The Southampton survey on asthma and ventilation: humidity measurements during winter.

As part of a collaborative trial on the effects of ventilation on house dust mites and asthma, 20 mechanical ventilation units were installed in houses in the Southampton area in southern England. The hypothesis is that continuous ventilation over winter months can maintain humidity below a mixing ratio of 7 g/kg, with a consequent reduction in house dust mite numbers. The systems served upstairs only, extracting from bathroom and landing and supplying fresh air to bedrooms. Temperature and humidity in the experimental houses and 20 control houses were recorded in the patients bedrooms at carpet level. Over the 4 months from December to March the mechanically ventilated houses had significantly lower absolute humidities. Of the 20 MV houses, 15 had a mean mixing ratio below 7 g/kg, compared with only 3/20 of the non-MV houses.