly used in environment. The aim is to identify worker groups having an increased risk of developing adverse health effects. These groups will be concerned by priority measures in improvment of industrial health prevention and specific medical survey. Airborne monitoring is mainly used by industrial hygienists but this method implies rigorous exposure assessment strategy and specific sampling material. Biological monitoring is developing, being more easy and better related to target dose.

Heterogeneity in sources of exposure variability among groups of workers exposed to inorganic mercury

Many exposure assessment strategies rely on the occupational group as the unit of analysis in which workers are classified on the basis of job title, location, or on other characteristics related to the workplace or the job. Although statistical methods that combine exposure data collected on workers from different occupational groups are more efficient, the underlying assumption that the degree of variation over time and among workers is the same for all groups has yet to be fully investigated. Given the utility of different modeling approaches when assessing exposures, we investigated assumptions of homogeneity of variance within and between workers using both random- and mixed-effects models. In our study of four groups of workers exposed to inorganic mercury (Hg) at a chloralkali plant, there was no evidence of significant heterogeneity in the levels of variation over time or between workers for air Hg levels. For the biological monitoring data, however, our findings indicate that groups did not share common levels of variability and that it was not appropriate to pool the data and obtain single estimates of the within- and between-worker variance components. Classification of job group as a random or fixed effect had no effect on the results and yielded the same conclusions when the models were compared. To illustrate effects related to the proper specification of a model, the likelihood of exceeding certain levels (which is a function of the parameters of the underlying distribution of the natural log-transformed exposures) was evaluated using the results obtained from the different models. Although the probability that workers' mean exposures exceeded occupational exposure limits for air, urine and blood Hg was generally low (<10%) for all groups except maintenance workers, the estimated values sometimes varied depending upon the particular model that was applied. Given the growing use of random- and mixed-effects models that combine data across occupational groups, additional studies are warranted to evaluate whether it is reasonable to assume common variances and covariances among measurements collected on workers from different groups.

Heterogeneity in sources of exposure variability among groups of workers exposed to inorganic mercury

Many exposure assessment strategies rely on the occupational group as the unit of analysis in which workers are classified on the basis of job title, location, or on other characteristics related to the workplace or the job. Although statistical methods that combine exposure data collected on workers from different occupational groups are more efficient, the underlying assumption that the degree of variation over time and among workers is the same for all groups has yet to be fully investigated. Given the utility of different modeling approaches when assessing exposures, we investigated assumptions of homogeneity of variance within and between workers using both random- and mixed-effects models. In our study of four groups of workers exposed to inorganic mercury (Hg) at a chloralkali plant, there was no evidence of significant heterogeneity in the levels of variation over time or between workers for air Hg levels. For the biological monitoring data, however, our findings indicate that groups did not share common levels of variability and that it was not appropriate to pool the data and obtain single estimates of the within- and between-worker variance components. Classification of job group as a random or fixed effect had no effect on the results and yielded the same conclusions when the models were compared. To illustrate effects related to the proper specification of a model, the likelihood of exceeding certain levels (which is a function of the parameters of the underlying distribution of the natural log-transformed exposures) was evaluated using the results obtained from the different models. Although the probability that workers' mean exposures exceeded occupational exposure limits for air, urine and blood Hg was generally low (<10%) for all groups except maintenance workers, the estimated values sometimes varied depending upon the particular model that was applied. Given the growing use of random- and mixed-effects models that combine data across occupational groups, additional studies are warranted to evaluate whether it is reasonable to assume common variances and covariances among measurements collected on workers from different groups.

