Relation of Occupational Exposure to Respiratory Symptoms and Asthma in a General Population Sample: Self-reported versus Interview-based Exposure Data
Per Sigvald Bakke,
Rolf Hanoa and
Amund Gulsvik
From the Department of Thoracic Medicine, University of Bergen, Bergen, Norway.
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ABSTRACT
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The objective of this study was to examine how the consistency of self-reported exposure to dust or gas, asbestos, and quartz varied between subjects with and those without respiratory symptoms and asthma in a Norwegian community sample (
) in 19871988. Exposure characterization obtained in a structured work history interview was used as the "gold standard." The authors also wanted to assess how the exposure-disease relation differed when the exposure was based on self-reported versus interview-obtained data. The prevalence of self-reported exposure to dust or gas, asbestos, and quartz was 28%, 5%, and 4%, respectively. The sensitivity of the self-reported exposure data varied from 21% to 64% and was higher in those with than in those without the respiratory disorders. The specificity varied from 78% to 100% and was lower in those with than in those without the respiratory disorders. The sex-, age-, and smoking-adjusted odds ratios of the respiratory disorders in those with exposure to dust or gas and to asbestos were only slightly reduced when misclassification was taken into account. The corresponding numbers for exposure to quartz were halved and lost their statistical significance when the misclassification was allowed for. In this general population sample, the self-reported occupational, airborne exposure data were differentially misclassified by disease status. Am J Epidemiol 2001;154:47783.
asthma; occupational exposure; respiration disorders
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INTRODUCTION
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During the last decade, several community studies examined the relation between occupational airborne exposure and various indices of obstructive lung disease (1





8
). Whereas information on the disease variables has been validated and standardization projects for providing the disease data have been established (9
), limited population-based data are available on the validity of the exposure information (10
) and how the consistency of the exposure data may effect the exposure-disease relation.
Most of the occupational exposure data obtained in the community studies on obstructive lung disease are self-reported. One could speculate that at least part of the observed exposure-disease relation could be due to reporting bias by overreporting of exposure in subjects with obstructive lung disease compared with those without the disease.
The objective of this report from a Norwegian general population survey was to assess the consistency of self-reported exposure to dust or gas and to asbestos and quartz in subjects with and those without respiratory symptoms and asthma. Finally, we wanted to assess how the exposure-disease relation changed by using self-reported exposure data compared with using structured interview-based data.
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MATERIALS AND METHODS
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Population
The methods of selection and characteristics of this population have been presented in detail elsewhere (11
, 12
). Briefly, the study was a two-phased, cross-sectional survey conducted in 19871988. The first phase was a questionnaire survey of a 1.9 percent random sample (n = 4,992) of the general population of the county of Hordaland on the southwest coast of Norway. The response rate was 90 percent of the sample. The questionnaire included queries on smoking habits, past or present occupational dust or gas exposure, respiratory symptoms, and a physician's diagnosis of asthma or emphysema. The eligible subjects for the second phase of the study were those living in the city of Bergen, Norway, and the 11 surrounding municipalities and included 3,370 of those who responded to the questionnaire survey. From the eligible subjects, a random sample of 91 percent of those who stated on the questionnaire that they had received a physician's diagnosis of asthma or emphysema and a random sample of 22 percent of those who denied having received such a diagnosis were invited to the Outpatient Chest Clinic at the University of Bergen. There were 714 subjects who responded to the invitation, representing 84 percent of those invited. The response rate was 85 percent for those with and 84 percent of those without asthma or emphysema.
The respondents to the first phase were found to be representative of the general population in terms of sex, age, smoking habits, and respiratory symptoms (11
). The respondents and those who failed to respond to the second phase were comparable with regard to sex, smoking habits, prevalence of asthma, and self-reported occupational dust or gas exposure (12
). The respondents to the second phase were 5 years older than those who did not respond. However, a response rate of 100 percent would have only changed the mean age of the respondents from 42 to 41 years (12
).
