In this population-based study of a potentially susceptible group of working-age adults with respect to respiratory infections, i.e. those with recently diagnosed asthma, we found that they experienced an increased risk of both URTIs and LRTIs in certain occupations when compared to office workers. This study showed that occupations, where the workers usually change their work environment according to the worksite that is under construction or is being otherwise worked on, for example in forestry, experienced particularly increased risk of common colds. Such workforces are rather mobile and may include workers from other parts of the same country or from other countries, which may spread infections especially during epidemics.
In addition, increased risk of common colds was experienced by students, who may change their studying area frequently and attend lectures with a large audience being relatively close to each other, so their multiple contacts during the day may facilitate catching infections. In addition, those unemployed showed significantly increased unadjusted IRR of common cold, which raises the question whether a stressful situation in life increases susceptibility to catch respiratory infections. However, neither of these two occupations were linked to significantly increased risk when adjusted for potential confounders.
Hairdressers and fur and leather workers experienced increased risk of tonsillitis and sinusitis, suggesting potential role of some chemicals used at work in facilitating catching infections. Replacing such chemicals and other potentially irritating substances with less irritating ones might reduce infections in these occupations.
Laboratory technicians showed increased risk of tonsillitis, sinusitis as well as of pneumonia, suggesting that close contact with the customers is a potential route of infections. Exposure to laboratory chemicals could also influence susceptibility of the airways to respiratory infections.
Increased risk of acute otitis media was detected among bakers and food processors, which raises the question whether this is due to frequent contacts with customers, or exposures related to preparation of food. Those on maternity leave showed increased risk of both tonsillitis and otitis media, which are common infections among infants.
In contrast, increased risk of acute bronchitis was detected in occupations where workers often work in small workshops, in which they may work close to each other and where the ventilation may sometimes be insufficient. In addition, their work seemed to include handling of chemicals, such as glues among leather workers, or irritating substances, such as mineral fibers.
Increased risk of pneumonia was detected in occupations where the workers often work in close contact with the customers, such as laboratory technicians and drivers, the latter also sharing a rather small space with the customers. In such situations, it might be possible to prevent spreading of infections by increasing the volume of the work area and/or possibly by increasing air exchange rate in the work area.
Metal workers showed significantly increased risk of lower respiratory tract infections and borderline significantly increased risk of upper respiratory tract infections. It would be interesting to explore in the future, which features of metal work underlie the increased risk of respiratory infections detected in this study. Can exposure to metal dust increase susceptibility to infections or is exposure to metal working fluids or welding fogs underlying such susceptibility.
Validity of the results
The study population was well defined because the diagnosis of incident asthma was based on reported asthma symptoms and objective findings of bronchial obstruction with significant bronchodilation effect in line with the national asthma guidelines applied at the time of the study period. The participation rate of cases of adult-onset asthma in the original population-based FEAS was good at 86%. All the information needed for the present analyses was available for 94.4% of the total FEAS asthma population. Thus, any major selection bias is unlikely in the present study.
Occupation at the time of diagnosis of asthma was the determinant of interest. The frequency and type of respiratory infections during the past 12 months formed the outcomes of interest. The outcomes were based on self-reporting in the FEAS questionnaire, which asked about both upper respiratory tract infections and lower respiratory tract infections in a systematic way. Those with asthma have been suggested to be more susceptible to experience respiratory infections in general.4 As this study was limited to those with recently diagnosed adult-onset asthma, comparison between different occupational groups can be considered valid.
In the multivariate regression analyses, we adjusted as potential confounders for age, gender, and smoking, all of which may be related to increased susceptibility to respiratory infections.8,9 This gives some assurance that the differences detected between occupations are related to the occupation or occupational environment linked to it.
The sample size in some of the occupations was small, which is reflected in wide confidence intervals. However, the lower 95% confidence limit was clearly above 1 for those occupations mentioned above.
Synthesis with previous knowledge
Previous literature on the association between occupation and respiratory tract infections is still rather limited and focuses on the risk of influenza or influenza like illness.10–14 The risk has been explained by different frequencies of contacts with other people, contact with contaminated surfaces at work or by work-related stress.14 A recent register-based study found that people working in occupations with high person-to-person contact, such as work in day care, sewers, public transportation, and nursing and home care, had an increased risk of being hospitalized with pneumonia or influenza compared to people working within public administration.14 These findings are in line with our results showing that drivers have increased risk of pneumonia and people working in day care centers have increased risk of URTIs. Furthermore, Pujol et al. suggested that manual workers, including people who are self-employed, workers in skilled technical occupations, workers qualified at the primary sector, and unskilled workers have a higher risk of hospitalization for influenza than people who are working in a non-manual occupation.13 Consistent with our study, occupational groups related to food preparation and serving, community and social services, personal care and services, and building and ground cleaning as well as maintenance were associated with increased occurrence of influenza-like illness, defined as fever and cough or sore throat, in two questionnaire-based studies.10,12 Studies have also reported that unemployed adults have increased risk of influenza and influenza-like illness compared with employed adults,11,12 which again is in line with our finding that unemployed study subjects had increased IRR for common cold.
Most of the previous studies were conducted in societies where access to health care services and the possibility to receive seasonal influenza vaccination could vary between different occupational groups. However, in Finland everyone with chronic diseases or working in health care has equal opportunity to receive vaccination within the national seasonal influenza vaccination program. Previous studies included general working aged populations, and some of them adjusted for co-morbidities, such as asthma, in the multivariate statistical models. However, none of the previous studies focused on a high-risk population as we did in our study that was based on a population of subjects with recently diagnosed adult-onset asthma.