A study was made of the house dust mite, Dermatophagoides pteronyssinus, in its natural surroundings for a year. Records of the relative humidity inside and outside the bed and of the temperature outside the bed were kept. An estimate of the number of mites on the surface of the mattress was made by brushing the mattress dust into a container at monthly intervals and then counting the mites. Suction cleaning of the mattress was also carried out to ascertain whether it is possible to control the number of mites by such measures. It was found that suction cleaning and brushing of the mattress considerably reduced the numbers of live and dead mites. The number of live mites remained low during the winter, but increased during the late spring and summer when temperature and relative humidity became higher. It is suggested that, for effective control, cleaning of mattresses should be carried out more frequently in the spring and summer than in the winter.
Records of the relative humidity inside and outside the bed confirmed the fact that when a bed is occupied the relative humidity falls, but attains equilibrium with the surrounding air again soon after the occupant has left the bed.