A Comparison and Critique of Historical and Current Exposure Assessment Methods for Beryllium: Implications for Evaluating Risk of Chronic Beryllium Disease

The primary beryllium industry has generated a large amount of data on airborne beryllium concentrations that has been used to characterize exposure by task-specific activities, job category, individual worker, and processing area using a variety of methods. These methods have included high-volume breathing zone sampling, high-volume process sampling, high- and low-volume respirable and area sampling, real-time monitoring, and personal sampling. Many of the beryllium studies have used these air sampling methods to assess inhalation exposure and chronic beryllium disease (CBD) risk to beryllium; however, available data do not show a consistent dose-response relationship between airborne concentrations of beryllium and the incidence of CBD. In this article, we describe the air sampling and exposure assessment methods that have been used, review the studies that have estimated worker exposures, discuss the uncertainties associated with the level of beryllium for which these studies have reported an increased risk of CBD, and identify future investigative exposure assessment strategies. Our evaluation indicated that studies of beryllium workers are often not directly comparable because they (1) used a variety of exposure assessment methods that are not necessarily representative of individual worker exposures, (2) rarely considered respirator use, and (3) have not evaluated changes in work practices. It appears that the current exposure metric for beryllium, total beryllium mass, may not be an appropriate measurement to predict the risk of CBD. Other exposure metrics such as mass of respirable particles, chemical form, and particle surface chemistry may be more related to the prevalence of CBD than total mass of airborne beryllium mass. In addition, assessing beryllium exposure by all routes of exposure (e.g., inhalation, dermal uptake, and ingestion) rather than only inhalation exposure in future studies may prove useful.

Identification of dermal exposure pathways in the rubber manufacturing industry

Current existing dermal exposure assessment strategies are predominantly based on regulatory protocols. In order to develop effective and efficient strategies more data driven approaches are needed. In a recently developed conceptual model for dermal exposure, compartments, barriers and mass transport processes relevant for dermal exposure were described. We systematically applied this conceptual model to the rubber manufacturing industry to assess dermal exposure to cyclohexane soluble matter (CSM) and used quantitative data to design an exposure assessment strategy. Identification of the spatial distribution of the dermal contamination showed high CSM surface concentrations for the upper body. Moreover, because of the high correlation between dermal exposure at the wrist and calculated total body exposure (r=0.89, P<0.01) an exposure assessment strategy based on only one pad sampler was employed to estimate CSM surface concentrations in the skin contaminant layer. Qualitative and quantitative evaluation of the relevant compartments and related mass transport processes demonstrated the importance of deposition of airborne contaminants and direct transfer of contaminants from sources and surfaces to the skin contaminant layer. Interestingly, the importance of the different exposure pathways varied considerably between production functions. The use of a model driven exposure assessment strategy in the rubber manufacturing industry revealed relevant skin regions, compartments and mass transport processes and enabled development of an effective and efficient strategy for dermal exposure assessment and hazard control in this particular occupational setting.

Identification of dermal exposure pathways in the rubber manufacturing industry

Current existing dermal exposure assessment strategies are predominantly based on regulatory protocols. In order to develop effective and efficient strategies more data driven approaches are needed. In a recently developed conceptual model for dermal exposure, compartments, barriers and mass transport processes relevant for dermal exposure were described. We systematically applied this conceptual model to the rubber manufacturing industry to assess dermal exposure to cyclohexane soluble matter (CSM) and used quantitative data to design an exposure assessment strategy. Identification of the spatial distribution of the dermal contamination showed high CSM surface concentrations for the upper body. Moreover, because of the high correlation between dermal exposure at the wrist and calculated total body exposure (r=0.89, P<0.01) an exposure assessment strategy based on only one pad sampler was employed to estimate CSM surface concentrations in the skin contaminant layer. Qualitative and quantitative evaluation of the relevant compartments and related mass transport processes demonstrated the importance of deposition of airborne contaminants and direct transfer of contaminants from sources and surfaces to the skin contaminant layer. Interestingly, the importance of the different exposure pathways varied considerably between production functions. The use of a model driven exposure assessment strategy in the rubber manufacturing industry revealed relevant skin regions, compartments and mass transport processes and enabled development of an effective and efficient strategy for dermal exposure assessment and hazard control in this particular occupational setting.

Evaluation of percutaneous penetration of natural rubber latex proteins.

