From the Division of Respiratory Disease Studies, National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, Morgantown, WV.
Received for publication November 21, 2001; accepted for publication May 28, 2002.
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ABSTRACT |
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airway obstruction; occupational diseases; occupational exposure; prevalence
Abbreviations: Abbreviations: CI, confidence interval; COPD, chronic obstructive pulmonary disease; FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity; NHANES III, Third National Health and Nutrition Examination Survey; OR, odds ratio; RR, relative risk.
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INTRODUCTION |
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The objective of the present study was to analyze the NHANES III survey data to estimate the prevalence, prevalence odds ratios, and attributable fractions for COPD and employment by specific industry sectors (industry) and occupational categories (occupation). The aim was to estimate the magnitude of the effect of occupational exposure on COPD in the US population and to identify industries and occupations at increased risk of COPD that can be targeted for etiologic studies or disease prevention programs.
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MATERIALS AND METHODS |
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The present study included subjects aged 3075 years (n = 11,447). These age cutoff points were chosen under the assumption that COPD due to occupational exposure would not generally be observed before midlife (20). In addition, many subjects less than age 30 years are still reaching their full lung function potential (21), and many older subjects develop age-related medical conditions that can affect lung function. Furthermore, we excluded 1,009 subjects because of lung function testing problems (missing tests (n = 826), unreliable flow volume curves (n = 97), or only one reliable maneuver (n = 86)), 506 subjects with physician-diagnosed current asthma, and 109 subjects for whom occupational codes were missing, leaving 9,823 subjects for the data analysis.
Definition of COPD
The definition of COPD was based on Global Initiative for Chronic Obstructive Lung Disease (GOLD) working group criteria (20) of forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) less than 70 percent and FEV1 less than 80 percent predicted. The FEV1 percent predicted values were calculated by using sex-specific reference equations for Caucasians, African Americans, and Mexican Americans, estimated from tests of asymptomatic nonsmokers in the NHANES III study (21). Lung function testing methods for the NHANES III study conformed to the 1987 American Thoracic Society acceptability and reproducibility criteria (22).
Cigarette smoking
The NHANES III survey included questions on current smoking status, number of cigarettes smoked, and age at which smoking started and stopped. Two smoking variables were created: smoking status and pack-years. For smoking status, subjects were categorized as never smokers, former smokers, or current smokers. Pack-years were estimated by multiplying the current number of packs of cigarettes smoked (or, for former smokers, the number of packs smoked when they last smoked) by the number of years of smoking. Never smokers were defined as those who smoked fewer than 100 cigarettes during their entire life.
Occupational coding
Occupational coding was based on two items on the NHANES III Adult Survey Questionnaire. That is, study subjects were asked to identify 1) the type of work they performed for the longest period of time (occupation) and 2) for this occupation, the type of business or industry they worked in the longest (industry). In the NHANES III data set, occupation was coded according to the 1990 Occupational Classification System used in the 1990 US Census (23). These codes were derived from the 1980 Standard Occupational Classification codes published by the US Department of Commerce (24). Industry was coded according to the 1990 Industrial Classification System used in the 1990 US Census, a modification of the Standard Industrial Classification codes (25). These detailed industry and occupation codes are collapsed in the NHANES III public use data set.
Industry categories used in the analysis
The NHANES III data set contains 44 industry and 40 occupation codes that include workers with a varied potential for exposure to respiratory hazards. A priori, we grouped industries in which workers had a similar potential for exposure to respiratory hazards believed to cause COPD as follows. We kept the original industry categories, but subjects with office work occupations were excluded from the industrial sectors. Office workers from all industry sectors were combined into a single category that was used as a comparison group. This "office workers" category included managers, secretaries and typists, clerks, and administrative workers as well as teachers, writers and artists, professionals, and workers from such nonmanufacturing sectors as banking, insurance, communications, education, social service, and public administration. We moved laborers, cleaners, material handlers, and maintenance workers from the banking, insurance, communications, education, social service, and public administration sectors into a new industry category called "office building services." Similarly, we moved construction, horticultural, medical, food preparation, and transportation workers from the nonmanufacturing sectors to the appropriate industry category with a similar potential for exposure to respiratory hazards.
This recoding resulted in 26 industry categories. For the final analysis, we combined categories in which odds ratios for COPD were not increased into one category of "other industries," which resulted in 16 final categories for the overall analysis.
Occupation categories used in the analysis
The NHANES III data set includes 40 occupation codes combined across all industries. A priori, we collapsed them into 25 occupational categories believed to have a similar potential for exposure to respiratory hazards and sufficient numbers for analysis. For the final analysis, occupational categories for which there was no evidence of an increased risk of COPD were combined in an "other occupations" category, resulting in 14 final categories.
