Controlling healthy worker effect in asthma

The healthy worker effect, i.e. the fact that sick workers tend to leave the workforce or to avoid jobs with hazardous exposures, usually leads to underestimation of the association between occupational exposure and asthma. A research with the participation of the CREAL estimated the effect of occupational exposure on asthma expression in a longitudinal study, using marginal structural modelling to control for the healthy worker effect. This study was published in Occupational and Environmental Medicine (OEM).

The analyses included 1284 participants (17–79 years, 48% men) from the follow-up (2003–2007) of the French Epidemiological study on the Genetics and Environment of Asthma. Age at asthma onset, periods with and without attacks over lifetime and occupational history were recorded retrospectively. “Exposures to known asthmagens, irritants or low level of chemicals and allergens were evaluated through a job-exposure matrix. The job history was reconstructed into 5-year intervals”, says Judith Garcia-Aymerich, researcher at CREAL and participant in the study.

“Thirty-one per cent of subjects had ever been exposed to occupational asthmagens. Among the 38% of subjects who had ever asthma, presence of attacks was reported in 52% of all time periods”, explains Francine Kauffmann, CREAL and INSERM researcher, who had also participated in this research. Using standard analyses, no association was observed between exposure to known asthmagens or to irritants and low level of chemicals/allergens and asthma attacks. However, using marginal structural models, those exposed to asthmagens had a 26% higher risk of asthma with presence of attacks, while those exposed to irritants or low level of chemicals/allergens had 56% higher risk than non-exposed workers.

“The healthy worker effect has an important impact in risk assessment in work-related asthma studies. Marginal structural models are useful to eliminate imbalances in exposure due to disease-driven selection. Results support the role of irritants in work-related asthma”, concludes Xavier Basagaña, CREAL researcher, who had also collaborated in this paper.

Reference

Dumas O, Le Moual N, Siroux V, Heederik D, Garcia-Aymerich J, Varraso R, Kauffmann F,  Basagaña X. Work related asthma. A causal analysis controlling the healthy worker effect. Occup Environ Med 2013; 0:1–8. doi.

Picture: Workers, by Melbournewaters (Flickr)