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Types of Allergens

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Pollen, molds, dust mites, or animal dander cause most allergic rhinitis and trigger asthma attacks in some people.


Plants make pollen. The pollens that can cause allergies are usually from trees, grasses, or weeds. These pollens are small, light, and dry, so the wind easily picks them up and carries them, often for long distances. Flowers also make pollen, which is spread by insects. Usually, only a little flower pollen gets into the air. So people usually do not have symptoms from this kind of pollen.

The pollen count measures the pollen in the air. When this number is high, you will be more likely to have an allergic reaction if you are allergic to pollens. Windy, dry weather often results in a high pollen count, while damp weather lowers it.

During the allergy season, TV stations, newspapers, or medical centers report local pollen counts. If you have allergies, find out where the pollen counts are advertised so you can avoid pollen when the counts are high. You may also look up the National Allergy Bureau's website for pollen count information at

The exact dates of pollen seasons vary in different parts of the United States, but usually:

  • Winter and spring bring tree pollens.
  • Late spring to midsummer brings grass pollens.
  • Late summer to early fall brings weed pollens. Ragweed pollen is a common cause of allergic rhinitis in the United States.


Mold looks like green, black, or white splotches on plants or on damp surfaces. Mold contains spores, which float in the air like pollen. If you are allergic to mold, breathing in the spores will cause a reaction.

Mold can live on soil, plants, rotting wood, or dead leaves. Indoors, it can live in damp places like basements, bathrooms, refrigerators, and window sills. Some outdoor mold spores are more likely to be in the air on dry, windy days; others are more likely to be present on rainy days.

Dust mites

Dust mites are tiny, eight-legged creatures (arachnids) that are so small you can only see them with a microscope. Dust mites eat dead flakes of human skin. They do not bite or spread disease. People are allergic to the insects' droppings, not the insects themselves. If you are allergic to dust mites, you usually have a problem all year.

Dust mites live mostly in carpeting, bedding, furniture, or stuffed animals. How clean your home is has nothing to do with whether you have dust mites. They thrive in warm, damp conditions where the humidity is more than 50%. They are not found in cold or dry climates.


WebMD Medical Reference from Healthwise

Last Updated: 2/, 014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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