Ten RCTs with 2,014 participants were included.
In terms of study quality, allocation concealment was adequate in 2 studies, unclear in seven and inadequate in one. Seven studies had inadequate description of drop-outs and only one clearly used an intention-to-treat analysis.
Passive versus active humidification: 9 RCTs compared passive against active humidification methods. Most comparisons were represented by only 1 study and showed no significant difference. One RCT comparing hydrophobic HME with HH-HWC significantly favoured HME (RR 0.41, 95% CI: 0.20, 0.86). Three RCTs comparing hygroscopic HME with HH-HWC found no significant difference (pooled RR 0.90, 95% CI: 0.53, 1.53).
Active humidification: 1 study comparing HH-HWC with HH found no significant difference.
Passive humidification: 1 study comparing hydrophobic HME with hygroscopic HME found no significant difference.
Tests of statistical heterogeneity were not significant for any comparison.