Hello,

I hope you're doing well and enjoying summer. I also hope that you'll find the following article about seasonal pollen by Shamika Edwards both interesting and useful. Shamika is an NAB Certified Pollen and Mold Spore Counter.

With best wishes,
Dave Gallup

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Each spring, summer, and fall, the season is filled with lush colors of blooming trees, grasses, and weeds, which release tiny particles. The tiny particles are known as pollen. Pollen has several vectors it uses for mobility; anemophily (movement via wind), entomophily (movement via insects), ornithophily (movement via birds), hydrophily (movement via water), chiropterophily (movement via bats), and zoophily (movement via other animals). The pollen grains proceed by hitching a ride on the currents of the air. Then, nature does its part to generate variation and speciation through cleistogamy (self-pollination), and allogamy (cross-pollination). Although the main purpose of pollen is to fertilize other plants, many times it never succeeds in making it to its intended target. Instead, pollen enters human noses and throats, triggering a type of seasonal allergic rhinitis typically called pollen allergy or hay fever.

Trees, grasses and weeds have a very distinct period of pollination that typically do not vary from year to year. Generally, the entire pollen season lasts from February through October with pine having an elevated pollen production throughout. Even though it is abundant, pine pollen is seldom an important allergen. The pollinating season, however, starts later in the spring the further north one goes. In warmer places, pollination can occur year-round. In most southern states, tree-pollinating season commences in late December and ends in May. In the South Texas region, a unique fall pollination of Ulmus (elm) and evergreens such as Juniperus (junipers). Also in Texas, mountain cedar pollen (Juniperus ashei) is another unique pollination period that occurs in December and January. Typically, grass pollen begins in late May followed by the weed season in June and July. Starting in August, weed pollen increases in the environment and by the end of August, ragweed pollen begins to dominate the air.

Figure 1: Microscopic photo of Pine (Family: Pinaceae; Genus: Pinus) pollen showing characteristic air bladders giving it the appearance of Mickey Mouse ears. Copyright © 2010 EMLab P&K

Figure 2: Microscopic photo of Elm (Family: Ulmaceae; Genus: Ulmus) pollen produced from January through April. Copyright © 2010 EMLab P&K

Seasonal allergic rhinitis is often caused by tree pollen in the early spring. The chemical makeup of pollen is the basic factor that determines whether the pollen is likely to cause any type of allergic symptom. During the late spring and early summer, grasses often cause symptoms. Hay fever is caused by weeds in the late summer and early fall. In the late fall, unique to Central Texas, is a seasonal allergic rhinitis known as cedar fever. Trees that produce allergenic pollen include oak, ash, elm, hickory, pecan, box elder, and mountain cedar. Among North American plants, weeds are the most prolific producers of allergenic pollen. Usually ragweed is the major culprit, but others of importance are sagebrush, redroot pigweed, lamb's quarters, Russian thistle (tumbleweed), and English plantain. Grasses are known to be a significant source of allergenic pollen. Timothy grass, Kentucky bluegrass, johnsongrass, Bermuda grass, redtop grass, orchard grass, and sweet vernal grass are all known to produce highly allergenic pollen.

Figure 3: Microscopic photo of Juniper (Family: Cupressaceae; Genus: Juniperus) pollen. Copyright © 2010 EMLab P&K

Figure 4: Microscopic photo of Grass (Family: Poaceae) pollen produced May through October. Copyright © 2010 EMLab P&K

A pollen count, which is familiar to many people from local weather reports, is a measure of how much pollen is in the air. This count represents the concentration of all the pollen, expressed as grains of pollen per cubic meter of air collected over 24 hours. This count is generated by certified Pollen Counters of the National Allergy Bureau (NAB). There are only 106 certified pollen counters in the country and 81 NAB-certified counting stations across the United States, Canada and Argentina. Each count comes from a NAB counting station, which is part of the American Academy of Allergy, Asthma and Immunology's (AAAAI) Aeroallergen Network, responsible for reporting current pollen and mold spore levels to the public.

Figure 5: Microscopic photo of Ash (Family: Oleaceae; Genus: Fraxinus) pollen produced from March through June. Copyright © 2010 EMLab P&K

Figure 6: Microscopic photo of Ragweed (Family: Asteracea; Genus: Ambrosia) pollen produced from August through October. Copyright © 2010 EMLab P&K

Weather has significant effects on pollen release. The most pollen will be released on warm, dry, sunny, and windy days. Cold temperatures and high humidity delay pollen release, and precipitation washes pollen out of the air. Certain weather conditions can increase or decrease the amount of pollination. If the winter is mild, then typically the allergy season will begin early because the trees will release their pollen earlier than normal. If, on the other hand, a mild spring occurs this will intensify the tree pollen release for the spring. Winds are another contributing factor, which can spread the pollen rapidly, thus increasing the pollination. The windier the conditions are, the higher distribution of pollen within the air. If the weather generates a late freeze, then tree pollination will be delayed or could possibly decrease. Increased rain amounts in fall or winter can cause an increase in spring tree pollination amounts. Increased rain amounts in spring can stimulate grass growth, thus producing more grass pollen. The lifeline of pollen depends on the weather, through dry days, breezy days, rainy days, foggy days and, humid days.

References:
1. Farrar JL, Trees of the Northern United States and Canada. Ames, IA: Iowa State University Press; 1995.

2. Smith, E. Grant, Sampling and Identifying Allergenic Pollen and Molds. TX: Blewstone Press; 2000.

3. Sharma Smrity. Allergy season at its worst in years in US. The Money Times. April 16, 2010.

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