Abstract: We included six trials from five publications involving a total of 387 participants. We included no new studies in this 2017 update. The 'Risk of bias' assessment suggested an unclear risk of bias in the domain of randomisation and a low risk of bias in performance, detection, attrition, and reporting. It was uncertain whether heated, humidified air provides symptomatic relief for the common cold, as the fixed-effect analysis showed evidence of an effect (odds ratio (OR) 0.30, 95% confidence interval (CI) 0.16 to 0.56; 2 studies, 149 participants), but the random-effects analysis showed no significant difference in the results (OR 0.22, 95% CI 0.03 to 1.95). There is an argument for using either form of analysis. No studies demonstrated an exacerbation of clinical symptom scores. One study conducted in the USA demonstrated worsened nasal resistance, but an earlier Israeli study showed improvement. One study examined viral shedding in nasal washings, finding no significant difference between treatment and placebo groups (OR 0.47, 95% CI 0.04 to 5.19). As judged by the subjective response to therapy (i.e. therapy did not help), the number of participants reporting resolution of symptoms was not significantly higher in the heated humidified group (OR 0.58, 95% CI 0.28 to 1.18; 2 studies, 124 participants). There was significant heterogeneity in the effects of heated, humidified air on different outcomes, therefore we graded the quality of the evidence as low. Some studies reported minor adverse events (including discomfort or irritation of the nose). The current evidence does not show any benefits or harms from the use of heated, humidified air delivered via the RhinoTherm device for the treatment of the common cold. There is a need for more double-blind, randomised trials that include standardised treatment modalities.

Plain-language summary: We included six randomised, double-blind trials from five publications involving a total of 387 participants published between 1987 and 1995 in the English language. All included trials used the RhinoTherm device, which delivered heated, humidified air for different lengths of time and at different flow rates to treat common cold symptoms. Three trials were conducted in the USA, two in the UK, and one in Israel. Most studies recruited people with naturally occurring colds, but one study induced colds by infecting participants. The RhinoTherm devices were provided by Netzer Sereni in four studies and A Beacham in two studies. One study was funded by Cleveland Clinic internal funding, and another was supported by authors' discretionary funds. The remaining studies did not mention funding sources. None of the included studies reported any worsening in clinical symptom scores after inhaling heated, humidified air. Participants in two trials showed a lack of persistent symptoms, however the results were inconsistent. Two studies reported minor adverse events. There was no effect of treatment on rhinovirus shedding. Using GRADE criteria, we assessed the quality of the evidence as low for the outcomes reduction in the clinical severity of the common cold (measured by decrease in the symptom score index); number of participants with the subjective response: therapy did not help; and number of participants with a positive viral culture in the nasal washings, due to risk of bias and inconsistency of the study results.