Mite allergens are considered one of the most potent allergens in house dust. The most common genus of mites found in house dust in north America and Europe is Dermatophagoides, of which there are two species, D. pteronyssinus and D. farinae. Their body parts and fecal excreta are initially 0.1 to 0.5 mm in diameter but break down into smaller fragments that become airborne when dust is disturbed. According to one study, more than half the weight of mite allergens within a home were found to be less than .5 mm in length. Because of their small size, fecal material are the primary health concern due to the fact that they can be inhaled into the lower airways of the lungs. If quantities of mite allergens are significant, IgE antibodies can form, leading to allergic reactions in the susceptible portion of the population.
Along with respiratory symptoms, high levels of dust mite allergens have also been correlated with atopic dermatitis (AD), characterized by itchy, irritated skin. In general, these studies suggest that those susceptible to mites (i.e., those likely to form IgE antibodies) are also likely to develop skin sensitization if exposed to high concentrations of mite allergens.
As one might expect, most mite allergens are formed by adult mite during their active phase. The survival of active adult mites (both male and female) is limited to 4 to 11 days at humidities below 50% RH at 25°C. The protonymph, however, which is one of the dormant larval forms, can survive for months at low humidities and then evolve to the more active forms when optimal conditions return. These protonymphs are particularly difficult to remove with normal vacuuming since they can bury themselves within surfaces.
Very high humidities can inhibit mite development due to the presence of mold growth. Humidity levels above 88%, in particular, lead to mold growth, which restrains mite development. Humidity levels which are very low can also inhibit mite development. Mites contain about 70% to 75% water by weight and must maintain this in order to reproduce. Their primary source of water is ambient water vapor.
Thus, for a given number of mites, the highest levels of allergens found in the environment usually correspond to optimal humidity conditions. Arlian (1992) examined the effect of RH on mite metabolism for a range of relative humidities between 22% and 95% and observed that feeding rates, and consequently the amount of fecal matter produced, increased with increasing RH. The effect was particularly significant between 75% and 85% RH, where there was a fivefold increase in the weight of food consumed for both D. pteronyssinus and D. farinae. Below the CEH (Critical Equilibrium Humidity), Arlian found that mites fed sparingly and produced little fecal matter. These results suggest that significant reductions in the level of mite allergens, which consist primarily of fecal matter, may occur if RH is reduced below the CEH. (For more detailed information on the mite life-cycle and metabolism, see Arlian .)
In a study of homes in Vancouver, Murray et al. (1979) detected significant numbers of mites only when the RH was greater than 50% for at least part of every day during the month of collection.
Carpeting can be a localized site of increased humidity and consequently may be an important reservoir for allergens in both homes and schools. Studies conducted in schools have demonstrated that carpets contain high levels of a variety of allergens including pollen, cat and dog danger and mite and mold allergens. This may be the primary source of exposure for young children, who generally live closer to the floor. Children do not have high exposures in bedding since they usually sleep on plastic-covered mattresses. The strong correlation between indoor relative humidity and dust mite population has led to recommendations to reduce indoor humidity. However, the exact upper limit is not obvious. Most of the field studies suggest that when indoor humidity is kept below 50% RH, mite populations do not grow to significant levels. A dedicated dehumidification system is required in most cases in order to achieve this.
Arlian, L.G., D. Bemstein, I.L. Bemstein, S. Friedman, A. Grant, P. Lieberman, et al. 1992. Prevalence of dust mites in the homes of people with asthma living in eight different geographic areas in the U.S. Journal of Allergy and Clinical Immunology 90 (3): 292-300.
Murray, A.B., and P. Zuk. 1979 The seasonal variation in a population of house dust mites in a North American city. Journal of Allergy and Clinical Immunology 64 (4): 266-269.
Genus: Dermatophagoides; Mites are Arachnids
Size: About 250 to 300 microns (mm) in length
Adult Mite Lifespan: Up to 3 months; (3 larval stages)
Reproduction: Female mites lay about 25 to 50 eggs
Habitat: Mites live in carpet, fabric upholstery, and mattresses
Diet: Human skin scale, animal dander and trace nutrients, mites needto absorb humidity, they cannot drink water
Allergen: Dust mite fecal material
Body Color: Translucent
Average Threshold before Developing Allergies: 100 mites/gm of dust
Allergic Reactions: asthma, rhinitis
Living ConditionsTemp. Range: approx. 59°F to 95°F (15°C to 35°C)Relative Humidity Range: approx. 55% to 85%
Dust mites are an important source of house dust allergens in homes worldwide. Exposure to house dust containing mite allergens can cause sneezing, nasal stuffiness, runny nose, itchy/watery eyes, and asthma. House dust allergies are particularly important because the symptoms tend to be year round rather than seasonal like pollen allergies. Previous research has shown that reducing mite allergen levels in homes can lead to an improvement in allergic symptoms and a reduction in medication requirements.
Many products are sold to reduce mite populations in homes, but it has remained difficult for allergy patients to reduce mite populations below the threshold for causing clinical symptoms. It has been concluded that reducing relative humidity levels reduces the number of dust mites in homes. This relationship between relative humidity and dust mite survival has been well documented. A study in the the summer of 1995 was done to determine the effectiveness of the Santa Fe ultra efficient dehumidifiers in reducing relative humidity and dust mite levels in homes.
The study ran from June 2 until September 25, 1995. It was conducted by Dr. Larry Arlian at Wright State University. Dr. Arlian is one of the nation’s leading experts on dust mites. His research and studies are often cited in medical journals and other health related publications.
The location of the study was Dayton, Ohio, where virtually 100% of all residences are inhabited by house dust mites. Fourteen homes were selected for the study after preliminary dust samples confirmed the presence of significant dust mite populations.
The study homes were divided into three groups (see adjacent chart): Santa Fe homes were air conditioned and equipped with free-standing Santa Fe dehumidifiers. Air Cond homes were only air conditioned. These homeowners agreed to operate their air conditioning throughout the study period. Non A/C homes did not have dehumidifiers or air conditioners.
Participants logged temperature and relative humidity conditions on a daily basis. The average relative humidity in the homes for the study period is shown in the chart above. Dust samples were taken at four week intervals from the family room sofa, family room carpet, and bedroom carpet. Counts of both live and dead dust mites were taken. The counts were performed on 0.15 grams of dust from the sample. The chart below reflects these counts extrapolated to mites per gram of dust. (This is the ratio referred to in most studies and journals.) 100 mites/gram is the standard average threshold for allergy symptoms begin to develop.
The summary of the study recognizes that air conditioning can reduce the relative humidity and the dust mite population in homes compared with homes without air conditioning or dehumidification. The reduction, however, does not prevent mite populations from growing beyond the threshold for inducing allergies. The Santa Fe ultra efficient dehumidifier used in conjunction with air conditioning was effective in reducing relative humidity in these homes below the threshold required for mite reproduction and population growth. After the fourth week of the study, 75% of the Santa Fe equipped homes had 0 (zero) live mites counted.
Air Quality, Home, Resources