The main triggers of outdoor allergies are pollen and mold.

Pollen is the microscopic granules that plants produce to fertilize other plants of the same species; they are the plant’s male reproductive cells. While several pollen grains together usually appear powdery in texture, most individual pollen grains are smaller in size than the width of a human hair. Pollen grains travel through the air easily and can also accumulate on outdoor surfaces such as patio furniture or cars.

Interestingly, the propensity of a plant to have allergy-causing pollen is related to its appearance. The pollen produced by bright flowering plants rarely causes allergies, while less eye-catching plants like trees, grasses, and weeds often have very allergenic pollen. This is due to the fact that flowing plants rely on birds and insects for fertilization (and the elaborate coloring serves to attract them), thus these plants have large, waxy pollen designed to attach itself to these visitors. Most trees, grasses, and weeds have small, dry pollen that is designed to be spread by the wind and it is these plants that tend to trigger allergy symptoms.

The type of pollen you encounter outdoors depends on the current season and the part of the country in which you live. Outdoor allergies in the early spring are often triggered by the pollen of the oak, western red cedar, elm, birch, ash, hickory, poplar, sycamore, maple, cypress, and walnut trees. For those with outdoor allergies in the late spring or early summer, pollen from grasses such as timothy, Bermuda, orchard, sweet vernal, red top, and some blue grasses are the main cause. In late summer and fall, ragweed is the most common source of allergenic pollen but other sources include sagebrush, pigweed, tumbleweed, Russian thistle and cockleweed.

For many people, outdoor allergies tend to occur at around the same time every year because plants tend to have a very regular pollination schedule. However, recent weather history and current conditions can dramatically affect the amount of pollen that is present in the air at a given time. Latitude is another major factor in the timing of pollination. In general, pollination seasons start later the farther North you go. In the South, pollen may show up as early as January, while in the Northern U.S. it may not begin until late in April.

The other main cause of outdoor allergies is mold. Mold is a microscopic fungi related to mushrooms that tends to grow in damp places. It reproduces by producing spores, and like pollen, mold spores can travel through the air. Unlike pollen, different molds do not have specific seasons, but the level of mold spores is heavily influenced by weather conditions such as wind, rain, and temperature. Mold spores are present year round in the South and on the West Coast, but in other areas they tend to peak in July in the warmer states and October in the colder states.

Common airborne molds include alternaria, cladosporium and aspergillus. Molds are present almost everywhere outdoors, including in soil, vegetation, and rotting wood. Mold can also be found indoors, especially in damp areas such as attics, basements, bathrooms, and inside refrigerators.