6. Louisville, KY
Approximately 40 million Americans have indoor/outdoor allergies as their primary allergy. (Many people with allergies usually have more than one type of allergy.) Approximately 10 million people are allergic to cat dander, the most common pet allergy. The most common indoor/outdoor allergy triggers are: tree, grass and weed pollen; mold spores; dust mite and cockroach allergen; and, cat, dog and rodent dander.