Methodological Study And Application Of Retrospective Occupational Exposure Assessment

Exposure assessment (EA) is qualitative or quantitative evaluation of exposure levels of chemicals or factors in one' s occupational history, which is an important component of occupational epidemiology. EA should have scientific methods to get correct and reliable results, reasonable dose-effect (dose-response) relationships, in order to protect workers' health and safety. Development of new EA methods is necessary with the increasing requirements on EA. Currently common methods include job-exposure matrix, expert subjective, physical model prediction, biological mark and so on, but there are some shortage in all of the methods, and it is necessary to futher improve. Expert subjective, for example, is prone to subjectivity, large variety of sensitivity and low repetition. In addition, there are many problems on EA which need to be solved, such as validation and comparison of results, choice of reasonable typical value and dose indexes. No agreements on those questions resulted in great differences of estimation for exposure levels and influence its unbias, validity and consistency.Benzene, a kind of widely used aromatic material, is a hazard to health. Studies showed that peripheral counts of white blood cell (WBC), red blood cell and platelet of workers exposed to benzene decreased significantly compared with that of the unexposed. Furthermore, low hemoglobin (Hb) concentrations were observed on subjects exposed to high level benzene in comparison with the unexposed. Inconsistent results are also reported on the adverse effect of benzene. The inconsistency is explainable giving that most studies had imprecise exposure assessment, or limited dose indexes, or incorrect typical value, or even the neglect of natural cycle of adverse effects.For the problems mentioned above, the present study assessed exposurelevels of subjects exposed to benzene from a rubber factory by data method, period method, model method on basis of historic measurements of area concentration. Personal exposure dose indexes included area concentration, time weighted average of 8 hours (TWA₈), yearly level, cumulative level. Results of area concentration and TWA₈ were compared and directly tested with field samples. Relationships between exposure levels and WBC, Hb were analyzed with mixed model. Accuracy of the 3 methods and choice of the best dose indexes were conducted by comparison of their information criteria values.Data method in present study meant that area concentrations were calculated by directly using historic measurements in association with similar exposure groups (SEGs) according to 2 level questionnaires on factory and tasks. If no data available for certain years, area concentrations were substitute by that of the nearest years. Period method took geometric means as area concentrations within similar exposure periods (SEPs) that were established on the basis of SEGs. Model method appraised area concentrations by the combination of historic measurements and expert objective with Bayesian statistics model. TWAh, yearly level and cumulative level were respectively assessed according to exposed hour daily and exposed month yearly.515 area concentration samples of historic measurements during past 20 years from 1964 to 2003 were collected, with geometric mean 11. 76mg/m3. 26292 person-months of 94 males and 53 females exposed to benzene, 14112 person-months of 40 males and 26 females unexposed, were observed.Results of the 3 methods showed that tasks of slurry-making, cloth-lining-assembling, slurry-applying had the highest area concentrations and TWA₈ of benzene. Averages of area concentrations of subjects exposed to benzene calculated by the 3 methods were 12. 19 mg/m3, 5. 10 mg/m3, 4. 26mg/m3respectively, and that of TWA₈ were 14. 75 mg/m3, 4. 69 mg/m3, 6.36mg/m3, yearly level 118.41 mg/m3. month, 72.9 mg/m3. month, 72. 63mg/m3. month, cumulative level 2371. 81 mg/m3. month, 783. 7 mg/m3. month, 835.14 mg/m3. month separately.Area concentration and TWA₈ of data method were significant higherthan that of period method and model method in most years. Furthermore, the former was more instable than the others. Numbers of area concentration of the 3 methods which were not obviously different with field measurements of 8 tasks were 2, 4, and 7 respectively and that of TWAh were 6, 6, and 7 compared with field measurements of 9 tasks. The results mentioned above suggested that there were less variety between model method output and field measurements than data method, and period method was medium.Mixed effect analysis on exposure level and WBC as the following were not able to achieve their convergence: area concentration and yearly level of data method; analysis on exposure levels and Hb as the following were not convergent: area concentration and TWA* of data method, cumulative levels of data method and period method. Analysis on all dose indexes of model method and WBC, Hb were convergent. It suggested that exposure assessment of model method fitted well, and that of data method and period method dispersed, fitted bad and was in need to be improved.The present results indicated that data method was practically simple and easy, but adequate historic measurements were in need; period method could save costs and be applicable to subjects of stable exposure and with certain measurements; model method was able to get precise assessment and fitted subjects with sparse instable measurements, but pertain information and profound experts were necessary.The study suggested that area concentration, TWA?, yearly level and cumulative level were all useful dose indexes because they all had dose effect relationships of WBC. Of all 4 dose indexes, TWA* had the smallest information criteria values, which were 4858.09, 4828.51, 4977.97, 5039.98 (according to -2 Restricted Log Likelihood of mixed analysis on TWA? and WBC). Therefore TWA? should be recommended as the most suitable dose indexes of benzene exposure.The finding in the present study indicated that there were significant negative correlations between benzene exposure levels and peripheral WBC. 3 coefficients of area concentration, TWA*, yearly level, cumulative level are-0.028> -0.022, -0.0019, -0.000076 (according to model method).