Latex allergy is recognized worldwide as a serious health risk. To date, exposure assessment and intervention strategies have focused primarily on respiratory protection; this work evaluates the potential role of dermal protein penetration in the development of latex allergy. In vitro penetration models using flow-through diffusion cells and both human surgical specimens and hairless guinea pig skin (CrL: IAF/HA) demonstrated iodinated latex proteins (ammoniated and non-ammoniated) penetrating into and through both intact and abraded skin. Although less than 1% penetration was observed with intact skin, up to 23% of latex proteins applied to abraded skin were recovered from receptor fluid within 24 h of exposure. Phosphoimaging of the concentrated effluent revealed proteins ranging in size from 3 to 26 kDa. Using a (3)H(2)O penetration assay to evaluate barrier integrity, the amount of latex protein penetration was found to positively correlate with the degree of dermabrasion. Immunohistochemistry of the skin localized latex proteins in the Langerhans cell-rich epidermis and in the dermis. Both in vitro penetration studies and immunohistochemistry supported the use of hairless guinea pig skin as a surrogate for human skin in evaluating latex protein penetration. In studies performed in vivo, 35% of hairless guinea pigs topically exposed to latex proteins (100 microg) 5 days per week for 3 months demonstrated elevations in latex-specific IgG1. The implication for these data is that the skin is not only a plausible route for latex sensitization but can be a major exposure route when the integument has been compromised.

Developing Reliable Ergonomic Exposure Assessment Strategies for Construction Work

The objective of this research was to provide guidelines for the reliable assessment of ergonomic exposures in construction work. Using a modified work-sampling approach, two or three observers collected a total of 4852 observations on tasks performed by three trades (iron workers, carpenters and laborers) for periods of several weeks. At each observation, recorded exposure variables included non-neutral trunk posture, trunk flexion, lateral bending or torsion, arm(s) at or above shoulder height, kneeling, squatting or leg bending, load handling, manual materials handling, hand-tool use and power-tool use. The frequency of exposure was calculated for each worker during each of the tasks on each of the days. ANOVA was used to assess the importance of task, between-worker variability and within-worker variability on exposures across days. A statistical resampling method (bootstrap) was then used to evaluate the reliability of exposure estimates for groups of workers performing the same task for different sampling periods. Most ergonomic exposures were found to vary significantly among construction tasks. In most cases, the between-worker component of variance within task was overshadowed by a large within-worker component of variance*** thought to consist largely of day-to-day differences in exposure and measurement error. When the average frequency of the ergonomic exposures was bootstrapped, the reliability of the most variable exposures tended to improve as the assessment periods approached 5 to 6 days, with marginal improvements for assessment periods longer than 6 days. Reliable estimates for the least variable exposures were obtained with 1or 2 days of observation. Based on this study, a task-based approach for classifying ergonomic exposures may benefit epidemiologic research and the evaluation of controls in construction work. A preliminary analysis of exposure variability is recommended when determining sampling strategies, and when multiple exposures are assessed simultaneously, sampling periods exceeding 6 days may be required for reliable measures.

Effect of different grouping strategies in developing estimates of personal exposures: specificity versus precision.

There has been increasing interest in recent years in developing strategies in epidemiology for the summarization of occupational exposures, strategies that serve to clarify observed relationships between occupational exposure and health outcomes. Where source occupational exposure data are scarce, it is common to assemble exposure groups with the goal of increasing the extent to which data-based exposure estimates are available for an entire cohort. There has been little guidance, however, on the effect of different grouping strategies on the observed fit between exposure and health outcome. This investigation examined the effect of the use of different exposure summarization strategies on observed relationships between dust exposure and lung function decline among coal mine workers. The dust exposure and spirometry data employed were gathered in the National Study of Coalworkers' Pneumoconiosis. An analysis of variance procedure was carried out to characterize the variability of the dust exposure data, employing single variables relating to mine identity, occupation, and year, as well as two- and three-way multivariate combinations of these variables. The resulting combinations were ranked according to the standard deviation of the observed exposure range to reflect the relative specificity of the various approaches. Sequential arrangements of single- and multiple-variable combinations were constructed, alternately employing highly specific codes or broad categories for mine, occupation, and year. Annual exposure estimates were constructed on the basis of these sequences and used in tandem with longitudinal change in forced expiratory volume (FEV1) in linear multiple regression procedures. Height, age, smoking status, and dust exposure were employed as predictor variables. The results show that the use of broad categorization approaches had a substantial impact on observed regression coefficients. The largest change was observed for categorization according to occupation, which resulted in two- to three-fold increases in the magnitude of observed regression coefficients. These results suggest that the use of highly specific exposure summarization approaches may result in regression outcomes which are marked by a high degree of attenuation, and that consideration of the precision of summarized exposure estimates is an important component of an effective exposure assessment strategy.

