An attributable risk model for exposures assumed to cause both chronic disease and its exacerbations.

BACKGROUND: Many chronic diseases are the product of an underlying pathologic condition and superimposed acute exacerbations. This model may apply to several conditions such as asthma, other obstructive lung diseases, or atherosclerosis. For exposures affecting both the development of chronic disease and its exacerbation, the usual methods to derive attributable risks (AR) are inappropriate.
METHODS: We expand traditional risk assessment methods to estimate the AR for exacerbations under a "chronic disease model." We use asthma in children as the chronic disease and air pollution as the exposure of interest. We estimate bronchitis symptom exacerbations attributable to air pollution, using data from the Children's Health Study to estimate asthma prevalence and symptom occurrence, and we examine the distribution of exposure and its acute and chronic effects.
RESULTS: In the combined AR model, 39.8% of exacerbations were attributable to air pollution, compared with 33.5% in the traditional model, which ignores a chronic effect of pollution on asthma development. Thus, there is a 1.19-fold higher estimated burden with the combined model. The difference is due to exacerbations caused by other factors (ie, not by air pollution) but nonetheless occurring among those assumed to have asthma that developed due to traffic-related pollution. The proposed model is applicable to other risk factors that play a role both in both the development of a chronic disease and its exacerbation.
CONCLUSIONS: Traditional approaches to the calculation of attributable risk may underestimate the health impact of long-term environmental or other exposures that produce both chronic and acute disease.