Overview of Indoor Allergies

An allergy is an abnormal immune response to a substance that does not cause a reaction in most people. Any substance that causes an allergic reaction is referred to as an allergen. When a person who is allergic comes in contact with an allergen, his or her immune system responds to the allergen as if it were a harmful substance, causing an allergic reaction.

Several different substances can cause allergies. Hay fever is a type of outdoor allergy caused by plant pollen (e.g., ragweed) in the air. Certain foods and medications also can cause allergies. In some cases, people develop allergies to substances that are present inside their homes. These types of allergies are called indoor allergies.

Common types of indoor allergens include the following:

  • Dust mite debris Dust mites are microscopic creatures that thrive in warm, humid conditions. They exist in large numbers in bedding, such as pillows and mattresses. Dust mite waste products are a common allergen. These substances can build up over time within bedding materials and can be inhaled during sleep.
  • Pet dander A protein in animal saliva is an allergen that triggers an immune response in people who have a pet allergy. This protein can be found on dead skin and hair debris from the pet and in the animal’s urine. This debris can become airborne in general house dust and then can be inhaled.
  • Cockroach debris Cockroaches are difficult to fully exterminate in certain areas (e.g., in the southern United States, in large, older apartment buildings in urban areas). When cockroaches die, their bodies and waste products break down into small particles that can become airborne and can be inhaled.
  • Mold Mold is a type of fungus that can grow on indoor as well as outdoor surfaces. Under the right conditions (e.g., damp, humid) within a home, mold can grow under flooring materials; behind walls; and on wood, sheet rock, or fabric. As it grows, mold produces spores that become airborne and can be inhaled.

Any of these allergens can cause allergy symptoms or an allergic reaction in people who are sensitive to them.

Incidence and Prevalence of Indoor Allergies

About 40 million people in the United States suffer from some type of indoor or outdoor allergy. Many allergy sufferers have more than one allergy.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 22 Oct 2008

Last Modified: 22 Sep 2015