For answers to additional questions see the links below. Pollen is the most likely cause. Your immune system has mistakenly learned to remember the pollen released by various plants as potentially harmful, and responds with the familiar allergic symptoms. Each species of plant releases pollen at about the same time every year. Trees are usually the first plants to release pollen, usually in the early spring.
Grasses come next in the spring and early summer.
Weeds generally release their pollen in late summer and fall. The specific types of pollen and the time of their release depends upon the local climate and varies around the country. People with seasonal rhinitis are often allergic to more than one type of pollen. Denver pollen levels . Generally, allergies last longer than a cold, often for a few weeks.