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While hay fever and allergies caused by trees are usually associated with springtime, seasonal allergies can also spike during the early fall months. Cool autumn air harbors irritants that can be just as unpleasant as pollen.

Allergens from trees and grasses float through the air in spring, summer and fall, according to the National Institutes of Health (NIH). These particles can enter a person's nose, eyes, ears and mouth, triggering an allergic reaction.

"The most common fall allergy is ragweed, which pollinates from August 15 to early October through most of the United States and parts of Europe," said Dr. Jay M. Portnoy, chief of allergy, asthma and immunology at Children's Mercy Hospitals & Clinics in Kansas City, Mich. "It causes hayfever, with symptoms that include sneezing, runny nose, stuffy nose, itchy nose and itchy, watery eyes."

Mold spores are also released in autumn, and become more common in the air as decaying leaves and other vegetation fall to the ground, Portnoy said. This worsens allergy symptoms because as mold particle counts climb higher, they become increasingly irritating to people with allergies. High mold counts also contribute to breathing problems among those with asthma, Portnoy said.

"Sadly, fall is also virus season, with increased colds and the flu," Portnoy told Life's Little Mysteries. "Since all of these are happening at the same time, it is often hard to tell what is due to allergies and what is due to infection."

Attempting to escape fall allergens by staying indoors probably won't work, Portnoy said, because low humidity inside homes is another major trigger of nasal and lung allergy symptoms. Low humidity dries out mucous membranes and leads to inflammation, while cold, dry air causes the lining of the nose to become swollen, resulting in a stuffy and runny nose.

While the ideal indoor humidity is from 35 to 50 percent, homes and offices may have a humidity level as low as 16 percent. Portnoy advises using a humidity meter, also known as a hygrometer, to keep track of the humidity levels. If levels are low, a humidifier may help relieve nasal problems.

Changing vent filters and servicing heating systems can also ease symptoms. Vacuuming and cleaning the house often to keep dust mites, pet dander or other indoor allergy triggers under control may help alleviate discomfort, according to the NIH.

But there's still some good news for those suffering from autumn allergies.

"Many people get better once the weather turns colder and stops fluctuating from warm and cold," Portnoy said. "By November, it often gets better."

This article was provided by Life's Little Mysteries, a sister site to LiveScience.