New avenues in research on musculoskeletal disorders.

Recent reviews on musculoskeletal disorders have clearly pointed out the need for more good-quality original research. Both symptom-based and objective outcome assessment methods should be improved. A variety of methods exists to assess exposure for short-term outcomes, whereas exposure assessment strategies for outcomes with long induction times should be developed. The change in exposure patterns--less force demands, more static, visual, cognitive, psychological and social load--will continue during the next millennium, and a likely result is a change in the disease panorama. The pathomechanisms of the effects of low-level static contractions and adverse psychosocial conditions need clarification. Genetic epidemiology is an emerging field of research, and it will be particularly interesting to study the interaction between genetically determined susceptibility and occupational risk factors. Because degeneration of the spine starts in adolescence, the importance of studying the etiology of degenerative disorders among young populations is emphasized.

Exposures of geotechnical laboratory workers to respirable crystalline silica.

Geotechnical laboratory testing involves the determination of the physical properties of soil, rock, and other building materials for engineering purposes. Individuals working in these laboratories are exposed to airborne soil, rock, and other dusts during the preparation and testing of these materials. Crystalline silica as quartz is a common constituent of these materials and represents a potential hazard to geotechnical laboratory workers when airborne as a respirable dust. The authors conducted an examination of the potential for geotechnical laboratory workers to be exposed to respirable dust and respirable quartz during the performance of three routine laboratory tasks. A task-based exposure assessment strategy was used. Although respirable dust was generated during the performance of each of these tasks, its impact on exposures was generally overridden by the presence of respirable quartz in the dust. Quartz content in the respirable dust ranged from below the detection limit to greater than 50 percent. Mean exposure to respirable quartz, based on the duration of the task and assuming no other exposures for the rest of the 8-hour day, exceeded the National Institute for Occupational Safety and Health (NIOSH) "action level" (the exposure level at which certain actions must be taken) of 0.025 mg/m3. If exposure was assumed to continue for the rest of the 8-hour day at the measured concentration, mean exposure to respirable quartz exceeded the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV) time-weighted average (TWA), the Occupational Safety and Health Administration (OSHA) PEL, and the NIOSH REL. Seven percent of 57 individual task exposure measurements exceeded the TLV-TWA and the PEL, 18 percent exceeded the REL, and another 12 percent exceeded excursion limits as defined by ACGIH. The results of this study support the conclusion that geotechnical laboratory workers are potentially exposed to respirable crystalline silica as quartz at levels that may be harmful. Because the quartz content of the materials being tested in these laboratories is highly variable and is almost never determined prior to testing, all materials being tested in the geotechnical laboratory should be assumed to contain quartz. Appropriate controls should be used to protect workers from inhaling dusts generated from these materials.

Validation of the Workplace Exposure Assessment Expert System (WORKBOOK)

A validation study was conducted to evaluate the accuracy, reliability, and effectiveness of the workplace exposure assessment expert system WORKBOOK (Version 1.0), a software system for evaluating and controlling occupational health risks. The application was developed to assist qualified people in performing exposure assessments under the guidance of professional hygienists. Fifty assessments were conducted, representing a wide variety of operations and involving technical staff at five U.S. and international facilities that conduct pharmaceutical research and development, manufacture bulk drug substances and dosage products, and produce medical devices. Important issues examined by the study include (1) assessing the competency and comprehension of users, (2) implementing exposure assessments, (3) examining the accuracy and reliability of WORKBOOK'S conclusions and recommendations, (4) improving the exposure assessment model and software, and (5) providing training and support services. Despite its original intention, WORKBOOK is a system for professional hygienists, rather than other technical professionals. Users need education and training as a professional hygienist, with at least 3 to 5 years of field experience. Although the exposure assessment model is valid and useful, its hygiene terminology and software features should be simplified and automated to make it more useful to technical professionals. Practical experiences from the validation study are directing WORKBOOK'S future development. However, employee training and technical support are essential to promote effective exposure assessment activities. Management support is required in the United States to shift from the traditional regulatory paradigm of exposure monitoring to more systematic, comprehensive exposure assessment strategies.

