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Abstract

Background: This study assessed whether higher levels of fine particulate matter (PM2.5) reduce the protective effects of leisure-time physical activity (LTPA) on all-cause, cardiovascular, and cancer mortality, and explored the PM2.5 threshold beyond which attenuation occurs.

Methods: We conducted two complementary investigations. First, a systematic review and meta-analysis (per PRISMA guidelines) identified eligible cohort studies from PubMed, Web of Science, Embase, and SPORTDiscus (from inception to 6 January 2025) that examined the independent or joint associations of LTPA and PM₂.₅ with mortality among adults (≥ 18 years). Second, an individual-level pooled analysis using harmonized data from three cohorts was performed using Cox regression modeling to assess the associations observed in the meta-analysis.

Results: In Study One, a total of seven cohort studies (n = 1,515,094; deaths = 115,196) were included in the meta-analysis, revealing that the reduction in all-cause mortality risk diminished with higher PM2.5 exposure. Meeting the recommended LTPA level (7.5-15 MET-h/week) reduced all-cause mortality risk by approximately 30% at PM2.5 < 25 μg/m3 but only 12-15% at 25 + μg/m3. Study Two (three cohorts; n = 869,038; deaths = 45,080) confirmed this pattern. Individuals meeting the recommended LTPA level (7.5-15 MET-h/week) had a lower risk of all-cause mortality compared to those in the highest-risk group (reference: < 1 MET-h/week and PM2.5: 35-50 μg/m3). Hazard ratios (HRs) varied by PM2.5 exposure, with lower HRs indicating a greater protective effect: 35-50 μg/m3 (HR = 0.75, 95% CI: 0.61-0.93), 25-35 μg/m3 (HR = 0.67, 95% CI: 0.57-0.79), 15-25 μg/m3 (HR = 0.34, 95% CI: 0.29-0.39), 10-15 μg/m3 (HR = 0.34, 95% CI: 0.28-0.41), and < 10 μg/m3 (HR = 0.30, 95% CI: 0.25-0.37). Higher levels of LTPA were generally associated with lower all-cause and cause-specific mortality across most PM₂.₅ exposure categories, but the protective effects were attenuated at PM₂.₅ levels 25 + μg/m3 for all outcomes and became non-significant for cancer mortality at 35-50 μg/m3.

Conclusions: LTPA is beneficial for all-cause, cardiovascular, and cancer mortality even at relatively high PM2.5 levels, with greater benefits observed under cleaner air conditions. However, its protective effects are attenuated at 25 + μg/m3 for all outcomes and become less evident at 35-50 μg/m3, particularly for cancer mortality.

Prospero registration number: CRD42023395364.

Keywords: Air pollution; Air quality; Combined effect; Death; Exercise; Joint association.

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Conflict of interest statement

Declarations. Ethics approval and consent to participate: This study was approved by the Institutional Review Board of Taichung Veterans General Hospital in Taiwan (CE24585C). Competing interests: The authors declare no competing interests.

Figures

Fig. 1
Fig. 1
Flowchart of study selection in the systematic review and meta-analysis
Fig. 2
Fig. 2
Subgroup analyses of meta-analysis by cohorts A unpublished results. PM2.5 fine particulate matter 2.5, LTPA leisure-time physical activity, HR hazard ratio, ‘n’ in the brackets represents the number of effect estimates in each study. LTPA was categorized into four levels in each study, while PM2.5 concentrations were categorized into quartiles, except for one study that used quintiles for PM2.5 [6]. Therefore, six studies yielded 72 effect estimates (6 studies * 3 levels * 4 categories), and one study (Elliot et al., 2020) yielded 15 estimates (1 study * 3 levels * 5 categories). Total number of effect estimates = 87. The data shown in the figures were derived from individual cohorts, and details on the covariates available and adjusted for in each cohort were provided in Additional file: Table S6. The category ranges of LTPA were defined as lower bound inclusive and upper bound exclusive, except for the highest category (15 + MET-h/week), which was open-ended
Fig. 3
Fig. 3
Subgroup analyses of meta-analysis by PM2.5 levels NHS Nurses’ Health Study; NHIS National Health Interview Survey; UKB UK Biobank; DDCHC Danish Diet, Cancer and Health Cohort; TWB Taiwan Biobank; MJ MJ Cohort Study; HKSAR Hong Kong Special Administrative Region; CEHS Chinese Elderly Health Service. The data shown in the figures were derived from individual cohorts, and details on the covariates available and adjusted for in each cohort were provided in Additional file: Table S6. All category ranges of LTPA and PM2.5 were defined as lower bound inclusive and upper bound exclusive, except for the highest category (15+ MET-h/week), which was open-ended
Fig. 4
Fig. 4
Joint associations of leisure-time physical activity and ambient PM2.5 with all-cause mortality in the pooled individual participant data (n = 869,038) CI confidence interval, HR hazard ratio, LTPA leisure-time physical activity, MET-h metabolic equivalent hours, PM2.5 particulate matter 2.5. Covariates: sex, age, educational levels, household income, smoking, alcohol consumption, body mass index, number of chronic diseases, and cohort. The category ranges of PM2.5 and LTPA were defined as lower bound inclusive and upper bound exclusive, except for the highest category (i.e., 15+ MET-h/wk), which was open-ended
Fig. 5
Fig. 5
Joint associations of leisure-time physical activity and PM2.5 with all-cause mortality across subgroup populations in the pooled individual participant data (total n = 869,038) PM2.5 particulate matter 2.5, LTPA leisure-time physical activity, HR hazard ratio. Covariates: in all models: sex, age, educational levels, household income, smoking, alcohol consumption, body mass index, number of chronic diseases, and cohort. The reference group was defined as individuals exposed to the highest risk levels of PM2.5 at a concentration of 35–50 μg/m3 and engaging in the least active group (< 1 MET-h/wk). To simplify the presentation and facilitate comparison across subgroups, only the results for participants meeting the WHO-recommended level (7.5+ MET-h/week) were shown, while lower activity groups were not included. The category ranges of PM2.5 and LTPA were defined as lower bound inclusive and upper bound exclusive, except for the highest category (i.e., 7.5+ MET-h/wk), which was open-ended
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