Rassegna bibliografica

Vol. 70, Iss. 9, September 2013

Work related asthma. A causal analysis controlling the healthy worker effect


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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