Challenges in assessing risk factors in epidemiologic studies on back disorders.

In epidemiologic studies on musculoskeletal disorders, some risk factors, especially physical load, cannot be determined independently from the worker. Posture, movement and external load are the result both of physical work requirements forced on the worker and of the worker's capacity to adopt particular techniques. Risk factors are also adjusted in relation to the worker's health. This paper presents a dynamic model that links exposure to risk factors for back pain and disability. Its aim is to help identify core elements in exposure assessment strategies for epidemiologic studies on back disorders. In this dynamic model, risk factors are determined relative to health status in order to distinguish between etiological and prognostic factors. Measurement techniques for various risk factors are classified into self-reports, observations, and direct instrumentation. Features of commonly used techniques are discussed with respect to feasibility, accuracy, and precision. In addition, consideration is given to the optimum allocation of measurements taking into account the effects of random and systematic variation in exposure due to tasks, workplaces, and workers.

Determination of Multiple Source Volatile Organic Compound Emission Factors in Offset Printing Shops

Abstract The printing industry emits large quantities of volatile organic compounds (VOCs) that impact workplace and ambient air quality. Emission factors are a useful way to describe these emissions. Emission factors describe the mass release of a pollutant in terms of a process activity related to the emissions, and are useful for concentration modeling, evaluation and prioritization of engineering control efforts, and development of logical exposure assessment strategies. Three offset printing facilities of various sizes were evaluated for VOC air concentrations, ventilation characteristics, and process activities. The air concentration and ventilation data were used in mass balance models to determine the hourly VOC emission rates. Emission rates were validated with measures of bulk solvent use. Average VOC emission rates at three sites ranged from 178 to 1076 g/h. On a mass VOC emitted/area printed basis, the smallest print shop emitted up to 12 times the amount from the other sites (29 versus 3.9 an...

An Investigation of Systematic Changes in Occupational Exposure

Nonstationary behavior in occupational exposure was examined among a number of job groups from different industries. A change in the mean level of exposure between two survey periods was evaluated by applying mixed-effects models. Overall, differences between surveys were observed in slightly more than one-third of the industries analyzed and in about one-quarter of the total number of comparisons performed. Exposures in the majority of cases decreased in the later survey. Further analyses examined the impact of nonstationary exposures on the estimation of the between- and within-worker components of variance. When changes in the mean exposure level were detected, point estimates of the variance components generated under the mixed model were compared with those estimates obtained under the one-way random-effects model, which assumes that the mean exposure level remains constant over time. The results indicate that the magnitude of the bias in the variance component estimates can be substantial when the misspecified model is applied. It is concluded that, in the absence of changes known to affect exposure, data collected within a year are likely to result in relatively valid inferences about mean and variance parameters using models that assume stationarity; for periods extending beyond a year, systematic changes in exposure are more likely to occur. Thus, exposure assessment strategies should be designed so that sufficient data are collected among groups of workers to investigate systematic changes and to ensure that appropriate statistical models are applied. In this way, occupational hygienists will be able to make reliable inferences about the underlying distribution of exposures pertinent to each occupational group.

An investigation of systematic changes in occupational exposure.

Nonstationary behavior in occupational exposure was examined among a number of job groups from different industries. A change in the mean level of exposure between two survey periods was evaluated by applying mixed-effects models. Overall, differences between surveys were observed in slightly more than one-third of the industries analyzed and in about one-quarter of the total number of comparisons performed. Exposures in the majority of cases decreased in the later survey. Further analyses examined the impact of nonstationary exposures on the estimation of the between- and within-worker components of variance. When changes in the mean exposure level were detected, point estimates of the variance components generated under the mixed model were compared with those estimates obtained under the one-way random-effects model, which assumes that the mean exposure level remains constant over time. The results indicate that the magnitude of the bias in the variance component estimates can be substantial when the misspecified model is applied. It is concluded that, in the absence of changes known to affect exposure, data collected within a year are likely to result in relatively valid inferences about mean and variance parameters using models that assume stationarity; for periods extending beyond a year, systematic changes in exposure are more likely to occur. Thus, exposure assessment strategies should be designed so that sufficient data are collected among groups of workers to investigate systematic changes and to ensure that appropriate statistical models are applied. In this way, occupational hygienists will be able to make reliable inferences about the underlying distribution of exposures pertinent to each occupational group.

