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You should keep your home cool and dry to decrease mold and dust mite growth and roach intrusions. After an 18-month study, Larry G. Arlian, PhD, professor of biological sciences and director of the microbiology and immunology graduate program at Wright State University in Dayton, OH, found that levels of mites and other allergens dropped significantly in homes where the relative humidity was kept below 50 percent. To maintain this level, you may need to use your air conditioner along with a dehumidifier. (If you use a dehumidifier, be sure to empty the water daily–or better yet, have the machine run directly into a drain, if possible.)