Respiratory symptoms and physician's diagnosis of asthma
The information on the respiratory symptoms and a physician's diagnosis of asthma was recorded by using a Norwegian respiratory symptom questionnaire. The wording of the questions used in these analyses was as follows: "Do you usually cough or clear your throat in the morning?", "Do you have a cough for three months or more altogether during a year?", "Do you have phlegm when coughing?", "Are you breathless when you climb two flights of stairs at an ordinary pace?" (breathlessness grade 2), and "Do you ever have wheezing in your chest?" The preprinted alternatives for answers were yes and no. Asthmatics were defined as those who answered affirmatively to the question "Have you ever been treated by a physician or in hospital for asthma?" The respiratory disorder information acquired in this way has previously been validated against forced expiratory volume in one second and bronchial responsiveness to methacholine (13
).
Exposure characterization
The self-reported, occupational, airborne exposure was based on the following questions (14
): "Have you ever had a working place with much dust or gas in the air?", "Have you ever been exposed to asbestos dust in your work?", and "Have you ever been exposed to quartz dust or stone dust with quartz at work?" The preprinted alternatives for answers were yes and no.
In the structured occupational interview, the subjects were first asked to state all jobs held since leaving school that lasted more than 6 months and to state their working tasks on each job. They were then interviewed by an occupational physician about their exposure to various airborne pollutants on each job, including exposures to asbestos and quartz. For each job, the occupational physician stated whether the subject had been exposed to any airborne pollutant. All subjects who were coded as having been exposed to an airborne pollutant for at least 6 months according to the occupational physician were characterized as exposed on the basis of the structured interview. The occupational physician did not know the subjects answers to the postal questionnaire regarding the respiratory disorders and the occupational exposures.
Smoking habits were divided into five groups: nonsmokers; ex-smokers; and smokers of 19, 1019, and 20 or more cigarettes per day. Nonsmokers were defined as subjects who had never smoked daily. Ex-smokers were subjects who had smoked daily, but had given up smoking. Subjects were classified as smokers if they smoked daily at the time of the study.
Data analysis
The prevalence estimates given in table 1 are representative of the community because they are corrected for the stratification as in a two-phased sampling procedure (15
). Hence, when the prevalence estimates are calculated, each person is weighted according to the sample fraction of the stratum to which he or she belongs. Frequencies were compared by using chi-square statistics. Unpaired t tests were used to compare mean values. Logistic regression analyses were applied to examine the relation of the occupational airborne exposures to asthma and respiratory symptoms after adjustment for sex, age, and smoking habits. For all analyses, a significance level of p = 0.05 was used. All analyses were performed with the BMDP package (16
).
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RESULTS
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The main characteristics of this population are given in table 1. The prevalences of occupational airborne exposures based on the structured work history interview were 1.42.9 times higher than the self-reported prevalences, with the difference between the two assessment methods being larger for the specific exposures (asbestos and quartz) than for those that were unspecific (dust or gas) (table 1).
The overall sensitivity of self-reported dust or gas exposure was 65 percent, while the specificity was 88 percent. For the self-reported asbestos exposure, the sensitivity and specificity were 28 and 98 percent, respectively. The corresponding numbers for self-reported quartz exposure were 45 and 98 percent, respectively.
Both the self-reported and the interview-based dust or gas exposure were higher among those with respiratory disorders than among those without (table 2). The sensitivity of self-reported dust or gas exposure was higher for those with respiratory disorders compared with those without. These differences reached the level of significance for all of the disorders except breathlessness grade 2 and asthma (table 2). The specificity for self-reported dust or gas exposure was significantly lower for all those with respiratory disorders compared with those without except for morning cough. For breathlessness grade 2, the difference was of borderline significance (table 2).
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TABLE 2. Self-reported and interview-based occupational dust or gas exposure and sensitivity and specificity of the self-reported exposure in those with and those without respiratory symptoms in a Norwegian general population study, 19871988
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For asbestos, the sensitivity and specificity of the self-reported exposure did not vary significantly between those with and those without the respiratory disorders (table 3), with the only exception being chronic cough, for which the sensitivity of the self-reported exposure was significantly higher in those with than in those without the symptom (table 3).
