Allergic rhinitis is typically caused by allergens that are airborne. These are roughly divided into outdoor airborne allergens and indoor airborne allergens. Common airborne allergens found outdoors include tree pollens, grass pollens, and weed pollens. Common airborne allergens found indoors include animal dander, dust mites, and mold.
Outdoor allergens are usually seasonal because plants release pollen at certain times of the year. Spring allergies are commonly due to tree pollens. Late spring and summer allergies are commonly due to grass pollens. Late summer and autumn allergies are commonly due to weed pollens, such as ragweed pollen. In fact, ragweed pollen is one of the primary causes of allergic rhinitis and 75% of people with pollen allergies are allergic to ragweed.
Indoor allergens may be present throughout the year. However, certain situations can make indoor allergies worse. For example, animal dander allergies may be worse in the winter because pets may spend more time indoors and there is less circulating fresh air. And mold allergies may be worse if there is a very rainy spring season.