Allergens are found in indoor and outdoor environments and may come from plants, food, pets, or even household products. Common allergens include:

• Pollen from grass, weeds, and trees produces hay fever, a common seasonal epidemic.
• Dust mites, or microscopic organisms that live in dust and in the fibers of upholstered furniture, rugs, pillows, and mattresses. They are especially prevalent in warm, humid areas. Cockroach allergens are common in urban, inner-city environments.
• Mold: These are microscopic fungi that produce airborne spores that enter the body when we breathe. Indoor mold is found in damp areas, such as basements, bathrooms, or rooms that have leaks or have been flooded. Mold can also be found outdoors in grass, leaf piles, and mulch, or around mushrooms.
• Animal dander and proteins: The most common animal allergen is animal dander, which is particles of proteins secreted by glands in the animal’s skin, then shed into the air. Proteins from an animal’s saliva, feces, urine, or feathers also can trigger allergic reactions for some people.
• Food components: Common food allergens in Western cultures include shellfish, peanuts, and tree nuts. In addition to those, children are sometimes allergic to milk, eggs, soy, and wheat.