Each spring, summer and fall, tiny particles are released from trees, weeds, and grasses. These particles, known as pollen, are transported in currents of air. These particles often enter the nose and throat, triggering an allergic response known as seasonal allergic rhinitis. Of all the things that can cause an allergy, pollen is one of the most widespread. Many of the foods, drugs or animals that cause allergies can be avoided to a great extent, and even insects and household dust are escapable. However, there is no easy way to avoid pollen. When the pollen count is high, staying indoors may not even help.
The types of pollen that most commonly cause allergic reactions are produced by trees, grasses and weeds that do not have showy flowers. These plants manufacture small, light, dry pollen granules that are custom-made for wind transport. Most allergic pollen comes from plants that produce it in huge quantities. For instance, a single ragweed plant can generate a million grains of pollen per day.
Among North American plants, weeds are the most prolific producers of allergenic pollen. Ragweed is the major culprit, but others of importance are lamb’s quarters and English plantain. Grasses and trees are also important sources of allergenic pollens. Although more than 1,000 species of grass grow in North America, only a few produce highly allergenic pollen. These include timothy grass, Kentucky bluegrass, Johnson grass, and Bermuda grass. Trees that produce allergenic pollen include oak, elm, hickory, pecan, and cedar.