How Many People Do Allergies Affect? Researchers think nasal allergies affect about 50 million people in the United States. Allergies are increasing. They affect as many as 30 percent of adults and 40 percent of children.1 Allergic disease, including asthma, is the fifth leading chronic disease in the U.S. in people of all ages. It is the third most common chronic disease in children under 18 years old.1