Statistical data analysis
We used the SUDAAN software program (26) to estimate the weighted population size, prevalence, and prevalence odds ratios for COPD by industry and occupation. This software adjusts for the sampling design in calculating the variance estimates (27) and uses individual sampling weights assigned by the National Center for Health Statistics to adjust for the number of persons in the population that each sampled person represents. The weighted estimates are unbiased estimators of the US population parameters (19).
The SUDAAN logistic program was used to estimate odds ratios for COPD and industry/occupation adjusted for the effect of age, sex, race/ethnic group, body mass index, smoking status, pack-years of cigarette smoking, education, and socioeconomic status (20). Socioeconomic status was adjusted by using the poverty income index, computed as the ratio of the family income and the poverty threshold value specific to the family size. Industry (or occupation) was included in the models as a set of 0 or 1 dummy variables representing each industry (or occupation). Because of large numbers and a low prevalence of COPD, office workers were used as the baseline category against which other occupational groups were compared. Preliminary analyses revealed differences between sexes and among race/ethnic groups in the size of the adjusted odds ratios; therefore, we also conducted sex- and race/ethnic-specific analyses for COPD to better identify subgroups at risk of COPD.
In an unweighted analysis, we examined trends in adjusted odds ratios for COPD and increasing duration of employment in the longest-held occupation (ascertained by asking the following question: Considering all your employers, for how long did you do this kind of work?). For that purpose, occupations were grouped, according to the size of the estimated odds ratio for COPD, into high- and low-risk groups. In these groups, the trend with duration of employment (0, 114, 15 years) was estimated by using office workers as the "unexposed" comparison group.
The population attributable fraction (AFP; i.e., the fraction of cases with COPD due to industrial exposure) was calculated according to the following formula (2830):
(ORi 1)/ORi
where k = number of industry sectors; pci = casesi/total cases, where casesi represents the estimated number of COPD cases in the ith industry and total cases represents the total number of cases summed across industry sectors; and ORi = the adjusted odds ratio for COPD estimated for the ith industry.
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RESULTS |
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The sex-specific analyses showed that for females, the adjusted odds ratios for COPD (number of cases/number of subjects exposed) were increased for rubber, plastics, and leather manufacturing overall (8/46; OR = 4.7, 95 percent confidence interval (CI): 2.7, 8.3) and for never smokers in that industry (2/31; OR = 3.8, 95 percent CI: 1.2, 12.3); for the textile mill products manufacturing industry overall (10/99; OR = 2.4, 95 percent CI: 1.2, 4.8) and for never smokers in that industry (3/53; OR = 6.0, 95 percent CI: 0.6, 62); for the agriculture industry overall (13/196; OR = 2.0, 95 percent CI: 0.9, 4.5) and for never smokers in that industry (6/134; OR = 2.3, 95 percent CI: 0.5, 10.4); for sales overall (40/512; OR = 1.6, 95 percent CI: 0.9, 2.9) and for never smokers in that industry (8/279; OR = 3.0, 95 percent CI: 0.7, 13); for food products manufacturing overall (9/123; OR = 1.5, 95 percent CI: 0.6, 4.0) and for never smokers in that industry (2/77; OR = 3.4, 95 percent CI: 0.3, 44); and for personal services overall (34/470; OR = 1.5, 95 percent CI: 0.7, 3.2). For the rubber, plastics, and leather manufacturing industry, the detailed analysis showed that occupations in this industry contributing the most cases were textile apparel machine operators as well as fabricators and assemblers. For sales, a detailed analysis showed that the adjusted odds ratios were increased for Caucasian females (34/285; OR = 1.9, 95 percent CI: 1.0, 3.5).
Industries presented in table 3 that employed mainly males were utilities, the armed forces, repair services and gas stations, and transportation and trucking. However, use of the male office workers group as a referent resulted in odds ratios for COPD similar to those reported in table 3. The prevalence of COPD among the male referent group was 5.1 percent (standard error, 0.9); for the overall referent group, it was 4.7 percent (standard error, 0.4).
Race/ethnic-specific adjusted odds ratios for COPD for Caucasians were similar to the overall odds ratios shown in table 3. Industries associated with increased adjusted odds ratios for COPD (number of cases/number of subjects exposed) mainly for African Americans and Mexican Americans were office building services, agriculture, and construction (office building servicesAfrican Americans: 6/48; OR = 2.9, 95 percent CI: 0.8, 10.6; Mexican Americans: 4/24; OR = 2.1, 95 percent CI: 0.4, 9.9; agricultureAfrican Americans: 14/76; OR = 2.1, 95 percent CI: 0.9, 5.1; Mexican Americans: 22/339; OR = 1.8, 95 percent CI: 0.7, 4.6; constructionAfrican Americans: 15/140; OR = 1.6, 95 percent CI: 0.7, 3.6; Mexican Americans: 9/160; OR = 1.6, 95 percent CI: 0.5, 5.9). Industries for which the odds ratios were increased mainly for Mexican Americans were personal services (15/161; OR = 2.7, 95 percent CI: 0.8, 9.7) and health care (4/118; OR = 3.7, 95 percent CI: 1.3, 10.7). The personal services industry included hairdressers and cosmetologists.
