Interventions to reduce the impact of outdoor air pollution on asthma: A systematic review

Exposure to air pollution can cause adverse health effects in people living with chronic lung disease. In people with asthma, it is not clear whether strategies to reduce outdoor air pollution can affect clinical symptoms and lung function.

To determine strategies to reduce air pollution exposure for people with asthma, and to describe the effect of reduced air pollution on asthma outcome.

A systematic review was conducted of six databases for English literature. Any study published between April 2012 and March 2022 that mentioned air pollution exposure reduction and asthma was reviewed. Two reviewers (STH and RMp) screened and extracted the data separately, using a standardised form based on the Cochrane data extraction tool. Risk of bias was assessed using the risk-of bias 2 tool. Outcome measures were the Asthma Control Test (ACT), the Childhood Asthma Control Test, exacerbations, and the forced expiratory volume in the 1st second (FEV₁ ), forced vital capacity (FVC) and FEV₁ /FVC ratio. The study was registered with PROSPERO (reg. no. CRD42022341648).

Of the 11 116 identified studies, eight met the inclusion criteria, with a total of 11 395 043 participants. Clean air policy implementation modestly improved lung function, as shown by an increase in FVC and FEV₁ of 0.02 L/year and 0.01 L/year, respectively. Reduction of exposure to outdoor smoke pollution with use of mobile application alerts resulted in behavioural change and improved ACT scores over 8 weeks (mean (standard deviation (SD)) 21.5 (2.3) compared with baseline (20.0 (2.4); p<0.001). Asthma control improved during low levels of pollution related to COVID-19 lockdown, as shown by mean (SD) ACT scores (17.3 (4.7) v. 19.7 (4.5); p<0.001) and associated declines in mean daily hospital admissions (4.5 (3.4) days v. 2.8 (2.5) days; p<0.001).

Air pollution is a major hazard, and strategies to reduce exposure have a positive outcome in terms of the asthma morbidity. This field would benefit from further high-quality randomised clinical trial evidence to inform policy and decision-making.

What the study adds. The prevalence and burden of asthma are increasing globally. Air pollution exposure is a major cause of worse asthma symptoms. Strategies to reduce air pollution or exposure to it may contribute towards improved quality of life. This study highlights potential strategies and their effect on asthma outcome.Implications of the findings. A combination of individual activities and actions by governments to reduce air pollution can improve asthma outcome. A focus on education together with behavioural changes can reduce exposure at the individual level. Implementation of clean air policies reduces air pollution exposure and improves lung health.