Update of a systematic review on associations between environmental exposures and asthma control and exacerbations in young children.

Objectives: To update our previous systematic review of the literature and describe associations published since 2014 between environmental exposure and asthma control and exacerbations in children.

Design: Systematic literature review following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines.

Data Sources: The search strategy from our earlier review was used to search the following databases: MEDLINE/OVID (1946-Present), Embase/OVID SP (1980-Present), CINAHL, Cochrane Centre Trials Register (CCTR), Cochrane Database of Systematic Reviews and Database of Abstracts of Reviews of Effects (DARE). Searches were carried out on 5 November 2021 and updated on 3 April 2023. Literature published since 2014 until the date of the search was included.

Eligibility Criteria: Studies that examined the association between environmental exposures and asthma exacerbations in children. Study participants were children with a mean age of ≤9 years. Outcomes of interest were indices of asthma symptoms, control and exacerbations. We included systematic reviews, intervention studies, time series and time-stratified case crossover studies and longitudinal cohort studies.

Data Extraction And Synthesis: Four researchers independently screened full text articles in English for inclusion and assessed them for quality using the Effective Public Health Practice Project Quality assessment tool. A meta-analysis was carried out for a subgroup of selected papers.

Results: There were 20 339 papers screened and 110 were eligible, including 23 rated strong, 81 rated medium and 6 low quality. There were 17 systematic reviews and 7 randomised controlled trials (RCTs). In most observational studies, exposure to exhaled tobacco smoke or respiratory virus and increasing exposures to outdoor air pollutants (including particulate matter, nitrogen dioxide, traffic-related air pollution and dust storms), fungal spores or tree pollen were associated with increased risk of worsening asthma outcomes. RCTs to reduce indoor air exposures were done in small populations and, although often able to reduce exposures, only one intervention changed asthma outcomes.

Conclusions: The number of eligible papers has increased to 110 from 27 described in our 2014 review, and the quality of papers has also increased. While associations between exposures and worsening outcomes are increasingly consistent, what remains unclear is whether interventions, particularly for indoor air and diet, may improve outcomes.

Prospero Registration Number: CRD42021290184.