Cleaner outdoor air diminishes the overall risk of intracerebral hemorrhage but brings differential benefits to subpopulations: a time-stratified case-crossover study

BACKGROUND: Short-term air pollution exposure and intracerebral hemorrhage (ICH) risk are related. However, the impact of the pollutant levels decline on this relationship, which attributes to clean air policy implementation and the COVID-19 pandemic lockdown, is unclear. In the present research, we explored the influence of different pollutant levels on ICH risk during eight years in a southwestern China megacity. METHODS: Our research used a time-stratified case-crossover design. We retrospectively analyzed ICH patients in a teaching hospital from January 1, 2014, to December 31, 2021, and divided 1571 eligible cases into two groups (1st group: 2014–2017; 2nd group: 2018–2021). We observed the trend of every pollutant in the entire study period and compared the pollution levels in each group, using air pollutants data (PM(2.5), PM(10), SO(2), NO(2), CO, and O(3)) documented by the local government. We further established a single pollutant model via conditional logistic regression to analyze the association between short-term air pollutants exposure and ICH risk. We also discussed the association of pollution levels and ICH risk in subpopulations according to individual factors and monthly mean temperature. RESULTS: We found that five air pollutants (PM(2.5), PM(10), SO(2), NO(2), CO) exhibited a continuous downward trend for the whole duration, and the daily concentration of all six pollutants decreased significantly in 2018–2021 compared with 2014–2017. Overall, the elevation of daily PM(2.5), SO(2), and CO was associated with increased ICH risk in the first group and was not positively associated with risk escalation in the second group. For patients in subgroups, the changes in the influence of lower pollutant levels on ICH risk were diverse. In the second group, for instance, PM(2.5) and PM(10) were associated with lower ICH risk in non-hypertension, smoking, and alcohol-drinking participants; however, SO(2) had associations with increased ICH risk for smokers, and O(3) had associations with raised risk in men, non-drinking, warm month population. CONCLUSIONS: Our study suggests that decreased pollution levels diminish the adverse effects of short-term air pollutants exposure and ICH risk in general. Nevertheless, the influence of lower air pollutants on ICH risk in subgroups is heterogeneous, indicating unequal benefits among subpopulations. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12889-023-16232-3.