Ambient air, when sampled close to ground level, contains gases, vapours and particulate matter derived either from natural sources, such as volcanoes and living organisms, or from human activities. Many of these can cause allergic reactions in some people under some conditions.
People normally show individual and different responses to aeroallergens. This makes appreciation of the problem rather complex and solutions ever more so. For example, spores, seeds, and pollen grains are natural constituents of ground level air. They are not pollutants (as are aeroallergens in car exhausts), and no government regulation or Clean Air Act can effectively deal with their presence. Plant pollen may cause ill health is some people (hay fever and asthma), but not all individuals are equally sensitive. Many people live their lives completely unaffected by pollens. The degree to which aeroallergens are a public health issue, or rather a personal lifestyle issue, is challenging for policymakers concerned with environmental health.
Cockroach allergen and dust mite allergen were found in around about 6% and 86%, respectively, of US homes (2001 study). Both allergens are linked to asthma. Homes where the annual income was less than $30,000 per year were about sevenfold more likely to have cockroach allergen and about twofold more likely to have dust mite allergens.