The most frequent outdoor aeroallergens are pollens and mould spores.
In the temperate areas these allergens are present in the air a limited period of time during the year (they are seasonal allergens) and consequently the symptoms they cause are also seasonal, peaking during the high season and subsiding, slower or faster (depending on the severity of the disease), at the end of the allergen season. In tropical areas these allergens (particularly pollens and moulds) can be present in the air all year round and consequently the symptoms they cause persist all year long.