Background It is not clear whether associations between respiratory symptoms and indoor mould are causal. Objective. A randomised controlled trial was conducted to see whether asthma improves when indoor mould is removed.
Methods Asthmatic patients' houses were randomly allocated into two groups. In one group, indoor mould was removed, fungicide was applied, and a fan was installed in the loft. In the control group, intervention was delayed for 12 months. At baseline, 6 months and 12 months questionnaires were administered and peak expiratory flow rate (PEFR) was measured.
Results Eighty-one houses were allocated to the intervention group and 83 to the control group; 95 participants in 68 intervention houses and 87 in 63 control houses supplied follow-up information. PEFR variability declined in both groups, with no significant differences between them. At 6 months, significantly more of the intervention group showed a net improvement in wheeze affecting activities (a difference of 25% between groups, 95% CI 3, 47; p=0.028), perceived improvement of breathing (52%, 95% CI 30, 74; p<0.0001) and perceived reduction in medication (59%, 95% CI 35, 81; p<0.0001). By 12 months the intervention group showed significantly greater reductions than the controls in preventer and reliever use, and improved more for rhinitis (24%, 95% CI 9, 39; p=0.001) and rhinoconjunctivitis (20%, 95% CI 5, 36; p=0.009).
Conclusions Although there was no objective evidence of benefit, symptoms of asthma and rhinitis improved and medication use declined following removal of indoor mould. It is unlikely that this was entirely a placebo effect.