Classification of worker exposures

The role of exposure reconstruction in occupational human health risk assessment: current methods and a recommended framework

Exposure reconstruction for substances of interest to human health is a process that has been used, with various levels of sophistication, as far back as the 1930s. The importance of robust and high-quality exposure reconstruction has been recognized by many researchers. It has been noted that misclassification of reconstructed exposures is relatively common and can result in potentially significant effects on the conclusions of a human health risk assessment or epidemiology study. In this analysis, a review of the key exposure reconstruction approaches described in over 400 papers in the peer-reviewed literature is presented. These approaches have been critically evaluated and classified according to quantitative, semiquantitative, and qualitative approaches. Our analysis indicates that much can still be done to improve the overall quality and consistency of exposure reconstructions and that a systematic framework would help to standardize the exposure reconstruction process in the future. The seven recommended steps in the exposure reconstruction process include identifying the goals of the reconstruction, organizing and ranking the available data, identifying key data gaps, selecting the best information sources and methodology for the reconstruction, incorporating probabilistic methods into the reconstruction, conducting an uncertainty analysis, and validating the results of the reconstruction. Influential emerging techniques, such as Bayesian data analysis, are highlighted. Important issues that will likely influence the conduct of exposure reconstruction into the future include improving statistical analysis methods, addressing the issue of chemical mixtures, evaluating aggregate exposures, and ensuring transparency with respect to variability and uncertainty in the reconstruction effort.

Epidemiologic evidence for chloroprene carcinogenicity: review of study quality and its application to risk assessment

This article evaluates the quality and weight of evidence associated with epidemiologic studies of cancer among occupational cohorts exposed to chloroprene. The focus is on liver, lung, and lymphohematopoietic cancers, which had been increased in early studies. Literature searches identified eight morbidity/mortality studies covering seven chloroprene-exposed cohorts from six countries. These studies were summarized and their quality was assessed using the 10 criteria suggested by the U.S. Environmental Protection Agency. The limitations within this literature (primarily the early studies) included crude exposure assessment, incomplete follow-up, uncertain baseline rates, and uncontrolled confounding by factors such as smoking, drinking, and co-exposure to benzene and vinyl chloride. Four cohorts were studied by the same group of investigators, who reported no overall increased associations for any cancers. This four-cohort study was by far the most rigorous, having the most comprehensive exposure assessment and follow-up and the most detailed documentation. This study also contained the two largest cohorts, including an American cohort from Louisville, Kentucky, that ranked at or near the top for each of the 10 quality criteria. There was evidence of a strong healthy worker effect in the four-cohort study, which could have hidden small excess risks. Small increased risks were suggested by internal or company-specific analyses, but these were most likely caused by uncontrolled confounding and low baseline rates. Overall, the weight of evidence does not support any substantial link between chloroprene exposure and cancer, but inconsistencies and a lack of control for major confounders preclude drawing firmer conclusions.

Mortality patterns among workers exposed to arsenic, cadmium, and other substances in a copper smelter

OBJECTIVE

To evaluate the long-term mortality experience of workers exposed to arsenic, cadmium, and other substances at a copper mine and smelter in Copperhill, Tennessee studied earlier as part of an industry-wide study.

METHODS

Subjects were 2,422 male workers employed three or more years in the smelter or mill between 1/1/46 until the plant strike and scale-down of operations in April 1996. Vital status was determined through 2000 for 99.4% of subjects and cause of death for 91.3% of 878 deaths. Historical exposures were estimated for lead, SO(2), arsenic, cadmium, dust, and cobalt. We computed standardized mortality ratios (SMRs) based on U.S. and local county rates and modeled internal relative risks (RRs).

RESULTS

We observed overall deficits in deaths based on national and local county comparisons from all causes, all cancers and most of the cause of death categories examined. We found limited evidence of increasing mortality risks from cerebrovascular disease with increasing duration and cumulative arsenic exposure, but no evidence of an exposure-response relationship for cadmium exposure and bronchitis.

CONCLUSIONS

Our limited evidence of an association between inhaled arsenic exposure and CVD is an exploratory finding not observed in other epidemiology studies of more highly exposed occupational populations. Possible alternative explanations include chance alone and uncontrolled confounding or effect modification by co-exposures or other factors correlated with arsenic exposure and unique to the Copperhill facility.

