By Dr. Brian McDonough, Medical Editor

PHILADELPHIA (CBS) – Dust mites are the most common cause of allergies from house dust.

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High levels of exposure to dust mites are an important factor in the development of asthma.

Dust mite particles are found mostly in pillows, mattresses and carpeting. They float into the air with virtually any disturbance – like when a person walks on a carpet or disturbs bedding or furniture.

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There may be as many as 19,000 dust mites in one gram of dust, but usually between 100 to 500 mites live in each gram.

By the way…there is a good reason why dust mites live in our homes. Mites eat particles of skin and dander, so they thrive in places where there are people and animals.

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Dust mites don’t bite, cannot spread diseases and usually do not live on people. They are harmful only to people who become allergic to them.