When found in homes mold mites are usually associated with some type of moisture issue which has caused mold growth. The moisture source can be as simple as a leaky pipe or as difficult as a leaky roof or foundation. The mites feed on the mold and populations can grow rapidly. The mites are tiny, and usually white or tan in color, and have very long "hairs", or setae.
See Mold Mites In Homes for additional life history and control information.
Mold mites are sometimes called "grain mites" because they also commonly occur in grain storage warehouses on the surface of grain sacks.
The mites are harmless. They don't bite or cause structural damage. However, the long body hairs can contribute to indoor allergens when they become airborne. The mites are generally an indication of a mold/moisture problem which is usually more of a concern than the mites themselves. No treatment is needed other than reduction in moisture.
See the "Mold Mites In Homes" article cited above for more information.