Substances you are exposed to only at certain times of the year, such as plant pollens, cause seasonal allergies. Of all the substances that can trigger an allergy, pollen is one of the most widespread. These tiny round or egg-shaped male plant cells hitch rides on air currents to fertilize other plant parts. However, not all pollen causes nasal allergies.
Allergy-causing pollens are:
- small and light enough to be carried on the wind
- produced in huge quantities
- found in areas where people live and play
In addition, the chemical makeup of the pollen determines if it will trigger an allergic reaction. For instance, pine trees produce huge amounts of pollen, making it a potentially strong allergen. But the chemical makeup of the pine pollen makes it less of an allergy trigger than other kinds of pollen. Also, because pine pollen is heavy, it tends to fall on the ground rather than drift in the air where it can reach sensitive noses.
In temperate climates such as the US, the most important sources of allergy-producing pollens are:
- trees in the spring
- grasses in late spring and summer
- weeds in late summer and fall
These plants make small, dry, light pollen granules that are perfect for riding on the wind and getting into your nose and causing allergy symptoms. Flowers, such as roses and fruit tree blossoms, rarely cause allergic reactions. Why? Because their pollens are too heavy to be carried by the wind. Instead, they are carried from plant to plant by insects, such as bees. Some people, though, who are repeatedly exposed to roses and other flowers, including florists and gardeners, can develop sensitivity to these pollens.
As appealing as it may sound to move somewhere that's pollen-free, it's not very likely you'll be able to do this. Airborne pollen is so light that air currents can carry it hundreds of miles from its source. For instance, ragweed pollen has been collected 400 miles out at sea and 2 miles high in the air. It doesn't matter if you live in the city or in the country. Pollen is everywhere. And it doesn't help to pull up your lawn or cut down your trees.
Some people believe they can move to the desert to escape pollen allergies, but it doesn't work. In Arizona, for instance, nonnative plants such as olive, cypress, juniper, and mulberry trees, as well as Bermuda grass, cause problems for sensitive people.
Types of Pollens
Tree Pollens and Allergies
Not every kind of tree produces pollen that causes nasal allergies. But enough do to make seasonal allergies from tree pollens a major health issue.
Common Allergy-Producing Trees
- box elder
- mulberry (nonfruiting)
- sweet gum
Grass Pollens and Allergies
Grass pollen is a common allergen. Yet of the more than 1,000 types of grass that grow in North America, only a few produce pollens that trigger nasal allergies.
Common Allergy-Producing Grasses
- blue (especially Kentucky)
- sugar beet
- sweet vernal
- wild oats
Weed Pollen and Allergies
Weeds are the largest source of allergy-triggering pollens in North America. In late summer and early fall in North America, ragweed pollen is the number one cause of hay fever. A single ragweed plant can produce millions of grains of allergy-producing pollen.
Common Allergy-Producing Weeds
- mash elder
- Russian thistle (tumbleweed)
- sages (mugworts)
- sheep sorrel
For more information about allergies and pollens, see the next page.
ABOUT THE AUTHOR
Written by Karen Serrano, MD Emergency Medicine resident at the University of Wisconsin-Madison.
Reviewed by Lisa V. Suffian, MD
Instructor of Clinical Pediatrics in the Division of Allergy and Pulmonary Medicine at Saint Louis Children's Hospital, Washington University School of Medicine
Assistant Clinical Professor in the Department of Pediatrics at Cardinal Glennon Children's Hospital, Saint Louis University
Board certified in Allergy and Immunology
Last updated June 2008