Q:

5/2/2012
What is the relation between humidity and house dust mite levels?

A:

Thank you for your inquiry.

As you can see from the three abstracts copied below, there is a direct relationship between relative humidity and house dust mite growth. In general, keeping the relative humidity below 50% considerably reduces the growth of dust mite. It is known that in areas of very low humidity such as exists in cities like Davos, Switzerland, and Denver, Colorado, there is little if any dust mite growth.

Thank you again for your inquiry and we hope this response is helpful to you.

Experimental and Applied Acarology
Volume 16, Numbers 1-2 (1992), 15-35, DOI: 10.1007/BF01201490
The house dust mites, Dermatophagoides farinae, D. pteronyssinus andEuroglyphus maynei, are prevalent in homes in humid geographical areas throughout the world. These mites thrive in humid environments in human dwellings where there is no liquid water to drink. However, their bodies contain 70¨C75% water by weight, which must be maintained in order to reproduce. Their primary source of water is water vapor which is actively extracted from unsaturated air. At relative humidities above 65¨C70%, adequate amounts of water can be extracted from unsaturated air to compensate for that lost by all avenues. Active uptake is associated with ingestion of a hyperosmotic solution which is secreted by the supracoxal glands. Active mites do not survive longer than 6¨C11 days at RHs 50%. They survive extended dry periods by forming a desiccation-resistant protonymphal stage which can survive for months at RHs below the critical humidity for active stages. Feeding rate and allergen production is directly influenced by RH. Mites feed, multiply, and produce more fecal matter at higher RHs than at lower ones.

J Allergy Clin Immunol 1999;104:852-6
Abstract
Background: Indoor relative humidity (RH) is the key factor that determines the survival and population development of the house dust mite Dermatophagoides farinae . Maintaining RH below 50% is one recommendation in a comprehensive plan to reduce house dust mites and mite allergen levels in homes. Even when mean daily RH is reduced below 50%, RH may rise above 50% intermittently for brief periods because of activities in the home (eg, cooking, bathing, and ventilation).
Objective: The purpose of this study was to determine how brief daily periods of moist air alternating with long spells of low ambient RH (0% or 35%) influence population survival and growth of D farinae.
Methods: Population growth was determined for D farinaeat daily RH regimens 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, respectively, at 35% RH provided sufficient moisture for small growths in population size. These growths after 10 weeks were reduced by 98.2%, 98.0%, and 97.3% for daily regimens of 4, 6, and 8 hours, respectively, at 75% RH (with the remainder of the day at 35% RH) compared with the growth of populations continuously exposed to 75% RH. Continuous exposure to 85% RH inhibited population growth, but alternating daily regimens of 16, 18, and 20 hours at 35% RH 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 completely prevent population growth of D farinae , RH must be maintained below 35% for at least 22 hours per day when the daily RH is 75% or 85% for the remainder of the day.

Journal of Medical Entomology, Volume 35, Number 1, January 1998 , pp. 46-53(8)
Abstract:
Experiments were conducted to determine the effects of relative humidity on the population dynamics of single and mixed species of Dermatophagoides farinae Hughes, D. pteronyssinus (Trouessart), and Euroglyphus maynei (Cooreman) at specific relative humidities maintained at 20¡ãC, with unlimited food. The population density of single and mixed species (D. farinae andD. pteronyssinus) increased exponentially when cultured at 65, 70, and 75% RH. The mean population growth rates were 17.3 ¡À 4.4 SD and 32.5% ¡À 4.7/wk for D. farinae and D. pteronyssinus, respectively. Mean population doubling times were 2.2 ¡À 0.3 and 4.2 ¡À 1.3 wk for D. pteronyssinus and D.farinae, respectively. Mixed species cultures, started with equal numbers of D. farinaeand D. pteronyssinus, resulted in higher percentages of D. farinae than D. pteronyssinus. In cultures started with 75% of one species and 25% of the other, the more numerous species remained dominant and in similar ratios throughout the experiment. Both D. farinae and D. pteronyssinus population densities maintained at 85% RH declined over a 12-wk culture period because of mold growth. E. maynei were unable to survive at 65, 70, 75, and 85% RH, which indicated that their climatic requirements were different from those of D. farinae and D. pteronyssinus. Population densities of D. farinae and D. pteronyssinus cultures declined when held at 21-22¡ãC and relative humidities of ¡Ü50%; however, at 50% RH, significant proportions of the populations survived for 10 wk. Half-life for desiccation of D. farinae and D. pteronyssinus at 45% RH was 11.5 and 1.2 wk, respectively, but at 50% RH was 86.3 and 4.0 wk, respectively. The data indicated that a ¡Ü5O% RH would have to be maintained for long periods to reduce both D. farinae and D. pteronyssinus by desiccation procedures. The results of this study show that D. farinae and D. pteronyssinus have high reproductive potentials and population growth rates, which indicate that mite reduction procedures must be thorough or mite densities will return to high levels quickly following remediation if adequate food and suitable microclimatic conditions exist.

Sincerely,
Phil Lieberman, M.D.

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