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.

The survival of active adult mites (both male and female) is limited to 4 to 11 days at humidities below 50% RH at 25 degrees. 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. As one might expect, most mite allergens are formed by adults during their active phase.

The upper humidity limit is constrained by the possibility of mold growth, particularly above 88%, which can 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. Ppteronyssinus 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 metabolic byproducts, may occur if RH is reduced below the CEH. (For more detailed information on the mite life-cycle and metabolism, see Arlian (19921).

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 dander. and mite and mold allergens. This may be the primary source of exposure for young children, who generally live closer to the floor. They 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. Dedicated dehumidification is required in most cases in order to achieve this.

Species: Dermatophagoides Farinae (DF)--commonly found in North America Dermatophagoides Pteronyssinus (DP)--commonly found in Europe

Size: About 250 to 300 microns (gm) in length

Adult Mite Life span: Up to 3 months; (3 larval stages)

Reproduction: Female mites lay about 25 to 50 eggs before they die

Habitat: Carpet, fabric, upholstery, and mattresses

Diet: Human skin scale, animal dander and trace nutrients. Mites need to absorb humidity, they cannot drink water.

Allergen: Dust mite fecal material

Body Color: Translucent.

Allergen Threshold: 100 mites/gm of dust

Allergic Reactions: Asthma, rhinitis

Relative Humidity Range: 55% to 85%

Number of People in the US Suffering from Allergies: 50 Million and rising