The role of environmental factors in asthma.

Although the everyday experience of asthmatic patients provides ample anecdotal evidence that environmental exposures provoke bronchospasm, it has proved more difficult to assess the impact of air quality on the timing of asthma attacks and the prevalence of asthma in populations. Spectacular 'asthma epidemic days' are sometimes attributable to exceptional outdoor aero-allergen exposures. By comparison, effects of inorganic particles and gaseous pollutants in outdoor air on the incidence of asthma attacks are subtle and poorly quantified. Environmental tobacco smoke and mould growth are the indoor factors most consistently associated with respiratory morbidity, but their roles in initiating allergic asthma remain uncertain. Evidence relating asthma risk to fumes from gas cooking, and to allergens from dust mites and household pets remains confused and controversial. It is unlikely that trends in either outdoor or indoor air pollution have contributed substantially to the rise in prevalence of asthma and allergic disease in recent decades.