Dr Jens Korsgaard, of University Hospital, Aarhus, in Denmark, told a conference in London on healthy homes that 'environmental treatment' achieved a 30 per cent increase in lung function among sufferers against 5-10 per cent through the most effective medicines available. Better ventilation cut drug usage by 60 per cent.
The incidence of asthma has been growing for two decades, with many specialists blaming the spread of fitted carpets, central heating and 'tight' houses built to conserve energy and cut out draughts. House dust mites have been identified as one of the chief causes of the asthma epidemic, which now affects two million people in Britain, 1 in 10 of them children, and accounts for 8 per cent of the NHS drugs budget.
However, Dr Korsgaard said studies in Denmark showed that fitted carpets and the frequency of vacuum cleaning did not affect mites. The sole factor was high humidity.
But opening a window for a while will not be enough, because of the huge 'reservoir' of water vapour in furnishings, fabrics and carpets. A family of four produces 12 litres (2.64 gallons) of water every 24 hours. In Denmark, mechanical ventilation, usually involving at least one complete change of air every hour, is officially recommended and in the case of severe sufferers can be paid for by the state.
The 'threshold' relative humidity level, above which the risk from mites increases, is 45 per cent, according to Dr Korsgaard. A decade ago two- thirds of houses in Denmark were below this level. Now, because of better insulation standards, four- fifths are above it.
An experiment in South Wales last February sponsored by the Building Research Establishment compared ordinary houses with those having mechanical ventilation and heat recovery and found that in the ventilated houses there were 10 mites per 100 milligrams (.0004oz) of carpet dust; in the others there were more than 100.
Yesterday's conference was sponsored by four electricity boards: the electricity industry believes that mechanical ventilation combined with devices such as heat exchangers can cut humidity and indoor pollution, and reduce energy costs.