Pollens are round or oval microscopic particles that act as engines for plant fertilization. In some cases, a plant uses the pollen in its own flower to fertilize itself. In other cases, one plant produces the pollen that fertilizes another plant of the same species, a process known as cross-pollination. Without pollens, plants could not reproduce.

In cross-pollination, the pollen must travel from one plant to another. Sometimes a host carrier delivers pollens to their eventual destination, such as when bees carry pollens from one flower to another. These types of pollen, usually from bright flowers, usually do not cause allergies. However, lighter pollens (from weeds, grass and trees) are carried by the wind to mate with their female counterparts.

These types of pollens have less desirable consequences for some humans. For millions of people, contact with certain types of pollen triggers the itchy eyes, runny nose, sneezing and nasal congestion associated with allergies.

Individual pollens are only active during certain times of the year. People who suffer from pollen-based allergies are likely to experience their symptoms during these same periods year after year.

Many who suffer from pollen-based allergies can minimize their symptoms by adjusting their activity schedule based on pollen counts. Those with more severe symptoms may have to seek medical help in alleviating their symptoms.