COPD by occupational categories
The overall analysis identified 12 occupational categories with increased odds ratios for COPD. Table 4 shows the estimated number of US workers employed in these occupations, the prevalence of COPD, the sample size, the number of cases with COPD, and the adjusted odds ratios and 95 percent confidence intervals for COPD for all subjects and for never smokers. Occupations for which there was no evidence of increased odds ratios were combined into an "other occupations" category. An occupational category generally included workers from several industries, but some occupational categories were similar to industries (the armed forces and agriculture) and were included for completeness. Occupations that had the highest odds ratios were freight, stock, and material handlers; the armed forces; vehicle mechanics; and records processing and distribution clerks. In addition, among never smokers, the odds ratios were also high for other machine operators and for construction trades and laborers.
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Attributable fraction for COPD
The overall attributable fractions for industry and occupation were calculated from equation 1 by using the estimated population size, prevalence, and adjusted odds ratios provided in table 3 and table 4. No cases were attributed to industries in which ORi 1.
For each industry, table 3 shows the number of attributable cases. The total estimated working population aged 3075 years was 110,300,000. Of these persons, 7,652,390 (6.9 percent) were estimated to have COPD. The number of cases attributable to employment in industries, after adjustment for confounding, was 1,467,290, resulting in an attributable fraction of 1,467,290/7,652,390 x 100 = 19.2 percent. We also estimated the overall adjusted odds ratio for COPD and the 14 industries with OR > 1 grouped into one category as 1.5 (95 percent CI: 1.1, 2.0). Based on this odds ratio, the attributable fraction was estimated to be 19.6 percent (95 percent CI: 7.0, 28) (31). For never smokers, the estimated working population was 46,490,000, of whom 1,131,840 (2.4 percent) were estimated to have COPD. The number of attributable cases was 352,167, which resulted in an attributable fraction of 31.1 percent. For never smokers, the overall odds ratio for COPD and the industries with OR > 1 grouped into one category was 1.9 (95 percent CI: 0.9, 4.1), and the attributable fraction was estimated as 27.8 percent (95 percent CI: 1.0, 47).
For each occupation, table 4 shows the number of attributable cases. The number of cases of COPD attributable to employment by occupation was 1,150,900 of 7,642,230 total cases, resulting in an attributable fraction of 15.1 percent. (Thirty-three subjects for whom there was no job code were excluded, resulting in a slightly different weighted total number of cases from the industry.) The overall adjusted odds ratio for occupations with OR > 1 grouped into one category was 1.4 (95 percent CI: 1.0, 2.0), and the corresponding attributable fraction was estimated as 15.1 percent (95 percent CI: 3.4, 25.8). For never smokers, the number of attributable cases was 307,299 of 1,123,370 total cases, resulting in an attributable fraction of 27.4 percent. The overall adjusted odds ratio for occupations with OR > 1 grouped into one category was 2.1 (95 percent CI: 0.9, 5.0), and the attributable fraction was 25.6 percent (95 percent CI: 2.3, 28.5). Because the occupations were summed over industries and some of the industries with increased odds ratios were not represented by the occupational categories with increased risk, the attributable fractions for the occupations were lower than those for the industry.
Trends with duration of employment
The average duration of working in the longest-held occupation was 15.9 (standard error, 0.1) years. For the high-risk occupations (for all subjects, OR 1.5) shown in table 4, the odds ratios for COPD by duration of employment0, 114, and
15 yearswere 1.0, 1.4 (95 percent CI: 0.8, 2.6), and 1.7 (95 percent CI: 1.1, 2.5), respectively. The greatest trend was observed among the armed forces, where the respective odds ratios were 1.0, 1.4 (95 percent CI: 0.3, 6.1), and 2.0 (95 percent CI: 1.0, 4.0). For the lower-risk occupations (for all subjects, 1 < OR < 1.5) shown in table 4, the respective odds ratios were 1.0, 1.4 (95 percent CI: 1.0, 1.9), and 1.5 (95 percent CI: 1.2, 2.0). The results suggest a trend associated with working 15 or more years in risky occupations.
