Causes

The exact cause of asthma is unknown. Asthma is most likely caused by a combination of genetic (inherited) factors and environmental triggers (such as allergens and infections). Asthma tends to run in families, so children whose parents have asthma are more likely to develop it themselves.

The Allergic Response (Allergens)

Nearly half of adults with asthma have an allergy-related condition, which, in most cases developed first in childhood. (In patients who first develop asthma during adulthood, the allergic response usually does not play a strong causal role.)

In people with allergies, the immune system overreacts to exposure to allergens. Allergic asthma is triggered by inhaling certain substances (allergens) such as:

• Dust mites, specifically mite feces, which are coated with enzymes that contain a powerful allergen. These are the primary allergens in the home.
• Animal dander. Cats harbor significant allergens, which can even be carried on clothing; dogs usually cause fewer problems. People with asthma who already have pets and are not allergic to them probably have a low risk for developing such allergies later on.
• Molds.
• Cockroaches. Cockroach dust is a major asthma trigger and may reduce lung function even in people without a history of asthma.
• Pollen, from plants.

Environmental Factors (Irritants)

An asthma attack can also be induced or aggravated by direct irritants to the lungs. Important irritants involved in asthma include cigarette smoke, indoor chemicals, and air pollution.

Infections

Respiratory viral and bacterial infections play a role in some cases of adult-onset asthma. In both children and adults with existing allergic asthma, an upper respiratory tract infection often worsens an attack.

Review Date: 05/03/2011
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)