Final Report: Factors Controlling the Dust Mite Population in the Indoor Environment

EPA Grant Number: R825250
Title: Factors Controlling the Dust Mite Population in the Indoor Environment
Investigators: Arlian, Larry G.
Institution: Wright State University - Main Campus
EPA Project Officer: Katz, Stacey
Project Period: December 1, 1996 through November 30, 1999 (Extended to November 30, 2000)
Project Amount: $480,000
RFA: Air Quality (1996) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Air

Objective:

The objectives of this research project were to:

1. Establish that the regulated use of dehumidifiers and air conditioning in homes with high mite levels reduces the relative humidity (RH) sufficiently to reduce mite and mite allergen levels.

2. Establish dust mite survival and population dynamics in fluctuating climatic conditions.

3. Define specific temperatures required to kill mites in water and evaluate various laundry detergents and carpet cleaning products for their efficacy in killing mites and removing allergens from various carpet types.

4. Elucidate the relationship between xerophilic fungi and house dust mites.

5. Elucidate how mites disperse and colonize dwellings.

Summary/Accomplishments (Outputs/Outcomes):

The findings are presented below by objective.

Objective 1. Establish that the regulated use of dehumidifiers and air conditioning in homes with high mite levels reduces the relative humidity (RH) sufficiently to reduce mite and mite allergen levels.

In homes that maintained indoor RH <51 percent, the average live mite count was 401.2 mites/g of dust at the beginning of the study then it declined to 8.2 mites/g by the end of the first heating season (March 99) and 6.7 mites/g at the end of the study in October 1999. In addition, reservoirs of dead mites gradually declined during the 17-month study. In contrast, the densities of live mites in homes in Groups A (average ambient RH >51 percent with air conditioning [with or without dehumidifier]) and C (average ambient RH >51 percent control homes [no air conditioning or dehumidifier]) increased in parallel with the summer increases in indoor ambient RH during both summers and then decreased in the late summer and fall as indoor ambient RH decreased. Dead mites in these high humidity homes paralleled live mite densities.

The mean concentrations of Der 1 (Der f 1 + Der p 1) for the 3 sampled sites continually decreased in the Low RH Group homes from 17.5 µg/g of dust at the beginning of the study to 4.0 µg/g of dust at the end of the study. In contrast, mean Der 1 concentrations in Group A and C homes showed similar trends to those seen for the mite counts and increased during the humid summer months then decreased during the winter months.

The ability to maintain RH<51 percent was independent of all the physical house characteristics. Homes that maintained indoor RH <51 percent showed no correlation in live mite counts between those built with either basements or slabs. In addition, there was no correlation between live mite counts and the presence or absence of pets. Similar results were seen with total Der 1 (Der f 1 + Der p 1) allergen.

Conclusion: In conclusion, our study showed that it is possible, practical and effective to reduce indoor RH to levels that will control dust mites. This coupled with regular vacuum cleaning by the participants of the study resulted in the reduction of allergen in surface dust to insignificant levels. In addition, there was no correlation between the physical home characteristics and the mite counts and allergen density in homes nor the physical home characteristics and the ability to maintain low RH. Presence or absence of pets was not a factor correlated with mite or mite allergen density in any group.

Objective 2. Establish dust mite survival and population dynamics in fluctuating climatic conditions.

Population Studies Under Fluctuating Relative Humidities: D. farinae populations declined to a few surviving mites when give daily regimes of 2, 4, 6, and 8 hours of moist air (75 or 85 percent RH) and 22, 20, 18 and 16 hours of dry air at 0 percent RH, respectively, for 10 weeks. In contrast, daily moisture regimes of 75 or 85 percent RH for 4, 6, and 8 hours and dry air of 35 percent for 20, 18, and 16 hours, respectively, provided sufficient moisture to support minimal population growth up to 14 weeks. These results indicate that long intervals at 0 percent RH inhibited mite survival, but raising the dehydrating RH to 35 percent supported population survival and some growth even though both conditions were dehydrating to the mites. Therefore, passive sorption of water at 35 percent RH provided the necessary moisture for survival, because hydrating conditions (75 or 85 percent RH) were identical for both experiments.

Mite survival was directly related to the time exposed to moist conditions. The experimental groups exposed to fluctuating RHs of 75 and 35 percent RH were ~ 98 percent smaller than the control cultures held at a constant 75 percent RH which indicated that long periods at 35 percent RH depressed normal population growth. In contrast, the mite population increases were inversely proportional to the amount of time in hydrating moist air at 85 percent RH (the remaining time at 35 percent RH) which indicated that too much moisture (high RH for long periods of time) also inhibited mite growth.

With or without water stress, there was no apparent accumulation of any active life stage in the population which indicated the test conditions did not induce formation of a diapause or prolonged quiescent nymphal life stage.