Mortality patterns among industrial workers exposed to chloroprene and other substances

We conducted an historical cohort study to investigate the mortality experience of industrial workers potentially exposed to chloroprene (CD) and other substances, including vinyl chloride (VC), with emphasis on cancer mortality, including respiratory system (RSC) and liver. In 1999, the International Agency for Research on Cancer (IARC) classified CD as a possible carcinogen (Group 2B); VC was classified in 1987 as a known human carcinogen (Group 1). Subjects were 12,430 workers ever employed at one of two U.S. industrial sites (Louisville, KY (n=5507) and Pontchartrain, LA (n=1357)) or two European sites (Maydown, Northern Ireland (n=4849) and Grenoble, France (n=717)), with earliest CD production dates ranging from 1942 (L) to 1969 (P). Two sites (L and M) synthesized CD with the acetylene process that produced VC exposures. We determined vital status through 2000 for 95% of subjects and cause of death for 95% of the deaths. Historical exposures for individual workers were estimated quantitatively for CD and VC. Workers ever exposed to CD ranged from 92.3% (M) to 100% (G); to VC from 5.5% (M) to 22.7% (L). We computed standardized mortality ratios (SMRs) (using national and regional standard populations) in relation to selected demographic, work history and exposure factors. We used worker pay type (white or blue collar) as a rough surrogate for lifetime smoking history. For the combined cohort, SMRs (95% CIs) for all causes combined, all cancers combined, RSC and liver cancer were, respectively, 0.72 (0.69-0.74), 0.73 (0.68-0.78), 0.75 (0.67-0.84) and 0.72 (0.43-1.13). Site-specific (L, M, P and G, respectively) SMRs were: for all cancers combined: 0.75 (0.69-0.80), 0.68 (0.56-0.80), 0.68 (0.47-0.95) and 0.59 (0.36-0.91); for RSC: 0.75 (0.66-0.85), 0.79 (0.58-1.05), 0.62 (0.32-1.09) and 0.85 (0.41-1.56); for liver cancer: 0.90 (0.53-1.44) (17 deaths), 0.24 (0.01-1.34) (1 death), 0.0 (0-2.39) (no deaths) and 0.56 (0.01-3.12) (1 death). Among all workers ever exposed to CD, SMRs were: for all cancers combined: 0.71 (0.66-0.76); for RSC: 0.75 (0.67-0.84); for liver cancer: 0.71 (0.42-1.14). We also observed no increased mortality risks among cohort subgroups defined by race, gender, worker pay type, worker service type (short/long term), time period, year of hire, age at hire, duration of employment, the time since first employment, and CD or VC exposure status (never/ever exposed). In summary, our study has many strengths and is the most definitive study of the human carcinogenic potential of exposure to CD conducted to date. We conclude that persons exposed to chloroprene or vinyl chloride at the levels encountered in the four study sites did not have elevated risks of mortality from any of the causes of death examined, including all cancers combined and lung and liver cancer, the cancer sites of a priori interest. This conclusion is corroborated by our detailed analyses of mortality in relation to qualitative and quantitative exposures to CD and VC at each of the four study sites, reported in our companion paper (Marsh et al., submitted for publication).

Chemical process based reconstruction of exposures for an epidemiological study

In the occupational hygiene component of occupational epidemiological studies the goal is to assign group average exposure levels that can be used to compute individual cumulative exposures. This task requires the availability of sufficient amounts of proper individual exposure level data. Typically, the required data are either sparse, completely lacking or happenstance data collected for purposes not suitable for the aims of the study. In the epidemiological study of mortality patterns among industrial workers exposed to chloroprene and other substances, we developed and used a process analysis and modeling based exposure reconstruction to augment, extrapolate, or interpolate the available exposure data. The models developed utilize equations based on the engineering principles and chemistry associated with the processes as determined from the process documentation and task performance habits as determined from interviews of knowledgeable personnel. The resulting equations are tractable and provide a general basis for calculating exposure levels for vapors. The validation of the results with available exposure measurements suggests that comprehensive process analysis and modeling may be used to reconstruct exposures or to evaluate exposure potential with scientifically defensible methods. Furthermore, even in the absence of validating data, the methodology developed has potentially very useful applications in predicting exposure levels to newly synthesized substances. Properly interpreted, the limitations of modeling can be minimized to obtain scientifically reasonable results.

Mortality patterns among industrial workers exposed to chloroprene and other substances

