They can travel through the air and are often invisible. They are usually inhaled, either alone or by attaching themselves to particles of dust and then entering the respiratory system. While the majority of biological pollutants are found indoors, some are of outdoor origin. Bacteria, fungi and molds find nourishment and can flourish in improperly maintained air ducts, air conditioners, humidifiers, dehumidifiers, air-cleaning filters, carpets and in improperly ventilated places where moisture is likely to collect, such as bathrooms, kitchens, laundry rooms and basements. Viruses can be carried indoors and spread by people, while plants, pets and insects are potential sources of pollen, dander, and other allergens.
Dust mites and other insects can thrive in sofas, stuffed chairs, carpets and bedding.
All of us are exposed to biological pollutants. However, the effects on our health depend upon the type and amount of biological pollution and the sensitivity of the individual. Allergic reactions can range from mildly uncomfortable to life-threatening, as in a severe asthma attack. These people have very sensitive airways that can react to various irritants, making breathing difficult. The number of people who have asthma has greatly increased in recent years up 61 percent between 1982 and 1994, to a total of 14.6 million persons in the United States.