Results

RESPONSE

The initial response to participate in the study was 88% (326 respondents). Baseline spirometric tests were completed by 306 workers (85%). During the follow up 23 workers (7%) left the company and 43 workers (13%) had incomplete data on causes of sickness absence, primarily due to non-response on the short questionnaire during sickness absence. The total study population with complete 2 year follow up consisted of 251 people. A non-response analysis showed that the 66 workers who did not complete follow up had a higher prevalence of CNSLD complaints at baseline than those who completed the follow up, 33% and 17% respectively. Workers with incomplete sickness absence data (n=43) had a higher sickness absence both in frequency and total duration than the workers in the study population, although any differences in respiratory sickness absence could not be determined. The 66 workers lost to follow up did not differ in individual characteristics and working experience from those who completed the follow up.

BASELINE CHARACTERISTICS

Table 1 shows the basic characteristics of the 251 workers with complete data in the study population. The population consisted predominantly of middle aged men, although age ranged from 19 to 59. The occupational groups did not differ for individual characteristics and distributions of duration of employment in current and previous jobs. About 86% had had previous jobs and most of the previous positions held were comparable with the current occupation. Between welders and metal workers there was no significant difference in current smokers and years of smoking but there was a significant difference between office workers and the two blue collar groups. Ex-smokers (25.5%) stopped smoking, on average, almost 10 years before the study in all the groups.

The predominant respiratory complaint in the 12 months before the study was wheezing (18%) followed by chronic sputum secretion (9%), prolonged wheezing (9%), and chronic cough (8%) (table 2). Other complaints were less common; the same was true for work related symptoms. One out of four smokers had respiratory complaints. Welders had more complaints than the other groups but these differences did not reach conventional levels for significance except for chronic cough (χ² test, p=0.02) and chest tightness (χ²test, p=0.02). The prevalence of CNSLD was significantly higher in welders (χ² test, p=0.05) but this was not so for asthmatic complaints and COPD. Smoking was correlated with complaints like chronic cough and chronic sputum secretion but not with chest tightness and prolonged wheezing. This resulted in a significantly higher prevalence of COPD (χ² test, p=0.01) and CNSLD (χ² test, p=0.04) in smokers but this was not so for asthma.

SICKNESS ABSENCE

Among the 251 people with sufficient data during the study, 39 (15.5%) did not take any sick leave at all. During the follow up over 2 years, 212 (84.5%) workers were absent from work due to sickness for one or more periods, resulting in a total of 816 sickness absence periods ranging between 1 and 14 periods per person (mean=3.89). During the 2 year follow up the sickness absence ratio in the study population was 7.7% with non-specific respiratory complaints accounting for 14.2% of the days lost. Musculoskeletal disorders accounted for 43.8% of all days lost, influenza 19.0%, other causes 19.9%, and unknown causes 3.1%.

Table 3 shows the prevalence, frequency, duration, and ratio of sickness absence due to respiratory complaints. These results have been calculated for people with at least one sickness period due to CNSLD. During the 2 year follow up the proportion of workers that experienced at least one period of sickness absence as a result of asthma-like symptoms was 18%, COPD-like symptoms 31% and, in general from CNSLD-like symptoms 35%. The distribution of time at work before a first sickness absence due to these complaints differed significantly between the occupational groups.

As can be seen from the frequencies of sickness absence in table 3, several workers were absent more than once for the same complaint. The probability of a recurrent sickness absence within 1 year was significantly higher than the probability of a first sickness absence within 1 year. For asthmatic complaints a 1 year recurrence rate of 25% was found, for complaints about COPD 35%, and for complaints about CNSLD 43%.

The distribution of the duration of sickness absence did not differ among the specific disorders. The figure shows the return to work for workers who lost any time off work because of a first spell of respiratory complaints (between occupational groups). Almost no difference was found in rates of return to work between first periods and subsequent periods of sickness absence due to respiratory complaints. Most workers (67%) returned to work within 5 working days (1 week) and almost all (90%) returned within 10 days (about 2 weeks), although those with asthmatic complaints returned to work later (55% in the first 5 days and 87% in the first 10 days). Only in three cases (2%) did the duration of sickness absence exceed 20 days (1 month) and all came for workers with more than one absence.

