Airborne pollen is the culprit behind seasonal allergies. Pollen is very small: about 50 microns in size (for comparison, the width of an average human hair is about 62 microns).
Although tiny, pollen can be quite a problem for an allergy sufferer. Not every plant releases its pollen into the air, but there are enough that do to cause a lot of sneezing each spring and fall.
Spring and fall are the worst times for seasonal allergy sufferers. That's because most pollen is produced during these two seasons.
In the spring, elms, oaks, maples, sycamores, and other trees are the biggest producers of pollen. In the late spring, grasses begin to release pollen and continue to do so throughout the summer. The fall brings in the weed pollens, particularly ragweed, one of the largest producers of pollen.