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General Pollen Information

Pollen and Mold

Missouri State Air Pollution Readings

Pollen and Mold

Reduce your exposure to aeroallergens, monitor the daily pollen and mold levels by visiting our Pollen and Mold Center or by calling our Pollen Information Line at 314-615-6825.

Both our Center and our Pollen Line are updated at 11:00 am each weekday with the latest pollen and mold counts and with the day's air quality level. 

The Center has extensive information on different pollens and molds, as well as research links to more information.

If you suffer from asthma or serious allergies, stay indoors or take other precautions during peak pollination periods.

We monitor atmospheric pollen and mold (aeroallergens) throughout the year, measure and record airborne pollen and mold levels. We collect, identify and count aeroallergens under microscopic magnification. This data is reported to news and weather forecasters, and to health organizations such as the American Lung Association.

The daily pollen and mold counts reflect the seasonal variation for the entire St. Louis metropolitan area. Meteorologists, allergy specialists, physicians and individuals have relied on this data since 1960.

When are pollens most prevalent?


Pollen season occurs in three phases:


Tree pollens are dominant in the spring.


Grasses occur in late spring and early summer.


Weeds like ragweed grow from late summer until the first hard frost.

Warm gentle breezes raise pollen levels.
Heavy rains temporarily cleanse the air of pollen.
A sudden temperature drop lowers the pollen count.


Seasonal pollens and their concentration vary with location. The type of vegetation in your own area, whether native trees, ornamental or agricultural, contributes to your exposure.

Pollen also possesses diurnal variation. This means pollen grains are generally shed in the early morning hours. Later in the day, weather conditions and wind speed affect how much pollen becomes airborne.

When are molds most prevalent?

Unlike pollen, molds (fungi) are not seasonal. Because mold is more greatly affected by day to day weather conditions, its overall prevalence in the air is harder to predict than pollen.

Molds occur all year, with peaks in spring and fall. They are closely tied to temperature and humidity. The mold count is very low during cold wather, especially when snow is on the ground.

Mold count is highest during late summer and early fall, due to decaying vegetation. Parasitic fungi such a smuts and rusts are numerous during harvest time, as their spores become airborne.

Several types of fungi comprise the daily mold count. Alternaria, Cladosporium, and Epicoccum are dry weather spores. During dry periods, fungi increase in number until noon and decrease throughout the evening. Ascospores and Basidiospores are considered wet weather spores, and are plentiful during light rainfall or in predawn hours when condensation is heavy.

Many fungi have internal mechanisms which actively discharge their spores into the air. Other fungi rely on air movement for their dispersal.

How are pollen and mold counted?

Pollen grains and mold spores are reported as the number of particles per cubic meter of air sampled.

Efficient equipment collects tiny particles. In January 2000, the Saint Louis County Department of Public Health replaced a roto-rod sampler that had been in use since 1985 with a more efficient, slit-type-volumetric spore trap. The device is mounted on the roof of a centrally located, Saint Louis County Department of Public Health building, where it collects airborne particles of pollen and mold. Pollen grains and mold spores, ranging in size between one and 100 microns, flow through the sampling apparatus and stick to a greased flat surface.

Note: Although counts for pollen grains are comparable, counts for mold spores may typically be three to ten times higher than numbers reported in the past, which were based on the roto-rod sampler.

Magnification: Before a pollen sample can be identified and counted, it must be magnified at least 400 times. Many mold spores are extremely samll, and need to be magnified 1,000 times for observation.

Precision: To assure the accuracy of our pollen counts, each analyst's performance is evaluated by the American Academy of Allergy Asthma and Immunology Quality Control Program.

Aeroallergens and asthma

Pollen and mold have been clearly implicated in asthma, allergic rhinitis and other respiratory diseases. About four percent of the United States population suffers from asthma. Each year, four to six thousand Americans die from asthma. Asthma often begins before age five, but can start at any age.

If you suffer from asthma or serious allergies, stay indoors or take other precautions during peak pollination periods.

Call 314-615-6825 or visit our Pollen and Mold Center for the daily pollen and mold count.