More than 50 million Americans have some form of allergy. Allergic rhinitis, the most common type of allergy, affects approximately 40 million Americans each year. Sensitivity often starts in childhood or young adulthood.

Although experts do not fully understand why some people develop allergies and others don't, allergies do tend to run in families. If both parents have allergies, a child's chances of developing allergies are 66%. If only one parent has allergies, a child's chances of developing them are about 50%.

Early exposure to irritants like cigarette smoke is also believed to increase a child's chances of developing allergies.