Plants produce microscopic round or oval pollen grains to reproduce. In some species, the pollens are used within the plant for fertilization, while in others pollens travel from one plant to other via air or other medium.

The types of pollen that are associated with pollen allergy are produced by the plain-looking plants such as, trees, grasses, and weeds. Normally these plants don’t have showy flowers. The pollens that these plants manufacture are small, light and dry that can be transported easily by wind.

Some of such pollens have been collected 400 miles out at sea and two miles high in the air. Further, most of the allergenic pollen comes from plants that produce it in huge quantities, for instance, a single ragweed plant can generate a million grains of pollen a day.