These particles, known as pollen, hitch rides on currents of air. Although their mission is to fertilize parts of other plants, many never reach their targets. Of all the things that can cause an allergy, pollen is one of the most widespread. People with pollen allergies often develop sensitivities to other troublemakers that are present all year, such as dust mites. Year-round airborne allergens cause perennial allergic rhinitis, as distinguished from seasonal allergic rhinitis.
Plants produce microscopic round or oval pollen grains to reproduce.
In some species, the plant uses the pollen from its own flowers to fertilize itself. Insects do this job for certain flowering plants, while other plants rely on wind transport. These plants manufacture small, light, dry pollen granules that are custom-made for wind transport. Samples of ragweed pollen have been collected 400 miles out at sea and 2 miles high in the air. In addition, most allergenic pollen comes from plants that produce it in huge quantities.