Topic Overview

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
  • Late spring to midsummer brings grass
  • 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.

Animal dander

Animal dander is loose skin cells (like dandruff) from animals such
as dogs and cats. These skin cells contain saliva or other material from the
animals. The cells float in the air. If you are allergic to animal dander, when
you breathe in the cells you may have an allergic reaction.

Animal dander causes the allergy, not animal fur or hair. The
length of the fur on an animal does not affect how allergic you are to an


Cockroaches cause an allergic reaction in many people. The reaction
results from breathing the air where there are dead cockroaches and their
droppings (feces).


ByHealthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD – Internal Medicine
Specialist Medical Reviewer Rohit K Katial, MD – Allergy and Immunology

Current as ofOctober 6, 2017