Riassunto

Objectives The healthy worker effect usually leads to underestimation of the association between occupational exposure and asthma. The role of irritants in work-related asthma is disputed. We estimated the effect of occupational exposure on asthma expression in a longitudinal study, using marginal structural modelling to control for the healthy worker effect.

Methods 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 (case-control study). Age at asthma onset, periods with/without attacks over lifetime and occupational history were recorded retrospectively. Exposures to known asthmagens, irritants or low level of chemicals/allergens were evaluated through a job-exposure matrix. The job history was reconstructed into 5-year intervals.

Results Thirty-one per cent of subjects had ever been exposed to occupational asthmagens. Among the 38% of subjects who had asthma (ever), presence of attacks was reported in 52% of all time periods. Using standard analyses, no association was observed between exposure to known asthmagens (OR (95% CI): 0.99 (0.72 to 1.36)) or to irritants/low level of chemicals/allergens (0.82 (0.56 to 1.20)) and asthma attacks. Using a marginal structural model, all associations increased with suggestive evidence for known asthmagens (1.26 (0.90 to 1.76)), and reaching statistical significance for irritants/low level of chemicals/allergens (1.56 (1.02 to 2.40)).

Conclusions 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.

Commento

L’effetto “lavoratore sano” è un fenomeno, principalmente osservato negli studi epidemiologici inerenti le malattie professionali, a causa del quale i lavoratori  esposti tendono a presentare tassi di morbilità e mortalità complessivi inferiori a quelli della popolazione generale. La spiegazione risiede nel fatto che gli individui sani sono più facilitati ad ottenere un impiego e a rimanere impiegati nel tempo, mentre quelli gravemente malati o disabili cronici sono progressivamente esclusi dal lavoro.

Il fenomeno è stato descritto in letteratura per la prima volta da William Ogle nel 1885 e pone un problema metodologico per chi studia coorti occupazionali esposte a rischi professionali, in quanto può mascherare eccessi di morbilità e mortalità correlati ai rischi stessi. Il problema è ulteriormente complicato dal fatto che gli effetti del fenomeno possono variare sensibilmente da una coorte occupazionale all'altra, considerando che le professioni più difficoltose tendono ad allontanare soggetti malati gravi o disabili, mentre quelle a carattere più semplice li attraggono.

Tenendo il problema nella dovuta considerazione, questo studio longitudinale di Dumas e collaboratori, si è proposto di stimare l’effetto delle esposizioni professionali a sostanze irritanti sull’incidenza dell’asma, utilizzando un modello per controllare l’effetto “lavoratore sano”.

Keywords

Allergy, asthma, healthy worker effect

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