On strategies for comparing occupational exposure data to limits.

Parametric statistical approaches to assessing workplace exposure levels have typically focused either on the probability that a single measurement exceeds a limit or on whether the mean exposure for a population of workers exceeds a limit. This article reviews and clarifies some methods that have been proposed for each of these two approaches, on the assumption that the exposure data represent a random sample from a lognormal distribution. For tests concerning the mean exposure level, the authors developed a potentially useful new procedure based on a bound for noncentral t critical values. Appropriate sample size calculations are emphasized, and computer simulation is used to compare competing methods for assessing mean exposure. The authors conclude that the new proposed method offers an appealing alternative to existing methods in many cases. The importance of employing an exposure assessment strategy that is in concert with underlying etiologic considerations is stressed.

On Strategies for Comparing Occupational Exposure Data to Limits

Parametric statistical approaches to assessing workplace exposure levels have typically focused either on the probability that a single measurement exceeds a limit or on whether the mean exposure for a population of workers exceeds a limit. This article reviews and clarifies some methods that have been proposed for each of these two approaches, on the assumption that the exposure data represent a random sample from a lognormal distribution. For tests concerning the mean exposure level, the authors developed a potentially useful new procedure based on a bound for noncentral t critical values. Appropriate sample size calculation are emphasized, and computer simulation is used to compare competing methods for assessing mean exposure. The authors conclude that the new proposed method offers an appealing alternative to existing methods in many cases. The importance of employiong an exposure assessment strategy that is is concert with underlying etiologic consideration is stressed.

Tiered exposure-assessment strategy in the Semiconductor Health Study.

The Semiconductor Health Study included 14 U.S. companies with 82 silicon-based wafer-fabrication rooms and a complex array of processes and chemicals. Its epidemiologic components were a historical examination of spontaneous abortion rates among 902 women, a prospective evaluation of reproductive outcomes in 483 women, and a cross-sectional review of male fertility and of respiratory, ergonomic, neurologic, or gastrointestinal problems among 3,175 men and women. Designing an exposure assessment strategy presented unique problems, and multiple agents had to be evaluated. A three-tiered approach to exposure assessment was developed to reflect increasing specificity of exposures. At the first tier, employees were divided into fabrication and nonfabrication groups. At the second tier, work groups with qualitatively different exposures were determined. At the third tier, intensity of exposures to specific chemical, physical, and ergonomic agents was evaluated. Evaluations were based on worker reports of tasks performed, moderated by fabrication-specific factors observed by study industrial hygienists during site visits.

Autocorrelation of Interday Exposures at an Automobile Assembly Plant

This article presents an analysis of interday autocorrelation in the exposure time series of workers in an automobile assembly plant. In all, serial 8-hour time-weighted average exposures for six workers, two each at three separate processes, were monitored for periods ranging from 20 to 110 consecutive days. Autocorrelation was assessed in each case. In general the results support the conclusions of previous investigations, in that relatively little autocorrelation was observed. However, uncertainties regarding the stationarity of the time series over the period of interest highlight potential difficulties in autocorrelation analysis. Particularly, nonrandom events in a plant, such as ventilation and process changes and extended down times, should be considered in any autocorrelation analysis inasmuch as these factors may affect the distributional parameters as well as the stationarity of the time series. In the workplace under study these nonrandom events appear to be more important in the overall assessment process than concern over within-week autocorrelation. Since exposure distributions vary over time, samples collected across the full range of procedures, activities, and environmental conditions that could influence exposures should be included in developing an exposure assessment strategy. Additionally, this study demontrates that formal statistical tests to assess stationarity should be used with care and should always be supported with visual investigation.