A prevalence of dust mites and mould in domestic properties is known to aggravate the symptoms of asthma. Therefore, measures to alleviate the conditions that allow mites to thrive (e.g. high humidity) should have a beneficial impact on the health of asthmatics. Many energy efficiency interventions achieve this, although this is often an unplanned, albeit useful, side effect of work aimed at reducing energy consumption and wastage. Some interventions may even increase the problem: by reducing ventilation, for example.