The number of better insulated, warmer and more humid homes has increased substantially over the past few decades in developed nations. At the same time, growing levels of asthma have been identified among their populations. It is known that allergens produced by domestic dust mites are a contributory factor to asthma and that these mites thrive in warm and humid environments. As a consequence, solutions to minimise mite levels while retaining warmth would have valuable health benefits.
Key research Question
The study sought to measure the effects of a specific ventilation apparatus on levels of dust mites (Dermatophagoides pteronyssinus) in domestic housing and what impact this intervention may have on the prevalence of asthma among residents.
Summary of activity
The research involved recruiting 120 households where at least one resident had a clinical diagnosis of asthma to participate in a randomised double-blind placebo-controlled parallel-group trial. Mechanical heat recovery and ventilation (MHRV) systems were installed in all the properties but were only activated in half of them. All the properties were steam-cleaned, and new bedlinen was provided to each household. A wide range of technical measurements were made at fixed time points over two years (2003–05), including household climate, volumes of allergens in dust samples, peak expiratory flow (PEF) readings and enzyme levels in blood. These data were subjected to statistical analysis (e.g. analysis of covariance models).
Over the first 12 months, there were identifiable health improvements in the MHRV group. PEF readings improved among residents of properties with activated MHRV in the evening, but not in the morning.
Indoor humidity was reduced significantly in properties with activated MHRV, but not by more than 50%. However, levels of dust mite allergens were similar across all properties. Therefore, the observable health improvements could not be related to dust mite levels.
Measurements of quality-adjusted life years recorded a small positive impact in terms of savings in health costs for residents of properties with activated MHRV, but the installation fee means this is only cost-effective when the savings are spread over the lifetime of the unit (10–20 years).
The authors suggest a number of possible explanations for the findings. These include: the identified health benefits of MHRV were due to better airflow, which removed other causes of disease such as damp, tobacco smoke and volatile organic compounds; or the mite levels persisted because the humidity levels did not drop by more than 50%.
Further research is needed. If further trials are undertaken, a humidistat controller connected to a variable-flow fan unit should be installed in the MHRV units to ensure humidity levels drop by more than 50% in order to assess the impact on dust mites. Other factors implicated in asthma should be measured. Aside from MHRV technology, other research indicates that work to eradicate damp housing is likely to have continuing positive effects on asthma levels.