These airborne particles may cause asthma or hay fever (allergic rhinitis). Pollen grains are coloured pink. The surface of the trachea is made up of cells with hair-like cilia (yellow). Together with mucus, these cilia serve to trap airborne particles and by beating upwards in a wave-like motion they remove foreign matter from the air tubes and lungs. In asthmatic or allergic patients, such particles may lead to a hyper-sensitive reaction causing breathing difficulties. Magnification: x2,000 at 6x7cm size. x6,500 at 8x10"

See also:
Indoor Air Quality - How lungs work ,courtesy of the National Heart, Lung, and Blood Institute (as a part of the National Institutes of Health and the U.S. Department of Health and Human Services

Allergies: The Radical Theory of Sneezing, Tina Adler, Environmental Health Perspectives, Volume 113, Number 11, November 2005