As part of an historical cohort study to investigate the mortality experience of industrial workers exposed to chloroprene (CD) and other substances, including vinyl chloride monomer (VC), we analyzed mortality from all cancers combined, respiratory system (RSC) and liver cancer in relation to CD and VC exposures. Subjects were 12,430 workers ever employed at one of two U.S. sites (Louisville, KY (n=5507) and Pontchartrain, LA (n=1357)) or two European sites (Maydown, Northern Ireland (n=4849) and Grenoble, France (n=717)). Historical exposures for individual workers were estimated quantitatively for CD and VC. For sites L, M, P and G, respectively, average intensity of CD exposures (median value of exposed workers in ppm) were 5.23, 0.16, 0.028 and 0.149 and median cumulative exposures (ppm years) were 18.35, 0.084, 0.133 and 1.01. For sites L and M, respectively, average intensity of VC exposures (median value of exposed workers in ppm) was 1.54 and 0.03 and median cumulative exposures (ppm years) were 1.54 and 0.094. We performed relative risk (RR) regression modeling to investigate the dependence of the internal cohort rates for all cancers combined, RSC and liver cancer on combinations of the categorical CD or VC exposure measures with adjustment for potential confounding factors. We categorized exposure measures into approximate quartiles based on the distribution of deaths from all cancers combined. We also considered 5- and 15-year lagged exposure measures and adjusted some RR models for worker pay type (white/blue collar) as a rough surrogate for lifetime smoking history. All modeling was site-specific to account for exposure heterogeneity. We also computed exposure category-specific standardized mortality ratios (SMRs) to assess absolute mortality rates. With the exception of a one statistically significant association with duration of exposure to CD and all cancers combined in plant M, we observed no evidence of a positive association with all cancers, RSC or liver cancer and exposure to CD and/or VC using both the unlagged and lagged exposure measures: duration, average intensity or cumulative exposure to CD or VC; time since first CD or VC exposure; and duration of CD exposure or time since first CD exposure in presence or absence of VC exposure. We observed elevated and statistically significantly elevated RRs for some analysis subgroups, but these were due to inordinately low death rates in the baseline categories. With the possible exception of all cancer mortality in plant G, our additional adjustment of RRs for pay type revealed no evidence of positive confounding by smoking. We conclude that exposures to CD or VC at the levels encountered in the four study sites do not elevate mortality risks from all cancers, RSC or liver cancer. This conclusion is corroborated by our analysis of general mortality patterns among the CD cohort reported in our companion paper [G. Marsh, A. Youk, J. Buchanich, M. Cunningham, N. Esmen, T. Hall, M. Phillips, Mortality patterns among industrial workers exposed to chloroprene and other substances. I. General mortality patterns, Chem.-Biol. Interact., submitted for publication].

International Symposium on the Evaluation of Butadiene and Chloroprene Health Risks

These proceedings represent nearly all the platform and poster presentations given during the International Symposium on Evaluation of Butadiene and Chloroprene Health Risks, held in Charleston, South Carolina, USA, on September 20-22, 2005. The Symposium was attended by 78 participants representing private industry (37), academia (21), government (11), not-for-profit organizations (5), and consulting (4). The program followed the format of previous symposia on butadiene, chloroprene, and isoprene in London UK (2000) and butadiene and isoprene in Blaine, Washington USA (1995). This format enabled the exchange of significant new scientific results and discussion of future research needs. Isoprene was not evaluated during the 2005 Symposium because of lack of new data. For background information, the reader is referred to the proceedings of the London 2000 meeting for a thorough historical perspective and overview of scientific and regulatory issues concerning butadiene, chloroprene, and isoprene [Chem.-Biol. Interact. (2001) 135-136:1-7]. The Symposium consisted of seven sessions: (1) Introduction and Opening Remarks, (2) Butadiene/styrene-butadiene rubber (SBR)--Process Overview, Exposure and Health Effects/Human Studies; (3) Chloroprene--Process Overview, Exposure and Health Effects/Human Studies; (4) Mode of Action/Key Events; (5) Risk Assessment; (6) Poster Presentations; and (7) Panel Discussion and Future Directions. The Symposium concluded with a discussion by all participants of issues that arose throughout the course of the Symposium. The Proceedings of the Symposium published in this Special Issue are organized according to the Sessions outlined above. The purpose of this foreword is to summarize the presentations and their key findings and recommend future research directions for each chemical.

Chemical process-based reconstruction of exposures for an epidemiological study. Part II. Estimated exposures to chloroprene and vinyl chloride

In a four-facility occupational epidemiology study of chloroprene monomer and polymer production workers, the chloroprene (CD) and vinyl chloride monomer (VCM) exposures were modeled for plant specific job title classes. In two facilities an acetylene-based process was used and in the other two plants only a butadiene-based process was used in the monomer synthesis. In the Acetylene process VCM was an undesirable by-product to be removed. In the newer butadiene-based process, VCM was not involved and the exposures to CD were considerably lower than they were in the earlier years. One of the limiting factors was the operator rotation within a number of job titles. This rotation and inability to differentiate between job titles subsumed in job classifications recorded in the work histories required an exposure classification scheme based on an order of magnitude separation of exposure classes. In the four facilities with considerable variation in the mix of the production methods, the CD exposures were remarkably similar in both calculated and measured values. The reductions in exposures were much more dependent upon the improvement of the production methods, rather than deliberate exposure control for occupational hygiene considerations. This is reasonable since the exposures were generally lower than the coeval exposure limits and/or guidelines. The estimated exposures were less than 100 ppm in the pre-1960 era and less than 10 ppm in the 1960-1980 era, less than 1 ppm 1980-1990 era and less than 0.5 ppm thereafter. The exposures were categorized in four classes for VCM and six classes for CD. The characteristic class exposure values were used to cumulate individual exposures over time with a quantification of the potential range for exposures that are reasonably certain to ascribe correct ranking to job classes.