Pollen is the most common cause of allergies. A small pinch of pollen contains thousands of round or egg-shaped grains. It is produced by seed-bearing plants and dispersed by wind, bees or insects. Small, light, dry grains of pollen are easily spread by the wind and inhaled, and may cause an allergic reaction. Waxy, heavy or large pollen grains of flowers, like roses, are spread by bees and insects and cannot be inhaled. People usually do not develop an allergy to these types of pollens, although florists and gardeners may because of extended close contact with the plants.
Weeds produce abundant amounts of pollen and are the largest source of allergy pollen in North America. Weed pollen is most common in the fall, with the peak occurring in September. Weed pollen counts are highest between 5 and 10 a.m. and on hot, dry, windy days. Ragweed is the most commonly known weed allergen. Lamb's quarter, redroot pigweed, Russian thistle and sagebrush are some other common weed allergens.
Only a small percentage of the 1,200 species of grass pollen cause allergies. The release of grass pollen varies by regions and seasons. Time of day, temperature and rain also affect the release of grass pollen. Some common grass allergens are Bermuda grass, Johnson grass, Kentucky bluegrass, orchard grass and Timothy grass.
Fewer than 100 tree species of more than 50,000 cause allergies. The wind distributes tree pollen miles away from the original source. Tree pollen season begins in midwinter in the south and in spring in the north. Ashes, birches, elms, hickories, oaks and pecans are common tree allergens.