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Time to return to work after a first period of sickness absence due to respiratory complaints

PROGNOSTIC FACTORS

Table 4 presents the factors associated with the occurrence of sickness absence due to respiratory complaints during the 2 year follow up. The presence of asthmatic complaints in the 12 months before the study was a significant predictor for sickness absence due to asthma-like symptoms (RR 3.96; 95% CI 1.99 to 7.90). However complaints about COPD and CNSLD were not associated with an increased probability of sickness absence during follow up. For the prognostic value of severe limitation of lung function, decrease in FVC (<80% of the reference value) contributed to the prediction of sickness absence due to asthma and CNSLD but not for COPD although the risk was increased (RR 1.77; 95% CI 0.71 to 4.42). The other indexes of lung function (FEV₁ and Tiffeneau index) were not significant predictors. In none of the multivariate models was age a significant risk factor. Smoking was not a significant factor for sickness absence from any cause. From the other individual characteristics only the duration of employment was associated with the probability of not taking sick leave (RR 0.94; 95% CI 0.91 to 0.97). This was true for all causes.

Table 5 shows the prognostic factors for return to work after a period of sickness absence. Metal workers and welders with COPD and CNSLD had a significantly decreased chance of returning to work quickly compared with the reference group despite the few office workers. Individual characteristics and work history were not associated with the probability of return to work.

Discussion

Data on the natural course of respiratory disorders and subsequent sickness absence are sparse.5-8 This study among blue collar and white collar workers showed that over a 2 year period about 35% of the workers attributed at least one episode of sickness absence to respiratory complaints. The impact of respiratory disorders on the workforce was also illustrated by the finding that these disorders accounted for 20% of all absence periods during the follow up but for 14.2% of the days lost. The 1 year rate of absence due to CNSLD was 21%, for COPD it was 17.5%, and for asthma it was 10.5%. These numbers comply well with the published information.2 8However, these studies were based on self reports rather than sickness absence registries and some bias may have occurred. As expected, there was also increased risk for recurrent periods of absence for CNSLD. The 1 year recurrence rates for particular respiratory sickness absences were about twice the 1 year period prevalences, indicating that workers with previous temporary disability from respiratory problems are at higher risk of subsequent sickness absence.

The prevalences of particular respiratory complaints in the 12 months before the baseline survey were between 5% (n=11) and 18% (n=44) in the total population (n=251). There were no significant differences between blue collar and white collar workers, and the prevalences were within the range of reported prevalences in occupational groups of blue collar workers.8 17 18 Of all workers with respiratory symptoms, about half reported only symptoms related to specific working conditions (20 out of 42). This is in agreement with published results.19 There was a substantial overlap between the asthma-like diagnosis and the COPD-like diagnosis, indicating that respiratory complaints do not distinguish sharply between these diagnoses. The lack of significant differences between blue collar and white collar workers may be due to the healthy worker effect. There was a difference between welders and metal workers which may reflect differences in work activities and working conditions. The non-response may also have contributed to the moderate prevalence of respiratory complaints because the analysis of non-response showed that in both groups sickness absence was higher in non-respondents than in the subjects participating in this study.

The well known role of smoking in the occurrence of respiratory complaints, was partially confirmed in the survey.20 21Current smoking was strongly associated with respiratory complaints only in metal workers. That was true especially for COPD-like complaints. There was a weaker association for welders and none for office workers. Former smokers resembled lifetime non-smokers, which could be explained by the finding that former smokers, on average, had stopped smoking for almost 10 years. The relation between age and respiratory complaints was ambiguous. Among the office workers there was a declining trend, whereas the blue collar workers had an increased prevalence with age. That was true especially for COPD. Interpretation of these patterns is difficult. A possible cause could be that the respiratory symptoms included asthma and chronic obstructive pulmonary disease, which have different associations with age. Asthma expresses itself generally at an early age, whereas chronic bronchitis arises more often in older subjects. More likely, different mechanisms of a healthy worker selection play an important part. Because age and duration of employment were strongly correlated, duration of employment had similar patterns of association with respiratory complaints.

In the regression analysis several factors were evaluated for their influence on the probability of occurrence of sickness absence due to respiratory disorders. Individual characteristics such as age, height, and weight were not predictive for respiratory disorders leading to absence from work. Current smoking did not affect absenteeism due to respiratory complaints. Duration of employment had a slight protective effect in absence due to respiratory complaints. Decreased FEV₁ was not predictive for respiratory absence, by contrast with the findings of other studies.5-8

The limited size of the study population hampers detailed analyses and also introduced large 95% CIs, which prevents risk factors from reaching significance. This was particularly the case in the analysis of factors associated with respiratory absence. Therefore, it was not feasible to investigate the influence of important factors such as overall health, morbidity, and work requirements on the sickness absence due to respiratory diseases.1

Workers with respiratory symptoms were absent more often due to respiratory symptoms than those without. This difference reached a conventional level of significance only for absence due to asthmatic complaints. These findings are not in full agreement with the few publications on this matter.5-8

Previous asthmatic complaints were very significant predictors of future sickness absence due to these complaints. Both complaints about CNSLD and work related CNSLD were also significant predictors of absence due to asthmatic complaints. Surprisingly, the occurrence of complaints about COPD and CNSLD in the 12 months before the study was not associated in a significant level with absence for these complaints. This contrasts with the common finding that a history of COPD or CNSLD is a strong risk factor for recurrent complaints and sickness absence.5-8 A possible explanation for this contradictory result might be that subjects whose complaints followed a less severe but unremitting chronic course were overrepresented among those lost to follow up. The non-response analysis showed that subjects who dropped out of the study during the follow up reported significantly more COPD during the baseline survey than those who completed the study.