Conclusion: The results of these experiments are significant because they demonstrated that D. farinae mite populations showed remarkable adaptations for surviving daily regimes of dry air for long periods with only brief periods of moist air (75 percent). However, population growth rates were significantly reduced compared to those for mites held for long periods in moist air. To effectively control dust mites under fluctuating hydrating and dehydrating RHs, daily humidity must be kept at 35 percent for at least 16 hours. Finally, a significant finding was that daily intervals of 85 percent RH for 4, 6, or 8 hours (remaining hours at 35 percent RH) or the constant 85 percent RH (control) reduced or inhibited mite population survival when compared to population growth at a constant 75 percent RH. We assumed that this was associated with microbial growth.

Life Cycle Study Under Fluctuating/Constant Relative Humidities: The average daily ambient RHs in homes in temperate climates where D. farinae is found are typically 25-45 percent RH for 3-5 months during the heating season. Therefore, the purpose of this life cycle study was to determine how providing long periods of 35 percent RH while providing short periods daily of 75 percent RH affected the development of D. farinae. In this study we found that the life cycle could be completed when the developing mites were given daily regimes of only 4 or 6 hours of moist air daily (75 percent RH) alternated with a dehydrating RH of 35 percent. Egg incubation times were similar for the 3 fluctuating regimes but they were significantly longer in duration than at continuous 75 or 35 percent RHs. Overall, lengths of time for development were inversely related to the lengths of time that moist air was provided. This study confirmed that short periods of 4 and 6 hours of moist air at 75 percent RH while the remainder of the day was at 35 percent RH provided sufficient moisture for some mites to survive and complete the life cycle. However, the longer developmental times would depress the rapid population growth and therefore allergen accumulation exhibited by D. farinae.

As a control, we investigated how different constant RHs influenced the life cycle. We determined the duration of the life cycle of D. farinae (fresh eggs to adults) at 22 C and constant 60 and 75 percent RH. It is important to understand how different hydrating RHs directly influence the rate of development which in turn affect the rate of population growth and allergen production. The life cycle was completed in 36.8 and 40.4 days for mites held at 75 and 60 percent RH, respectively. There were no differences in the sex ratios of the resulting males and females.

Conclusions: The results of these studies demonstrate that the lengths of time for development of D. farinae were inversely related to the length of time that moist air was provided. D. farinae mites were unable to complete the life cycle when exposed to long periods of dehydrating conditions of 0 percent RH. However, when dehydrating conditions were 35 percent RH, only brief periods of moist air (4-6 hours) were needed for some mites to complete development. These results prove that RH is a key factor in mite development.

Multiple Matings by D. farinae: An additional experiment was conducted under this objective but at a constant RH. Previous life cycle studies found that after an initial mating, the reproductive periods (egg production) for D. farinae and D. pteronyssinus were similar and were 31 and 26 days, respectively. However, D. pteronyssinus females died within 2 days after cessation of egg production while D. farinae females continued to live for approximately 63 days. The long period after egg production before the death of D. farinae suggested that it might be possible for females to mate a second time and produce more eggs, thus increasing fecundity and population growth. Therefore, the purpose of this study was to determine if a second mating and reproductive period would occur for D. farinae females.

The first two experiments of this study revealed that after an initial mating and egg producing period, D. farinae females mated again and then produced a second small batch of fertile eggs. This was direct evidence that D. farinae was capable of successful multiple matings and determined that a second successful insemination was possible. However, in their natural environment or in cultures females have a continuous availability of males. Therefore, the third experiment determined that the females' reproductive potential was increased (30.7 percent) by multiple matings when they were continuously exposed to males, compared to a single mating. The egg producing period was increased by about 10 days or 32 percent.

Conclusion: These findings indirectly indicated that in natural or cultured populations two or more successful inseminations must occur.

Objective 3. Define specific temperatures required to kill mites in water and evaluate various laundry detergents and carpet cleaning products for their efficacy in killing mites and removing allergens from various carpet types.

The three mite species tested showed very different results. At hot water temperatures (50 C), D. farinae was the most sensitive mite species exhibiting 100 percent mortality after a 7.5-minute soak. The addition of detergents at this temperature for D. farinae did not seem to affect percent mortality. However, E. maynei and D. pteronyssinus seemed to be more resistant to these temperatures and the addition of detergents did seem to increase their mortality. Immersion of all 3 species in warm water (35 C) with or without detergents for 4 hours resulted in less than 50 percent mortality. D. farinae was the only species that had 100 percent mortality when submerged in recommended amount of bleach for 4 hours; D. pteronyssinus and E. maynei had 44 and 26 percent mortality, respectively, at these same conditions. The addition of bleach to the recommended concentrations of detergents at 35 C increased mortality for D. farinae only after a 4-hour soak.

