Allergens are classified as seasonal (present during only one season of the year) or perennial (present year-round). Some people suffer from one or the other, but many suffer with both: year-round allergy symptoms that are worse in certain pollen seasons.
In the seasonal allergen class are weed, tree and grass pollens.
Dust mites, cockroach and molds are present all through the year. And animal danders occur whereever and whenever the animals are.