During the spring, the majority of the pollen in the air is released from trees.  Different people have different reactions to different types of tree pollen.  Surprisingly, the pollen that many people associate with spring allergies--the yellow pine pollen that often can be seen on cars and driveways during the spring--does not normally cause allergic reactions.  Most pine pollen grains are too large to pass through the body's natural filters to enter the respiratory system.  Pollen levels are ranked as None, Slight, Moderate, Heavy, or Extremely Heavy.

During the summer, grass produces the majority of the pollen.  Ragweed, which is highly allergenic, is triggered by the changing length of days.

To track historical patterns of pollen counts check out the Bureau's online archives.