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TABLE 3. Self-reported and interview-based occupational asbestos exposure and sensitivity and specificity of the self-reported exposure in those with and those without respiratory symptoms in a Norwegian general population study, 19871988
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Both self-reported and interview-based quartz exposure were 24 times higher in those with respiratory disorders than in those without (table 4). Except for breathlessness, the sensitivity of self-reported quartz exposure was significantly higher in those who were suffering from the respiratory symptoms and asthma than those who were not. The specificity of self-reported quartz exposure did not vary overtly between those who reported the disorders and those not did not (table 4).
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TABLE 4. Self-reported and interview-based occupational quartz exposure and sensitivity and specificity of the self-reported exposure in those with and those without respiratory symptoms in a Norwegian general population study, 19871988
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Table 5 shows the sex-, age-, and smoking-adjusted odds ratios for the respiratory disorders when occupational exposure was based on self-reporting and when exposure was interview based. For almost all of the exposure-disease relations, the adjusted odds ratios decreased when the exposure was interview based compared with when it was self-reported. The two exceptions were the relations between morning cough and dust or gas exposure and those between morning cough and asbestos exposure. The reductions in odds ratios tended to be higher for quartz than for asbestos and dust or gas. For quartz exposure, half of the significant, self-reported exposure disease relations lost their level of significance when exposure was interview based (table 5). For dust or gas exposure and asbestos exposure, none of the self-reported exposure-disease relations lost their significance level when exposure was interview based. No interaction between smoking and the occupational exposures and the respiratory symptoms and asthma was observed, when using either self-reported or interview-based exposure data.
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TABLE 5. Adjusted* odds ratios with 95% confidence intervals for respiratory symptoms and asthma by occupational airborne exposure as obtained through self-reporting and through a structured interview, Norwegian general population study, 19871988
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DISCUSSION
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This report from a Norwegian community study showed that self-reported occupational airborne exposure data compared with data obtained in a structured job history interview had a higher sensitivity and a lower specificity in subjects with respiratory disorders than in those without. The sex-, age-, and smoking-adjusted relations of dust or gas and asbestos exposure were only slightly changed when using self-reported versus interview-based exposure data. For quartz exposure, the exposure-disease relations were markedly weakened when exposure data were applied based on a structured work history interview instead of on self-reported data.
Before we discuss these findings in more detail, there are some methodological aspects that should be considered. First, the structured work history interview was used as a "gold standard." These interview data are not necessarily correct. Another expert might have reached another exposure characterization (17
). However, an expert assessment is regarded as the best alternative of exposure characterization in population-based retrospective studies (18
). Exposure data from large workplaces might have been used as a gold standard for previous exposure. However, because this study covered the entire working history of a general population sample, these record data would probably have been available for only some of the study participants. We have no data on any use of protective breathing equipment or on whether those who used it have taken that into account when stating their self-reported exposure. Second, the self-reported occupational exposure was based on questions about ever being exposed, while the expert coded the exposure on a job-specific basis. If we had also asked for job-specific exposure on the self-administered questionnaire, the sensitivity would probably have been higher and the specificity lower than that observed in this analysis. The expert was blinded to the subjects' self-reported statements both on occupational exposure and on respiratory symptoms and asthma.
Third, response bias could have influenced our results. However, an overall response as high as 90 percent in the first phase and 84 percent in the second phase would tend to reduce the effect of any response bias. Furthermore, no significant difference in self-reported dust or gas exposure was observed between responders and nonresponders to the second phase (12
).
Fourth, the observed relations between self-reported exposure and respiratory symptoms might have been flawed not only with regard to recall bias of the exposure, but also with regard to recall bias of the symptoms. Exposed subjects might have overreported respiratory symptoms (type 2 bias) (19
). However, when the subjects completed the questionnaires in the first phase of our study, they were first asked about respiratory symptoms and asthma, and at the end of the two-sheet questionnaire, they were asked about occupational airborne exposure (20
). This should reduce any type 2 recall bias. The generalizability of our findings may be restricted to communities with the same industrial structures as those in our study area.
