Reducing relative humidity is a practical way to control dust mites and their allergens in homes in temperate climates

Background: Maintaining a relative humidity [RH) of less than 50% is one recommendation fur reducing numbers of house dust mites and their allergens in homes, Objective: The purpose of this study was to determine whether, in a humid temperate climate, indoor RH could be sufficiently lowered to control dust mites and their allergens. Methods: During a period spanning 2 humid summers (May 1998 to October 1999), dust mite and allergen densities were determined in 3 groups of homes. One group (low RH group, n = 23) maintained an RN of less than 51%, Most of these homes used a high-efficiency dehumidifier and air conditioning. A second group of homes (group A) used air conditioning only (n = 19) or air conditioning and dehumidification (n = 5) but did not maintain an RH of less than 51%, A third group of homes (group C, n = 24) controlled climate by opening windows and had an RH of greater than 51%. Normal housecleaning was maintained in all homes during the study, Results: The low RH group homes started in June with a mean +/- SE of 401 +/- 124 live mites and 17 +/- 3 mug of total Der 1 allergen per gram of dust. After 17 months of maintaining an RH of less than 51%, these declined significantly to 8 +/- 3 live mites per gram LP =.004) and 4 +/- 1 mug of Der 1 per gram of dust (P <.001). In contrast, group A and C homes exhibited seasonal peaks of 500 to 1000 mites and 40 to 70 <mu>g of Der 1 per gram of dust At all time points after the baseline sample, the low RH group homes had significantly less (P <.001) allergen than the group A and C homes, After 17 months, allergen levels were more than 10 Limes lo,Per in low Rfi homes compared with humid homes, Conclusion: This study showed that it is practical to maintain an indoor RH of less than 51% during the humid summer season in a temperate climate, and this resulted in significant reductions in mite and allergen levels.