Outdoor allergens are substances that trigger symptoms in individuals with certain allergies and asthma. Pollen and mold are the most common outdoor allergens, but there are many other allergens present outdoors.

These allergens usually gain access to the body through the nose (inhaled) and trigger symptoms of allergic rhinitis (inflammation of the inner lining of the nose due to contact with an allergen). However, they can also enter by ingestion (eaten) or skin contact (touched) and trigger various other symptoms.

In people with allergies or allergic asthma (an allergic inflammation in the lower airway of the respiratory system), the body mistakes these harmless substances as “foreign invaders” and creates antibodies (proteins that attach to foreign substances and help destroy them) to defend against them. This process, called an allergic cascade, leads to the symptoms associated with allergic reactions.

Allergens carry proteins that trigger symptoms in allergic individuals. Outdoor allergens that carry these proteins include:

• Pollens. Small, powdery grains of flowering plants that can easily become airborne. These are most often produced by trees, grass and weeds. Many plants carry pollens that can trigger allergies, but ragweed is the most common. In fact, 75 percent of people with seasonal allergies are allergic to ragweed, according to the Asthma and Allergy Foundation of America.
  
  

  

• Molds and mildews. Tiny fungus spores that can become airborne. These often thrive outdoors in soil, vegetation and rotting wood. They can be found indoors as well, especially in damp areas, such as basements and bathrooms.
  
  

  

Because the airborne levels of pollen and mold change with the seasons, allergic individuals are more likely to experience symptoms during specific times of the year. Reactions to outdoor allergens usually flare up in the spring when plants begin to bloom and release pollen. Some trees begin to pollinate as early as January in the southern United States and February in the northern states. By late spring, grasses are pollinating – followed by weeds in the summer and fall. Warmer climates will see outdoor mold spore growth peak by midsummer.

A person's symptoms can also vary day-to-day or within the same day depending on the weather. Allergy symptoms are often minimal on rainy, cloudy or windless days because pollen does not move around during these conditions. However, hot, dry and windy weather results in increased allergy symptoms due to greater pollen and mold distribution. Pollen counts also tend to be lowest in the evening hours and highest in the mornings.

Though most outdoor allergens vary by season, there are certain locations where pollen or mold is present all year. Therefore, people in these areas may have outdoor allergies that are perennial (symptoms occur year-round). However, it is much more common for people with perennial allergies to be allergic to indoor allergens than for outdoor allergens to trigger year-round symptoms.