Seasonal allergies cause symptoms during certain times of the year as opposed to perennial allergies, in which symptoms occur year round. These patterns are usually predictable from year to year and correspond to periods when pollens or molds are in full bloom. 

Those with seasonal allergies experience symptoms when an allergen gains access to the body, usually through the nose (inhaled). However, allergens can also affect a person if they are ingested (eaten) or come into contact with the skin (touched). People can develop seasonal allergies at any age.

Seasonal allergies are usually triggered when an allergic individual comes into contact with proteins found in certain airborne particles during a certain time of year. These proteins can come from a variety of sources, but usually include:

• Pollens. Small, powdery grains of flowering plants that are involved in fertilization and can easily become airborne. These are most often produced by trees, grass and weeds. Ragweed is a major culprit for the majority of people who suffer from pollen-related symptoms.
  

  

   
• 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, basements and bathrooms. Because molds depend on damp and dark conditions, they have a less predictable season than pollens.
  
  

  

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. Plants begin to bloom and release pollen in the spring, however not all areas of the country begin allergy season at the same time. Some trees begin to pollinate as early as January in the southern United States and April in the northern states. By late spring, grasses are pollinating – followed by weeds in the summer and fall. Leaf mold during the fall is also a major trigger of seasonal allergies. Warmer climates will see outdoor mold spore growth peak by midsummer. “Seasonal” allergies can also be present year-round in some warmer climates.

By contrast people susceptible to indoor molds, dust mites and pet dander usually suffer from perennial allergic rhinitis, which is a form of allergy with symptoms that manifest throughout the year. However, it is important to note that allergens that trigger seasonal allergies can be tracked indoors.

Pollens can also cross-react, creating a condition known as oral allergy syndrome (OAS). In OAS, the body mistakes a substance in food for an allergen to which it is sensitive. The proteins in some foods (usually fresh fruits, vegetables and nuts) are similar to proteins found in some types of pollen. At times when pollen counts are high, people with pollen allergies may experience itching or swelling around the mouth, throat and lips after eating fresh fruits, vegetables or nuts. Patients with this condition must avoid any offending foods.