Some of the categories they fall into are inhalants, ingestants, injectant, contactant, and infectants. Inhalants are more common sources of allergy symptoms. Inhalants include the particles we breathe like dust, vapors, animal danders, and feathers. Some of the beverages, medicines, and vitamins we use can set off the sniffles, as well as clothing, metals leather, and pollen grasses, especially. The most common culprit of all our allergies is pollen.
Pollen is light and is very easily picked up by wind.
Seed bearing plants like evergreens and some flower bearing plants are allergens. In order to be considered an allergen, a large amount of pollen must be produced. Spore producing plants like mosses and ferns are not allergens. Molds also can cause the drips . They are everywhere, inside, outside and are hard to remove.