Association between nocturnal ozone enhancement and daily cardiovascular mortality: A multi-city study in China

The health effects of ambient ozone (O₃) pollution have been well documented, but most studies have focused on daytime exposure, with limited research on nighttime O₃. Nocturnal O₃ enhancement (NOE) refers to unexpected increases in nighttime O₃ concentrations over a few hours, yet the health effects of this phenomenon remain unknown. This study evaluated the short-term association between NOE days and cardiovascular mortality by conducting a multi-city time-series analysis in China (2013-2015). Nine definitions were adopted to identify site-level NOE events, considering both average O₃ levels and peak increments at night. A city-level NOE day was defined as a day when over 60% of the monitoring stations within a city recorded at least one site-level NOE event. City-level associations were analyzed using over-dispersed generalized additive models, and national estimates were pooled using meta-analysis. We also conducted stratified analyses by age, sex, season, and region. We found significant associations between NOE days and increased mortality due to total cardiovascular disease (CVD) and coronary heart disease (CHD). Under the strictest NOE definition, mortality risk increased by 1.7% (95% CI: 0.8%, 2.5%) for total CVD and 2.1% (95% CI: 0.9%, 3.3%) for CHD on the day after NOE. Relatively higher risk estimates were found in individuals aged 5-64 years, females, the warm season, and the southern region. No stable associations were found with stroke mortality. These findings offer new insights into the health impacts of NOE and underscore the need for time-phased actions to control both daytime and nighttime O₃ air pollution.