To update an existing systematic review by McDonald et al, which was conducted 20 years ago, by reviewing the clinical effectiveness of the air purification strategy for the treatment of allergic asthma.

Participants included children and adults with physician-diagnosed bronchial allergic asthma with positive testing to house dust mite, dog, cat, or cockroach.

A meta-analysis of trials evaluating the impact of housing or mobile ventilation systems, including HEPA filters but not vacuum cleaners, on allergic asthma. Studies were included only if they documented aeroallergen sensitization, history of asthma symptoms, and pulmonary function testing. Trials were also limited to peer-reviewed, randomized controlled trials with blinding, published in English. The initial article appraisal included the ten trials in the previously conducted systematic review by McDonald et al [Chest 2002;122(5):1535–1542]. An updated search of literature published since Jan 2000 was also performed in Embase, Medline, and the Cochrane Central Register of Controlled Trials (CENTRAL). Data extracted included asthma symptoms scores (ASS), medication use, forced expiratory volume in 1 second as a percentage of the predicted value (FEV1%pred), histamine provocative concentration causing a 20% reduction in FEV1 (PC20), Asthma Quality of Life Questionnaire (AQLQ) scores, and fractional exhaled nitric oxide (FeNO) levels.

The ten trials in the meta-analysis by McDonald et al were screened and three of these studies were excluded due to incomplete data. In the new literature search, 1000 titles were identified. However, 997 titles did not meet inclusion criteria (most of these lacked randomization and/or blinding). Therefore, three new full-text articles were included, giving a total of ten articles in this meta-analysis.

Statistically significant mean differences were found in the AQLQ scores and FeNO levels. These results were obtained in patients with predominantly mild to moderate asthma.

There is significant heterogeneity in the asthma patient population which is one factor often leading to seemingly conflicting research study findings. Large compilations of data such as this meta-analysis are important as they may identify effective treatments over a large set of patients, allowing potential extrapolation to clinical practice. This meta-analysis was able to identify some benefit to using air purification in mild to moderate asthma. It should also be noted that although a benefit of air purification in more severe allergic asthmatics was not elucidated specifically in the overall analysis, one large study of 189 subjects which was included here demonstrated a significant benefit in such patients. Therefore, air purification strategies may be beneficial, and additional study is needed specifically in moderate to severe allergic asthmatics.