The seasonal variation in a population of house dust mites in a North American city.

Mattress dust collected at monthly intervals for 2 1/2 yr was examined for mites. Both Dermatophagoides pteronyssinus and D. farinae were found. There was a significant association between the presence of live mites and the relative humidity (RH): Live mites were seen only when the RH had been greater than or equal to 50% for at least part of every day during the month of collection. There was a seasonal variation in that live mites were found only in the warmer months and not in the winter. However, the peak in the mite population was consistent neither for month nor for numbers: In 1977 a slight peak in mite numbers occurred in October, while in 1978 the peak was higher and occurred in July. Use of data from previously published studies to preduct mite levels may therefore be misleading.