Zwemer 1973.

Study characteristics
Methods	Cross‐over trial Randomisation method: not described Double‐blind Physical
Participants	N = 18 (12 in analyses) Age range 6 to 16 years Positive skin tests to house dust
Interventions	Test: active laminar air flow system (Pure‐zone system) Control: dummy filter Each period lasted 4 weeks
Outcomes	Asthma symptoms Three patients had sick days in the control group, none in the experimental group
Notes	No assessment of mite reduction. Daytime wheeze was selected blindly as the most relevant variable (other variables yielded closely similar results). Since the data were extremely skewed, the logarithm of the scores was used.
Risk of bias
Bias	Authors' judgement	Support for judgement
Allocation concealment (selection bias)	Unclear risk	Information not available

D pter: Dermatophagoides pteronyssinus; D far: Dermatophagoides farinae; DP1: D(2) receptor type 1; ELISA: enzyme‐linked immunosorbent assay; FEV1: forced expiratory volume in 1 second; FVC: forced vital capacity; HEPA: high‐efficiency particulate air; PD20 provocative dose producing a 20% fall in FEV1; PEFR: peak expiratory flow rate; RAST: radioallergosorbent test; SD: standard deviation; SEM: standard error of the mean; SPT: skin prick test; VAS: visual analogue scale