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)

Asthma and allergies often coexist, and the allergic response plays a strong role in childhood asthma. About 70 - 85% of children with asthma also have allergies. Some studies suggest that children who have allergies are also at greater risk for developing asthma as adults. However, only a minority of children with allergies have asthma.

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 trigger and may reduce lung function even in people without a history of asthma.
• Pollen, from plants.

Environmental Factors (Irritants)

An asthma attack can be induced or aggravated by direct irritants to the lungs. Studies indicate that the more indoor allergens a child is allergic to, the higher the risk for severe asthma. Important irritants include:

• Smoking: Parental smoking has been shown to increase the airway responsiveness of infants as early as the first 2 - 10 weeks of life. This extends even to the fetus of pregnant women who smoke. These mothers tend to have babies born at a low birth weight, which affects lung function and increases their baby's risks for asthma
• Food allergies. About 8 - 10% of children with asthma also have food allergies. Some research suggests that peanut and milk allergies may increase asthma severity. Indoor chemicals. Chemicals used in household cleaning products and furniture materials can be asthma triggers.
• Air pollution. Fossil fuels and chemicals that contribute to air pollution may worsen asthma symptoms.

Infections

The role of early childhood respiratory and intestinal infections is very complex. Viral respiratory infections certainly worsen existing asthma, but the most common ones are unlikely to cause childhood asthma. In fact, early respiratory and intestinal infections may offer some protection against asthma.

Studies suggest that most respiratory infections are not important causes of asthma in children, except in certain cases. An important exception is the respiratory syncytial virus (RSV), which has been implicated in the development of asthma. RSV is the major viral cause of infant pneumonia. Studies also indicate that infants who have reduced lung function within a few days after birth are at increased risk of developing asthma by the time they are 10 years old.

Common respiratory infection viruses that cause colds (such as the rhinovirus) may be associated with the development of asthma in some people. More likely, these viruses do not directly cause asthma, but worsen asthma in children who already have it. Rhinovirus has been reported to be the most common infection associated with asthma attacks.

Research indicates that children who have a viral-induced wheezing during infancy may be at increased risk for later development of asthma. However, many children outgrow attacks of intermittent wheezing.