From Kathleen MacNaughton, R.N.,
Your Guide to Asthma.
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About.com Health's Disease and Condition content is reviewed by Sanja Jelic, MD

Pollen is an airborne allergen that may set off numerous asthma symptoms in sensitive people at certain times of the year. Pollen is an outdoor allergen.

Pollen are tiny, egg-shaped male cells found in flowering plants. You may know pollen better as the tiny, powdery granules that plants use during the fertilization process. The size of a typical pollen spore is smaller in diameter than a human hair.

There are different types of pollen. Some are large and waxy, while others are light, small and dry. The first type is found on flowers such as roses and is mostly carried from plant to plant by birds and insects. This is not the type of pollen that triggers allergies or allergic asthma.

Light, dry pollens, which are found on certain trees, grasses and low weeds like ragweed, are easily picked up and disseminated by air currents and wind. This is the type of pollen that triggers allergies and allergic asthma.

Sources of Pollen

In the early spring, the pollens usually most responsible for causing asthma symptoms are tree pollens. The types of trees most likely to produce pollens that cause allergic symptoms are:

• Ash
• Birch
• Cypress
• Elm
• Hickory
• Maple
• Oak
• Poplar
• Sycamore
• Walnut
• Western red cedar

During the late spring and summer, grass pollens are more likely to trigger your symptoms. Common grass allergens include:

• Bermuda grass
• Bluegrass
• Orchard grass
• Red top grass
• Sweet vernal grass
• Timothy grass

Weed pollens, especially ragweed pollen, are among the most common pollen allergens around and tend to be at their highest levels during late summer and fall. Some other common weed allergens are:

• Cockleweeds
• Pigweed
• Russian thistle
• Sagebrush
• Tumbleweed

Plants generally pollinate around the same time each year, so we can predict approximate seasonal allergy and asthma periods. However, the amount of pollen in the air at any given time can vary, depending on a variety of conditions, including heat, humidity and wind.

There are steps you can take to avoid pollen.

^{Source: AAAAI, "Tips to remember: outdoor allergens." American Academy of Allergy Asthma & Immunology. 01 January 2006. AAAAI. 18 Oct 2007 http://www.aaaai.org/patients/publicedmat/tips/outdoorallergens.stm}