House dust mite allergen content in two areas with large differences in relative humidity

Striking differences in mite counts and mite-allergen levels have been documented between dwellings located at sea level and high altitude. Apart from relative humidity (RH), several other factors, ie, temperature, UV exposure, and altitude per se could account for this difference.

To evaluate whether RH by itself could influence mite infestation by comparing mite-allergens levels in two towns differing only with respect to RH.

We compared group I allergen content in two Moroccan towns: Casablanca, located on the seashore and Marrakech located at 1404 feet. Mean (+/- SD) RH in years 1990 and 1991 was 81.2 +/- 2.9% in Casablanca and 56.0 +/- 7.6% in Marrakech. Mean annual temperatures were 17.7 +/- 4.0 degrees C and 20.2 +/- 6.4 degrees C in Casablanca and Marrakech, respectively. In each town, 20 asymptomatic subjects agreed to participate in the study. Their mattresses were vacuum-cleaned for a standardized duration (2 min/m2). Mite allergen-content was evaluated using monoclonal antibodies and ELISA and results expressed as micrograms of group I (Der pI+Der f I) allergens per gram of dust (micrograms/g dust).

Mean (+/- SD) group I allergen level was 8.3 +/- 8.8 micrograms/g in Casablanca and 0.6 +/- 0.6 micrograms/g dust in Marrakech, a difference that is highly significant (P = .001). In both areas, mean Der f I allergen level was low (0.7 +/- 0.5 and < 0.1 micrograms/g dust, in Casablanca and Marrakech, respectively).

This study shows that house dust mite allergen content in households depends on RH rather than on temperature.