Scanning electron micrograph (SEM) of bedroom household dust. Dust contains an assortment of substances which can cause asthma or other allergic reactions. The allergens seen here include: fabric fibers, part of a dead mite, skin flakes, fungal spores, and unidentified particles are all potential allergens. Asthma or hay fever are often a hypersensitive and irritant response to substances that, in most people, are harmless. Airborne allergens may come into contact with the respiratory airways, skin or surface of the eye. Micr. magn. 750x, image width: 150 um.