The cause of asthma is unknown. Health experts believe that inherited, environmental, and immune system factors combine to cause inflammation of the bronchial tubes, which carry air to the lungs. This can lead to asthma symptoms and asthma attacks.

• Asthma may run in families (inherited). If this is the case in your family, your child may be more likely than other children to develop long-lasting (chronic) inflammation in the bronchial tubes.
• In some children, immune system cells release chemicals that cause inflammation in response to certain substances (allergens) that cause allergic reactions. Studies show that exposure to allergens such as dust mites, cockroaches, and animal dander may influence asthma's development.¹ Asthma is much more common in children with allergies (atopic children), though not all children with allergies develop asthma. And not all children with asthma have allergies.
• Environmental factors and today's germ-conscious lifestyle may play a role in the development of asthma. Some experts believe there are more cases of asthma because of pollution and less exposure to certain types of harmful bacteria and other "germs."² As a result, children's immune systems may develop in a way that makes it more likely they will also develop allergies and asthma.