REDUCTION IN HUMIDITY AS A METHOD OF CONTROLLING MITES AND MITE ALLERGENS - THE USE OF MECHANICAL VENTILATION IN BRITISH DOMESTIC DWELLINGS

Background Low humidity is an important limiting factor for mite popul ation growth. Reducing humidity can therefore be used as a method to c ontrol mites within the home. Objective This study investigated the ef fect of mechanical ventilation heat recovery (MVHR) units on house dus t mites and mite allergen Der p 1 in typical homes in the North-West o f England. Methods Mite counts and Der p 1 levels were measured at 3-m onthly intervals over a period of 1 year in 18 houses (nine with MVHR units and nine architecturally matched control houses). Paired dust sa mples were collected using a vacuum cleaner with an air-flow rate 451/ sec, adapted to collect the sample onto a preweighed filter paper. A 1 m(2) area of bedroom carpet, living room carpet and mattress was samp led for 2 min. Indoor temperature and relative humidity (RH) levels we re recorded for a period of 1 week before and after the winter period (November and February: 3 and 6 months data sets). The environmental q uestionnaire was completed at the beginning and at the end of the stud y. Results No difference in either Der p 1 concentrations or mite coun ts in any of the sampling sites at 3, 6, 9 and 12 months as compared w ith the baseline values was found, both within and between the groups (P > 0.1). The measured levels of RH performed in autumn and winter we re found to be lower in the MVHR houses compared to the architectural controls. The indoor temperature during each period did not differ bet ween the groups. Questionnaire data showed that the severity of conden sation improved in the MVHR homes, whilst during the winter period, th e severity of condensation had increased in the architectural control group. Conclusions The MVHR unit does not reduce indoor humidity to le vels capable of retarding mite population growth and decreasing mite a llergens in the type of houses predominantly found in the mild and hum id climate of the North-West of England.