The effect of domestic mechanical heat recovery ventilation on asthma control of patients allergic to the house dust mite

The prevalence of asthma has increased over the last generation, in parallel with a warm indoor microclimate. Central heating, fitted carpets and tight building construction have improved standards of heating and energy efficiency in homes, at the expense of ventilation. A warm, humid environment favours the growth of the house dust mite population. Allergy to the house dust mite is the most common allergy associated with asthma in the UK. Studies of occupational asthma, seasonal asthma and at altitude infer that the environment may directly affect symptoms of asthma. Allergen avoidance has been advocated as an important aspect of asthma management, yet the evidence for its efficacy has not been clear. Large studies of conventional measures to eradicate dust mites, such as mattress covers, have not shown a benefit for symptoms of asthma. As house dust mites are sensitive to humidity, an additional strategy would be to reduce indoor air humidity by improving ventilation. A randomised, double-blind placebo-controlled study examined the effect of the installation of domestic mechanical heat recovery ventilation on asthma control in the homes of 119 adults sensitive to house dust mite allergen. The study involved collaboration between the University Departments of Architecture, Respiratory Medicine and Immunology, local General Practices, the district general hospital, the local councils and industry. 100 participants completed follow-up. At twelve months, there was a clinically significant improvement in evening peak expiratory flow in the mechanical ventilation group and fewer admissions to hospital with asthma. There was a non-significant improvement in the mechanical ventilation group in the primary outcome, morning peak expiratory flow. There was a significant reduction in the asthma control questionnaire score at 3 months, but this was not sustained to 12 months. Rhinitis visual analogue scores for sneezing, nasal discharge and nasal blockage significantly improved in the group with mechanical ventilation compared to the control group at 6 months, but not at 12 months. There was no difference in exhaled nitric oxide, a measure of airway inflammation, between the two groups at 12 months. However, these clinical improvements could not be explained by reduced allergen exposure, as although indoor air humidity was reduced during the winter months, there was no difference between the house dust mite levels between the groups, nor in the levels of specific IgE to the house dust mite. Other mechanisms, such as mould, endotoxin, viral infection and environmental tobacco smoke should be considered in future work. In the mechanical ventilation group there was a modest individual gain of 0.02 Quality-adjusted life years over 12 months. However, it may still prove a cost-effective intervention if the clinical effects are sustained. Further research is required to establish if the clinical effects are sustained for greater than one year and to investigate the mechanism of the effect of improved home ventilation on respiratory health