Conclusion: These experiments show that washing clothes and bedding in hot water (50 C) for >30 minutes is required to kill D. pteronyssinus and E. maynei whereas a 7.5-minute wash will kill D. farinae. One-hundred percent mortality can be reached for D. pteronyssinus and E. maynei after 12- and 5-minute soaks, respectively, at 53 C. A 4-hour wash in warm water (35 C) kills less than half of the mites.

Objective 4. Elucidate the relationship between xerophilic fungi and house dust mites.

In order to assess this aim, we tested for the presence of fungus in the mites' culture medium or in/on the mites themselves. Studies also were done to investigate methods for killing mold in mite cultures while not directly affecting the mites. Various concentrations of fungizone (amphotericin B) were used to treat moldy cultures, as well as treating the culture medium that was offered to D. farinae.

Conclusion: From this series of experiments, it appears that mites thrive on mold-free culture medium. Mold does not normally grow in thriving cultures as evidenced by the fact that when mites and culture medium from thriving cultures were used to inoculate plates used for culturing molds, no mold colonies grew. Therefore, there is no obligate mite-fungus association when mites are cultured on some lab diets. However, the role of fungi for mites feeding on human skin scales in the natural environment remains to be answered.

Objective 5. Elucidate how mites disperse and colonize dwellings.

1. Eight homes that had live mites prior to replacing furnishings (carpet or furniture) were investigated. After replacing the carpeting or couch/chair, ~63 percent of the homes had live mites on the new articles in < 1 month. Two of the remaining homes (with ultra-efficient dehumidifiers) had dead mites in <2 months. One couch without mites had live mites 8 months after being moved to a home with live mites in the carpet.

2. Two potential factors of mite dispersal were investigated. The numbers of mites were analyzed on clothing and on automobile driver seats along with specific locations in homes (sofas, carpets) of the drivers. One hundred fifty (67.3 percent) automobile seats contained at least one mite/50 mg dust sample analyzed. More than 29 percent of the automobiles contained at least one live mite/50 mg dust. Seventy-seven percent of the 144 homes that had mite densities >100 mites/g of dust (5 mites/50 mg) had mites on their automobile seats. In contrast, mites were recovered from only 12.2 percent of the clothing sampled.

Conclusions:

It appears that automobile seats and clothing are contributing factors to mite dispersal. Automobile seats had mites at densities that could induce allergic symptoms in sensitive individuals. Clothing articles contained mites that could be carried from place to place within or between homes. Mites also were found to populate new furniture or carpet within one month.


Journal Articles on this Report : 4 Displayed | Download in RIS Format

Other project views: All 35 publications 4 publications in selected types All 4 journal articles
Type Citation Project Document Sources
Journal Article Arlian LG, Neal JS, Bacon SW. Survival, fecundity, and development of Dermatophagoides farinae (Acari : Pyroglyphidae) at fluctuating relative humidity. Journal of Medical Entomology 1998;35(6):962-966. R825250 (2000)
R825250 (Final)
  • Abstract from PubMed
  • Journal Article Arlian LG, Neal JS, Vyszenski-Moher DL. Fluctuating hydrating and dehydrating relative humidities effects on the life cycle of Dermatophagoides farinae (Acari: Pyroglyphidae). Journal of Medical Entomology 1999;36(4):457-461. R825250 (2000)
    R825250 (Final)
  • Abstract from PubMed
  • Journal Article Arlian LG, Neal JS, Vyszenski-Moher DL. Reducing relative humidity to control the house dust mite Dermatophagoides farinae. Journal of Allergy and Clinical Immunology 1999;104(4)852-856. R825250 (2000)
    R825250 (Final)
  • Abstract from PubMed
  • Full-text: Science Direct Full Text
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  • Journal Article Arlian LG, Neal JS, Morgan MS, Vyszenski-Moher DL, Rapp CM, Alexander AK. Reducing relative humidity is a practical way to control dust mites and their allergens in homes in temperate climates. Journal of Allergy and Clinical Immunology 2001;107(1):99-104. R825250 (2000)
    R825250 (Final)
  • Abstract from PubMed
  • Supplemental Keywords:

    indoor air, life-cycle, relative humidity, development, fungi, mites, allergens, detergents., Health, Scientific Discipline, Air, Health Risk Assessment, Risk Assessments, Allergens/Asthma, indoor air, asthma, dust mite, lungs, fungi, allergic rhinitis, laundry, exposure, carpet cleaning, airway inflammation, human exposure, inhalation, indoor air quality, climate factors, humidity

    Progress and Final Reports:

    Original Abstract
    2000 Progress Report