In this population of non-smoking blue-collar trucking industry workers, we observed statistically significant relationships of recent SHS exposure with self-reported respiratory symptoms, including chronic cough, chronic phlegm, and any wheeze. Workplace appeared to be an important source of current exposure to SHS that influenced the workers’ respiratory health. In addition, childhood exposure to SHS was statistically significantly associated with chronic phlegm, whereas life-time duration of SHS exposure at home was positively associated with chronic phlegm and any wheeze. We also found evidence of effect modification by obesity and urban residence on the association between childhood SHS exposure and respiratory symptoms.
Our findings are fairly consistent with results from previous studies, although the evidence has been less conclusive in studies of adults than in studies of children. Studies have considered many combinations of SHS exposure experienced in childhood, adulthood, at home, at work, and outside of home or work. In addition, multiple outcome measures have been assessed (e.g. cough, phlegm, wheeze, chest tightness, breathlessness/dyspnea, headache, sinusitis, etc.), either by self-reports or doctor diagnosis. Therefore, it is difficult to make a concise comparison of studies.
Studies evaluating relationships between home SHS exposures (i.e. living with a smoker as an adult) and respiratory symptoms have observed inconsistent results, ranging from no association to an almost four-fold risk of chronic or usual cough [4, 9, 26–31]. In our study, no associations between home exposure and symptoms were found after controlling for current workplace exposure, but there was a significant association between total duration of life-time home exposure and chronic phlegm. Fewer studies have evaluated the effect of childhood SHS exposure on adult respiratory symptoms [26, 32–34]. In a study of never smoking Singaporeans aged 45–74 years, positive associations were observed for childhood exposure with chronic phlegm and cough in adulthood . A multicenter study in India showed positive association between exposure during childhood and both during childhood and adulthood with the prevalence of asthma . A study in Western Norway revealed a significantly positive association between maternal smoking in childhood and wheeze in adulthood, but the results were not statistically significant for phlegm and cough . The association between childhood exposure and adult wheeze was also observed in a multi-area European study, but this association was no longer statistically significant when restricted to never smokers . In our present study of non-current smoking workers, childhood SHS exposure had a significant association with chronic phlegm and a borderline association with wheeze, after adjusting for current SHS exposure.
Analyses of self-reported workplace exposures have revealed more consistent associations with elevated risks of respiratory symptoms [6, 9, 35, 36]. In a prospective diary study as part of the SAPALDIA study in never smoking Swiss adults, SHS exposure at work was associated with bronchitis symptoms (cough or phlegm) and asthmatic symptoms (wheeze, dyspnea, or chest tightness) . Another study in 559 casino workers in London observed a significant relationship of intensity of workplace SHS exposure with shortness of breath and wheeze, controlling for home exposure . Among workers in Spanish hospitality venues (bars, restaurants, hotels, and discotheques), respiratory symptoms decreased significantly [−98.1% (95% CI: -51% to −99.9%) in multivariate analysis] after smoking was totally banned in their workplace, whereas there was no significant change among workers employed at hospitality venues where only partial smoking bans or no restrictions were implemented . A study of Norwegian restaurant employees found a complete smoking ban in restaurants contributed to fewer reports of respiratory symptoms . A small cross-sectional study in the former Yugoslav Republic of Macedonia found a significantly higher prevalence of wheezing and breathlessness in nonsmoking women who worked in offices with smokers as opposed to those who did not . A study of Italian women found exposure at work resulted in higher adjusted odds of wheeze, shortness of breath and asthma symptoms . A prospective telephone survey in Hong Kong focusing on workplace SHS exposure reported elevated risks of frequent cough and phlegm for those exposed at work compared to those unexposed . Our study also found workplace SHS exposure to have the strongest effect on respiratory symptoms, as compared to home exposure or recreational exposure.
Workers in the trucking industry are exposed to occupational sources of airborne particles and gasses. Therefore, there is the potential for confounding by or synergism with other exposures on the association of SHS exposure with health outcomes. Previous evidence has shown that SHS exposure is independently associated with respiratory symptoms and decreased lung function after adjusting for other sources of indoor particulate pollutants (e.g. wood stoves, candles, and gas cooking) in the general Danish population  and Singaporean women . In this study we observed significant associations between SHS exposure and respiratory symptoms even with adjustment for job title, a fairly reasonable indicator of occupational exposure to air pollutants in this industry .
We recognize that this study has several limitations. It is a cross-sectional survey, limiting our ability to assess causality. The response rate, although comparable to other occupational studies [6, 36], was only 64% of the people who received the second questionnaire. However, the responders and nonresponders were similar in terms of age, race, job title and active smoking. Both the exposures and the outcomes were self reported; therefore, the potential that recall bias could be responsible for our positive results cannot be ignored. In a study of workers in the same trucking companies we previously have shown evidence that self-reported workplace exposures to SHS are predictive of vapor-phase nicotine exposure as measured by a passive monitor worn on the lapel , providing evidence of validity of the self report of workplace exposure. It is unlikely that exposure only in the last 7 days would be causally related to chronic respiratory symptoms. However, it is likely that the recent exposure is reflective of more chronic exposures in the workplace, home and recreational facilities. Finally, there might be confounding by socioeconomic status and/or social stress. However, our study population is a group of blue-collar workers with fairly homogeneous backgrounds, and additional adjustment for educational level did not influence the results (data not shown). Thus, social factors are not likely to affect the results observed in this study.