Aims: This study systematically reviewed literature regarding the impact of ambient air pollution on physical activity among children and adults. Methods: Keyword and reference search was conducted in PubMed and Web of Science to systematically identify articles meeting all of the following criteria – study designs: interventions or experiments, retrospective or prospective cohort studies, cross-sectional studies, and case-control studies; subjects: adults; exposures: specific air pollutants and overall air quality; outcomes: physical activity and sedentary behaviour; article types: peer-reviewed publications; and language: articles written in English. Meta-analysis was performed to estimate the pooled effect size of ambient PM _2.5 air pollution on physical inactivity. Results: Seven studies met the inclusion criteria. Among them, six were conducted in the United States, and one was conducted in the United Kingdom. Six adopted a cross-sectional study design, and one used a prospective cohort design. Six had a sample size larger than 10,000. Specific air pollutants assessed included PM _2.5 , PM ₁₀ , O ₃ , and NO _x , whereas two studies focused on overall air quality. All studies found air pollution level to be negatively associated with physical activity and positively associated with leisure-time physical inactivity. Study participants, and particularly those with respiratory disease, self-reported a reduction in outdoor activities to mitigate the detrimental impact of air pollution. Meta-analysis revealed a one unit (μg/m ³ ) increase in ambient PM _2.5 concentration to be associated with an increase in the odds of physical inactivity by 1.1% (odds ratio = 1.011; 95% confidence interval = 1.001, 1.021; p -value < .001) among US adults. Conclusions: Existing literature in general suggested that air pollution discouraged physical activity. Current literature predominantly adopted a cross-sectional design and focused on the United States. Future studies are warranted to implement a longitudinal study design and evaluate the impact of air pollution on physical activity in heavily polluted developing countries.

The coronavirus disease-2019 (COVID-19) pandemic in the United States led to nationwide stay-at-home orders and school closures. Declines in energy expenditure resulting from canceled physical education classes and reduced physical activity may elevate childhood obesity risk. This study estimated the impact of COVID-19 on childhood obesity. A microsimulation model simulated the trajectory of a nationally representative kindergarten cohort's body mass index z-scores and childhood obesity prevalence from April 2020 to March 2021 under the control scenario without COVID-19 and under the 4 alternative scenarios with COVID-19-Scenario 1: 2-month nationwide school closure in April and May 2020; Scenario 2: Scenario 1 followed by a 10% reduction in daily physical activity in the summer from June to August; Scenario 3: Scenario 2 followed by 2-month school closure in September and October; and Scenario 4: Scenario 3 followed by an additional 2-month school closure in November and December. Relative to the control scenario without COVID-19, Scenarios 1, 2, 3, and 4 were associated with an increase in the mean body mass index z-scores by 0.056 (95% confidence interval (95%CI): 0.055-0.056), 0.084 (95%CI: 0.084-0.085), 0.141 (95%CI: 0.140-0.142), and 0.198 (95%CI: 0.197-0.199), respectively, and an increase in childhood obesity prevalence by 0.640 (95%CI: 0.515-0.765), 0.972 (95%CI: 0.819-1.126), 1.676 (95%CI: 1.475-1.877), and 2.373 (95%CI: 2.135-2.612) percentage points, respectively. Compared to girls and non-Hispanic whites and Asians, the impact of COVID-19 on childhood obesity was modestly larger among boys and non-Hispanic blacks and Hispanics, respectively. Public health interventions are urgently called to promote an active lifestyle and engagement in physical activity among children to mitigate the adverse impact of COVID-19 on unhealthy weight gains and childhood obesity.

Neighborhood built environment may profoundly influence children's physical activity (PA) and body weight. This study systematically reviewed scientific evidence regarding the impact of built environment on PA and obesity among children and adolescents in China. A keyword and reference search was conducted in Active Living Research, Cochrane Library, PubMed, and Web of Science. Studies that met all of the following criteria were included in the review: (1) study designs-experimental studies, observational studies, and qualitative studies; (2) study subjects-Chinese children and/or adolescents aged ≤17 years; (3) exposures-neighborhood built environment; (4) outcomes-PA and/or body weight status; (5) article type-peer-reviewed publications; (6) time window of search-from the inception of an electronicbibliographic database to May 31, 2018; (7) country-China; and (8) language-articles written in English. A total of 20 studies, including 16 cross-sectional studies, 3 longitudinal studies, and 1 descriptive study, met the predetermined selection criteria and were included in the review. A total of 13 studies adopted subjective built environment measures reported by parents and/or children,2 adopted objective measures (e.g., geographical information system, field observations), and 5 adopted both objective and subjective measures. PA behaviors included PA, physical inactivity, sedentary behavior, active/passive commuting from/to school, and park visits. Among the 16 studies that provided some quantitative estimates of the influence of built environment on PA and body weight status, all reported a statistically significant relationship in the expected direction. Availability and accessibility in proximity to greenspaces, parks, recreational facilities, and sidewalks were found to be associated with increased PA levels, reduced sedentary behaviors, and/or active commuting among Chinese childrenand adolescents. In contrast, the absence of bike lanes and living in higher density residential areas were associated with increased likelihood of childhood overweight and obesity. Neighborhood built environment plays an important role in Chinese children's PA engagement and weight outcomes. Building new exercise facilities and enhancing the accessibility of existing facilities hold the potential to enhance PA engagement among Chinese children and adolescents. In addition, urban designs that incorporate sidewalks, bike lanes, walking paths, less motorized traffic, and lower residential density are likely to promote PA and prevent childhood obesity in China.