The exact cause of asthma isn't known. Researchers think a combination of factors (family genes and certain environmental exposures) interact to cause asthma to develop, most often early in life. These factors include:
• An inherited tendency to develop allergies, called atopy (AT-o-pe)
• Parents who have asthma
• Certain respiratory infections during childhood
• Contact with some airborne allergens or exposure to some viral infections in infancy or in early childhood when the immune system is developing
If asthma or atopy runs in your family, exposure to airborne allergens (for example, house dust mites, cockroaches, and possibly cat or dog dander) and irritants (for example, tobacco smoke) may make your airways more reactive to substances in the air you breathe.
Different factors may be more likely to cause asthma in some people than in others. Researchers continue to explore what causes asthma.
The "Hygiene Hypothesis"
One theory researchers have for what causes asthma is the "hygiene hypothesis." They believe that our Western lifestyle—with its emphasis on hygiene and sanitation—has resulted in changes in our living conditions and an overall decline in infections in early childhood.
Many young children no longer experience the same types of environmental exposures and infections as children did in the past. This affects the way that the immune systems in today's young children develop during very early childhood, and it may increase their risk for atopy and asthma. This is especially true for children who have close family members with one or both of these conditions.