To our knowledge, this epidemiologic study is the first to examine the association of PM2.5 limits with all-cause mortality in Asia. The results indicated that, except for the PM2.5 threshold of 115 µg/m³, the concentrations of 35 µg/m³, 75 µg/m³, and 150 µg/m³ were significantly associated with mortality from all causes, and the effects of PM2.5 were stronger as the levels increased. In addition, consistent with other studies, there was no evidence of a limit at which PM2.5 exposure does not affect mortality, even for concentrations lower than 35 µg/m³, demonstrating that PM2.5 is a significant risk factor for all-cause mortality, and the adverse impacts on public health do not decrease as pollutant levels decrease 21–23.
The association between all-cause deaths and PM2.5 exposure was statistically significant at 35 µg/m³, 75 µg/m³, and 150 µg/m³, and this result maybe because of the relatively fewer deaths between 75 µg/m³ and 115 µg/m³. Furthermore, the daily temperature corresponding to each of these concentrations was higher than that of other thresholds. The impact of different PM2.5 limits on mortality may be due to high temperatures 24, 25.
The results of a previous study on the gender-specific effects of particulate matter were inconsistent 26. The results of the gender-stratified analysis demonstrated that female patients were more sensitive to the PM2.5 levels of 75 µg/m³, 115 µg/m³, and 150 µg/m³, whereas male patients were more sensitive to a concentration of 35 µg/m³, indicating that men are more susceptible to lower PM2.5 concentrations than women. Smoking is a critical environmental risk factor, and one study suggested that the estimated impact of air pollution might be stronger in nonsmokers than smokers 27. A potential reason for this difference may be that women have slightly stronger airway reactivity and smaller airways than men 28. Moreover, the adverse impacts of additional exposure to PM2.5 may be overcome by the oxidative and inflammatory effects of smoking 29.
Older individuals had increased susceptibility to PM2.5 levels of 75 µg/m³, 115 µg/m³, and 150 µg/m³ compared with younger individuals, possibly because the former group has a weaker immune system and higher sensitivity to these particles 30, 31. However, there was no significant association between PM2.5 exposure and all-cause mortality in individuals aged < 60 years, indicating that the general population should avoid high levels of PM2.5 (≥75 µg/m³).
This study has some limitations. First, the study selected the mean air pollutant concentration from each monitoring site in Jinan as the exposure concentration; nonetheless, individual exposure may depend on other factors, including the type of outdoor activity, physical fitness, and living habits, potentially causing exposure measurement errors or underestimating the impact of air pollution. In addition, this study belongs to the field of ecological research, and the conclusions cannot prove causality but merely indicate the relationship between air pollutants and all-cause mortality.