Exposure to submicron particulate matter and long-term survival: Cross-cohort analysis of 3 Chinese national surveys

Background

Cohort evidence linking increased mortality with airborne fine particulate matter (PM_2.5, particulate matter [PM] with aerodynamic diameter ≤2.5 μm) exposure was extensively validated worldwide. Nevertheless, long-term survival associated with submicron particulate matter (PM₁, PM with aerodynamic diameter ≤1 μm) exposure remained largely unstudied, particularly in highly exposed populations.

Methods

We performed a population-based investigation involving 86844 adults aged 16+ years from 3 national dynamic cohorts spanning from 2005 to 2018. Residential annual exposure to PM₁ and PM_2.5 was assigned for each follow-up year using satellite-derived spatiotemporal estimates at a 1-km² resolution. The concentration of PM_1-2.5 (PM with aerodynamic diameter between 1 and 2.5 μm) was calculated by subtracting PM₁ from PM_2.5. Time-independent Cox proportional hazards regression models were applied to assess the associations of all-cause mortality with long-term exposure to size-specific particles. To investigate the effect of PM₁ on PM_2.5-mortality associations, we categorized participants into low, medium, and high groups based on PM₁/PM_2.5 ratio and examined the risk of PM_2.5-associated mortality in each stratum. Effect modifications were checked via subgroup analyses.

Results

A total of 18722 deaths occurred during 497069.2 person-years of follow-up (median 5.7 years). Participants were exposed to an average annual concentration of 31.8 μg/m³ (range: 7.6–66.8 μg/m³) for PM₁, 56.3 μg/m³ (range: 19.8–127.2 μg/m³) for PM_2.5, and 24.5 μg/m³ (range: 7.3–60.3 μg/m³) for PM_1-2.5. PM₁, PM_2.5, and PM_1-2.5 were consistently associated with elevated mortality risks, with a hazard ratio (HR) of 1.029 (95% confidence interval [CI]: 1.013–1.046), 1.014 (95% CI: 1.005–1.023), and 1.019 (95% CI: 1.001–1.038) for each 10-μg/m³ increase in exposure, respectively. Compared with low (HR = 0.986, 95% CI: 0.967–1.004) and medium (HR = 1.015, 95% CI: 1.002–1.029) PM₁/PM_2.5 ratio groups, PM_2.5-related risk of mortality was more pronounced in high PM₁/PM_2.5 ratio stratum (HR = 1.041, 95% CI: 1.019–1.064). Greater risks of mortality associated with size-specific particles were found among the elderly (>80 years old), southeastern participants, and those living in warmer areas.

Conclusions

This study demonstrated that long-term exposure to PM₁, PM_2.5, and PM_1-2.5 was associated with heightened mortality, and PM₁ may play a predominant role in PM_2.5-induced risk. Our results emphasized the population health implications of establishing ambient PM₁ air quality guidelines to mitigate the burden of premature mortality stemming from particulate air pollution.