Job titles were predictors for sickness absence for all kinds of respiratory complaints. This may reflect differences in work activities and working conditions between jobs. Other prognostic factors described in the scientific literature—such as poor working relations, management policy, and worker's avoidance behaviour,1 22are less likely to explain these results as these risk factors would have entertained a similar systematic effect on all three categories of respiratory complaints.

The available information on return to work after an episode of respiratory sickness absence is based on small numbers. Hence, the statistical analysis provided limited insight into possible prognostic factors for return to work. Moreover, the study population of welders and metal workers may lack sufficient contrast in exposure and health status to ascertain their impact on return to work. It should be borne in mind that the current study included all cases of respiratory absence, irrespective of the duration of absence and whether the subjects off work sought medical care. Within the framework of the study it was not feasible to conduct a clinical evaluation of every case at the onset of absence. Hence, the results in this longitudinal study among workers may differ from inception cohort studies based on groups of patients enrolled in general practice or compensation systems shortly after the onset of symptoms. Studies on patients with CNSLD have identified predictors for delayed return to work. These include several clinical variables of the complaint and effect modifiers—such as mental health, poor job satisfaction, and pursuit of compensation.22

The risks of becoming ill due to CNSLD were similar but less than the risks of delayed return to work. Only job title was a significant predictor for delayed return to work. By contrast with incidence of absence, metal workers had a higher risk of delayed return to work than welders or office personnel. No significant differences were found among the distributions of duration of sickness absence for particular respiratory complaints. On average, about 67% of the workers returned to work within 1 week (5 working days), and about 90% returned to work within 2 weeks (10 working days; figure). This high return to work within the first weeks suggests that any medical programme aimed at early intervention will most likely be only cost effective when the treatment has a very strong effect. The differences between blue collar and white collar workers were based on very small numbers. Although there was a small difference between the distributions of return to work after a first period of sickness absence and after a repeat period of sickness absence, it was not significant. Although the information on duration of recurrent episodes of sickness absence was based on only 39 events, this finding does not support the hypothesis that subjects off work for recurrent periods have passed through earlier stages of disability with less lost time. Among welders and metal workers, subjects with respiratory sickness absence were at increased risk of a recurrent episode but this second episode did not delay return to work. In this study it was not possible to distinguish between complete recovery from respiratory complaints and return to work, as the presence of residual respiratory complaints when resuming work was not ascertained. Perhaps full recovery and return to work are outcomes that should be differentiated. In this study return to work and recovery may have been similar as workers did not experience any cut in their regular wages within the first few weeks of sickness absence.

This longitudinal study has several limitations, most of them related to the inability to perform clinical assessments of the complaints underlying respiratory sickness absence. However, the results showed that a history of complaints about asthma and CNSLD in the 12 months before the study was associated with subsequent sickness absence for asthmatic complaints during the follow up. Also, workers with absence due to respiratory disorders were at higher risk of subsequent sickness absence in the next year. It is important to study temporal associations between the occurrence of respiratory complaints and subsequent disability. In this respect, health complaints, sickness absence, and return to work should be studied simultaneously in the same occupational populations.

Many journals, including the BMJ, have moved to a system of open reviewing, whereby authors know the names of reviewers of their papers. Research has shown that named reviews, although not of better quality than anonymous reviews, are not of worse quality either. Therefore in the interests of transparency, it seems fair to let authors know who has reviewed their paper. AtOccupational and Environmental Medicine we have considered the issue carefully. There are some concerns that reviewers, especially those who are more junior, might feel intimidated and not wish to make negative comments about papers submitted by senior people in the field. On the other hand, some reviewers might hide behind the cloak of anonymity to make unfair criticisms so as to reduce the chances of publication by rivals. We have decided to introduce initially a system of open reviewing if the reviewers agree explicitly. So when a reviewer is sent a paper, he or she is asked to indicate whether we can disclose their name or not when sending the authors their comments. We will be monitoring this to see how many of our reviewers are happy to be named. If it is most of them, we will move to a system of open reviewing as the norm, with a possible “opt out” clause for reviewers.