Pollens, the main troublemakers for most cases of seasonal allergic rhinitis, are actually microscopic, fertilizing, male cells in the reproductive cycle of many kinds of flowering plants and trees. Because of their minute size and extreme dryness, pollen grains are easily blown about by the wind and may be carried hundreds of miles by air currents. A single ragweed plant may discharge into the air as many as eight billion grains of pollen, and in a typical season more than a quarter of a million tons may blow about the countryside to the dismay of allergy sufferers. This is why local small-scale destruction of ragweed plants usually has little effect overall in lessening the problem.

Mold, another very common respiratory allergen, is the microscopic form of a nonflowering parasitic plant life known as fungi. Like other forms of fungi, mold lacks roots, stems, leaves, and chlorophyll. As a result, it must obtain its nourishment from other plant and animal materials. Well-known for its ability to spoil foods, mold grows on corn, wheat, and oats; it is therefore a particular problem in the grain-producing regions of the Midwest. Molds propagate by producing spores, which, like pollens, are dispersed in huge numbers by the wind.

For the benefit of its local populace, many regional hospitals provide "pollen count" updates to the local radio and television stations.

A pollen count is a measurement of the number of pollen grains in each cubic yard of air collected in a twenty-four-hour period. Specially coated glass rods are used to capture the pollen grains, and special stains are used to identify them. Typical pollen counts during an average season may range from two hundred to five hundred, with readings of more than one hundred causing discomfort for the majority of allergy sufferers.

It is important to keep in mind that the pollen count heard on the morning news reflects the previous day's count and is not necessarily a prediction of what the new day will bring. Also, mold counts are sometimes obtained with the use of special gathering devices, but they are less commonly reported.