It isn't known why some people get hay fever, or seasonal allergic rhinitis, and others don’t. Seasonal allergy symptoms include runny nose and itchy eyes when certain allergens contained in plant pollens or molds are inhaled. Trees, weeds, leaves and grasses—not flowers—are the most likely sources of allergens. When a person suffers from seasonal allergies, his body mistakenly send antibodies to fight these normally harmless substances. This prompts the release of histamines, which cause the physical symptoms common to all types of pollen and mold allergies.
Weed Pollen
Ragweed pollen allergy is the leading source of hay fever in the United States. The NIH reports that ragweed in the east, and sage and goosefoot in other parts of the country, provoke allergy symptoms anywhere from August through November. These allergenic plants make up for their short life cycles by producing up to one billion pollen grains per plant, according to the Asthma and Allergy Foundation of America.
Leaf Mold
The fungi that grow on downed leaves may also cause seasonal allergic rhinitis symptoms. The University of Maryland Medical Center relates that some people react to the allergenic spores of leaf molds that are released into the air on contact. Mold allergy patients can keep track of the likelihood of problems via local pollen and mold count announcements.
Tree Pollen
Trees release large amounts of pollen into the atmosphere, which disperses the allergens for up to hundreds of miles. Sizable trees have fewer obstructions than ground-dwelling plants to block their pollen from the wind. For this reason, the National Institutes of Health (NIH) points out, allergy symptoms can arise in people who have no allergenic trees growing in their environments. Hot, dry, windy weather sends some tree pollens into distant states, such as the mountain cedar tree, which is the most allergenic tree in Central Texas. Still, most trees do not cause hay fever, and among those that do, only the males of some species pose a threat. Trees known to trigger allergic rhinitis in the spring include walnut, poplar, willow, alder, juniper and cypress.
Grass Pollen
Grass pollen counts have similar criteria as tree pollen levels. High temperatures and strong breezes fill the air with grass pollen, causing allergic rhinitis wherever it travels. Temperature and humidity especially affect the pollen release of low-growing grasses. A good rain prevents patients from ingesting much grass pollen, which sticks to the ground. Cool weather restricts airborne pollen travel. As the Cleveland Clinic reports, allergy symptoms occur in some people when fields of Kentucky blue, timothy or orchard grasses are pollinating, generally in the summer.
