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Plants produce round pollen grains. Individual grains are too tiny to see with the naked eye, but some can form large, visible clusters. For fertilization to take place and seeds to form in some plants, pollen must be moved from the flower of one plant to that of another of the same species—for example, from one oak tree to another oak tree—by a process called cross-pollination. Insects do this job for certain flowering plants, while other plants, such as ragweed, rely on wind to transport their pollen grains.
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Most of the pollen that causes allergic reactions comes from plants that don’t have showy flowers, such as trees, weeds, and grasses. These plants make small, light, and dry pollen grains that are made to be carried by wind.
Because airborne pollen can drift for many miles, removing an offending plant may not help. Amazingly, scientists have collected samples of ragweed pollen 400 miles out at sea and 2 miles high in the air. In addition, most allergy-causing pollen comes from plants that produce it in huge quantities. For example, a single ragweed plant can generate a million grains of pollen every day.
The components of a pollen grain are the main factors that determine whether that pollen is likely to cause allergic rhinitis. For example, pine tree pollen is produced in large amounts by a common tree, but it is not a major cause of pollen allergy because the components of pine pollen are less likely to cause an allergic reaction.
Among North American plants, weeds produce the largest amounts of allergenic pollen. Ragweed is the major culprit, but other important sources of weed pollen come from sagebrush, redroot pigweed, lamb’s quarters, Russian thistle (tumbleweed), and English plantain. Some species of grasses and trees also produce highly allergenic pollen.
Although some people may think they are allergic to colorful or scented flowers like roses, it’s not usually the case. Only florists, gardeners, and others who have close contact with flowers over a long period of time are likely to be sensitive to pollen from these plants. In fact, most people have little contact with the large, heavy, and waxy pollen grains of flowering plants because this type of pollen is not carried by wind, but by insects such as butterflies and bees.
One of the obvious features of pollen allergy is its seasonal nature—people have symptoms only when the pollen grains to which they are allergic are in the air. Each plant pollinates more or less at the same time from year to year. Exactly when a plant starts to pollinate seems to depend on the relative length of night and day—and therefore on geographical location—rather than on the weather. But weather conditions during pollination can affect the amount of pollen produced and carried by the wind in a specific year. For example, in the Northern Hemisphere, areas farther north experience a later start to the pollinating period and the pollen allergy season.
A pollen count, often reported by local weather broadcasts or allergy websites each year, is a measure of how much pollen is in the air. This count represents the concentration of all the pollen (or of one particular type, like ragweed) in the air in a certain area at a specific time. It is reported as grains of pollen per cubic meter of air collected over 24 hours.
Pollen counts tend to be the highest early in the morning on warm, dry, breezy days and the lowest during chilly, wet periods. Although the pollen count changes, it is useful as a general guide for when it may be wise for you to stay indoors and avoid contact with that pollen.
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Last Updated October 17, 2011
Last Reviewed October 07, 2011