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DISCUSSION |
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The present study generated hypotheses aimed at 1) identifying industries associated with an increased risk of COPD that might be targeted for further research and 2) estimating the attributable fraction for occupational COPD. The estimated attributable fraction of 19 percent was based on increased odds ratios for COPD in 14 industries listed in table 3. Additional supporting evidence for an increased risk of COPD in these industries came from increased odds ratios for never smokers, for sex- and race/ethnic-specific sub- groups, and by occupational categories. In preliminary analyses in which a less stringent cutoff point of 75 percent was used for the FEV1/FVC ratio, we observed a higher degree of consistency among the three major race/ethnic groups for the adjusted odds ratios for industries and occupations. Use of Global Initiative for Chronic Obstructive Lung Disease criteria resulted in a reduced number of COPD cases and wider confidence intervals but also in decreased odds ratios for African Americans and, to a lesser degree, Mexican Americans. This result may be related to higher age-related mean FEV1/FVC values observed for the two race/ethnic groups in the NHANES III study (21).
An increased risk of COPD has been reported for many of the associations observed in the present study. Several general population-based studies reported associations with specific occupational exposures. A study conducted in Tucson, Arizona, found an association between airflow obstruction and self-reported exposure to silica, fiberglass, sawdust, Freon (E. I. du Pont de Nemours & Co., Inc., Wilmington, Delaware), automobile exhaust, solvents, and construction work (5). The US Six-City study found associations of chronic respiratory symptoms and COPD with working with dusts, fumes, and gases (6).
In addition, studies in Norway found significant associations between airflow obstruction and occupational exposure to quartz (1113), ammonia, nitrous gas, sulfur dioxide gas, metal fumes, and anhydrides (13). A study of New Zealand subjects aged 2044 years found associations between airflow obstruction (FEV1/FVC <75 percent) and working as cleaners, bakers, spray painters, laboratory technicians, and plastics and rubber workers and in construction and mining. In addition, shortness of breath was reported more frequently by hairdressers and chemical processors. Chronic bronchitis was reported more often in food processors, chemical processors, and spray painters and in construction and mining. In a large international study of subjects aged 2044 years, self-reported occupational exposure to vapors, gas, dust, or fumes was associated with chronic bronchitis in agricultural, textile, paper, wood, chemical, and food processing workers (15). In the Netherlands, the Zutphen study found an increased risk of nonspecific lung disease for textile workers and tailors (relative risk (RR) = 2.4), construction and cement workers (RR = 2.3), transportation workers (RR = 2.1), furnace workers (RR = 2.1), wood and paper workers (RR = 1.7), and farmers (RR = 1.6) (33). Industry-based studies of COPD have found an increased risk in textile mills (34), the rubber industry (35), mining, construction, chemical production, food products manufacturing, and agriculture (3639).
The US Work-Related Lung Disease Surveillance Report lists industries associated with significantly elevated proportional mortality ratios for COPD (40). In support of our findings, the proportional mortality ratios for taxicab services, trucking services, automotive and miscellaneous repair services, construction, gasoline service stations, and the military were among the highest. On the other hand, mining and potteries, which had the highest proportional mortality ratios listed in the Work-Related Lung Disease Surveillance Report, were not identified by the present study primarily because of limitations described in the paragraph that follows.
To our knowledge, previous epidemiologic studies have not reported an increased risk of COPD for industries such as utilities, the armed forces, office building services, sales, and health care and for occupations such as records processing clerks as well as freight, stock, and material handlers. (The records processing clerks category includes mail handlers.) The National Institute for Occupational Safety and Health conducted several Health Hazard Evaluations in mail processing and distribution plants in response to workers complaints of work-related upper and lower respiratory symptoms and asthma (41). In addition, an epidemiologic study found decrements in lung function associated with more than 10 years of high exposure to paper dust (42).
The strengths of the present study include reliable lung function data, a large sample size that allowed for unbiased estimation of the effect of the confounding factors, and varied industrial and job exposures. Application of the sampling weights resulted in odds ratios that were similar to those obtained from unweighted analyses. The limitations of the study are mainly due to the study design and include the following: 1) the NHANES III survey was not designed to be representative of US employment patterns, thus potentially obscuring the importance of some high-risk work; 2) industry and occupation categories were not always homogenous with respect to exposure to respiratory hazards; 3) lack of detailed industry/occupation codes undoubtedly resulted in some misclassification of exposure; and 4) the cross-sectional study design has limitations for the ascertainment of correct causal exposure.
In conclusion, the results of the present study show that employment in specific industries and occupations is associated with an increased risk of developing COPD, and they add further evidence that occupational exposures contribute significantly to the overall burden of COPD. Part of this burden may be due to the synergistic effect of tobacco smoking and occupational exposure (43). More research is needed to confirm that the newly identified industries and occupations are associated with an increased risk of COPD and to identify causal risk factors. To maximize the impact of this research on reducing the prevalence of COPD in the US population, etiologic research and preventive interventions should focus on industries that have high odds ratios and a large number of cases attributable to workplace exposure.
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NOTES |
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REFERENCES |
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