To our knowledge, this is the first general population study to examine how the consistency of self-reported occupational airborne exposure data may differ with respiratory symptom status and how the relation between self-reported exposure and respiratory disorders might vary when this consistency is taken into account. In a French community study (21
) that included 13,500 subjects, self-reported exposures to dusts, gases, or chemical fumes were compared with data obtained by a job exposure matrix. However, this study was restricted to nonmanual workers. With this reservation in mind, the French study also observed that the relation between exposure and respiratory symptoms was higher with the self-reported assessment compared with using the job exposure matrix. Case-control studies on nonrespiratory diseases have not found any important differences in the accuracy of self-reported exposure between cases and controls (17
, 22
).
In occupational groups, several studies have examined the validity of self-reported exposure to airborne pollutants (23
25
). However, there are few studies that have examined how this validity assessment influences the risk estimates of disease due to exposure (26
).
Several of the community surveys that have examined the relation between occupational airborne exposure and respiratory disorders (1
, 3
, 4
, 6
, 7
) have assumed that the exposure misclassification is nondifferential and state that, consequently, any bias in the exposure-disease relation is toward the null. This statement is not necessarily correct (27
), and our study indicates that the assumption may be wrong.
Our finding that sensitivity is higher and specificity is lower in symptomatic subjects compared with those without symptoms may be due to recall or reporting bias. Healthy subjects may tend to remember and report only a high degree or a long duration of exposure to a greater extent than would diseased subjects, while the latter group may tend to overreport low-degree or short-time exposure. When this misclassification is corrected for, a reduction in the odds ratios is to be expected. We observed that the reduction in the odds ratios for the association of quartz with the respiratory disorders tended to be stronger than that of asbestos with the disorders (table 5). This could be due to the above-mentioned recall and reporting bias being stronger for quartz exposure than for asbestos exposure. Another explanation could be that subjects who falsely reported no quartz exposure had experienced a lesser degree or a shorter duration of exposure than subjects who falsely reported no asbestos exposure. However, interpretation of the degree of the reduction in the adjusted odds ratios in table 5 should be cautious because of the wide confidence intervals of the estimates.
To reduce the potential problem of reporting or recall bias in occupational exposure characterization in community studies, interview by experts or experts coding questionnaire information may be used (18
, 28
). However, this is a much more expensive and time-consuming method than use of questionnaire information only, and the method may be subject to observer bias and misclassification (29
). To our knowledge, the method has not been tried in surveys on general population samples but rather has been used for community-based case-control studies (17
). Another alternative is the use of job-exposure matrices in which job titles are coded into specific exposures. The method is cheaper than interview-based exposure characterization. The job titles are probably less subject to differential misclassification by disease than is self-reported exposure. However, misclassification may occur due to coding errors or unspecific coding of the job titles (30
). The job exposure matrices may not be designed for the population and disease to be examined. The latter problem may be avoided in part by using a population-specific rather than an external job exposure matrix (28
).
In conclusion, this study indicates that self-reported occupational exposure characterization may be misclassified by disease status in general population surveys of respiratory disorders when exposure information is compared with that obtained in a structured work history interview. However, after this misclassification is taken into account, a significant sex-, age-, and smoking-adjusted exposure-disease relation still persisted.
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ACKNOWLEDGMENTS
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Supported by the Royal Norwegian Council for Scientific and Industrial Research, the Norwegian Research Council for Science and the Humanities, and the Norwegian Asthma and Allergy Association.
The authors thank Lene Svendsen and Bjørg Meidell for technical assistance during the survey and Valborg Baste for preparing the database for analysis.
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NOTES
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Correspondence to Dr. Per Sigvald Bakke, Department of Thoracic Medicine, N-5021 Haukeland University Hospital, University of Bergen, Bergen, Norway.
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Received for publication August 8, 2000.
Accepted for publication January 17, 2001.