Reducing relative humidity to control the house dust mite Dermatophagoidesfarinae

Background: Indoor relative humidity (RH) is the key factor that determines the survival and population development of the house dust mite Dermatophag oides farinae. Maintaining RH below 50% is one recommendation in a comprehe nsive plan to reduce house dust mites and mite allergen levels in homes, Ev en when mean daily RH is reduced below 50%, RH may rise above 50% intermitt ently for brief periods because of activities in the home (eg, cooking, bat hing, and ventilation). Objective: The purpose of this study was to determine how brief daily perio ds of moist air alternating with long spells of low ambient RH (0% of 35%) influence population survival and gros th of D farinae. Methods: Population growth was determined for D farinae at daily RH regimen s of 2, 4, 6, and 8 hours at 75% or 85% RH alternating with 22, 20, 18, and 16 hours at 0% or 35% RH. Results: D farinae populations declined at daily regimens of 2 hours at 75% or 85% RH alternating with 22 hours at 0% or 35% RH, Daily regimens of 4, 6, and 8 hours at 75% RH alternating with 20, 18, and 16 hours, respectivel y, at 35% RH provided sufficient moisture for small growths in population s ize. These growths after 10 weeks were reduced by 98.2%, 98.0%, and 97.3% f or daily regimens of 4, 6, and 8 hours, respectively, at 75% RH (with the r emainder of the day at 35% RH) compared with the growth of populations cont inuously exposed to 75% RH. Continuous exposure to 85% RH inhibited populat ion growth, but alternating daily regimens of 16, 18, and 20 hours at 35% R H allowed small populations to develop, although they were reduced by 99.4% , 98.8%, and 99.1% compared with population growth at a continuous 75% RH. Conclusion: This study indicates that maintaining mean daily RH below 50%, even when RH rises above 50% for 2 to 8 hours daily, effectively restricts population growth of these mites and thus the production of allergen. To co mpletely prevent population growth of D farinae, RH must be maintained belo w 35% for at least 22 hours per day when the daily RH is 75% or 85% for the remainder of the day.