Effects of the interaction between cold spells and fine particulate matter on mortality risk in Xining: a case-crossover study at high altitude

Abstract

Background: With global climate change, the health impacts of cold spells and air pollution caused by PM_2.5 are increasingly aggravated, especially in high-altitude areas, which are particularly sensitive. Exploring their interactions is crucial for public health.

Methods: We collected time-series data on meteorology, air pollution, and various causes of death in Xining. This study employed a time-stratified case-crossover design and conditional logistic regression models to explore the association between cold spells, PM_2.5 exposure, and various causes of death, and to assess their interaction. We quantitatively analyzed the interaction using the relative excess odds due to interaction (REOI), attributable proportion due to interaction (AP), and synergy index (S). Moreover, we conducted stratified analyses by average altitude, sex, age, and educational level to identify potential vulnerable groups.

Results: We found significant associations between cold spells, PM_2.5, and various causes of death, with noticeable effects on respiratory disease mortality and COPD mortality. We identified significant synergistic effects (REOI>0, AP > 0, S > 1) between cold spells and PM_2.5 on various causes of death, which generally weakened with a stricter definition of cold spells and longer duration. It was estimated that up to 9.56% of non-accidental deaths could be attributed to concurrent exposure to cold spells and high-level PM_2.5. High-altitude areas, males, the older adults, and individuals with lower educational levels were more sensitive. The interaction mainly varied among age groups, indicating significant impacts and a synergistic action that increased mortality risk.

Conclusion: Our study found that in high-altitude areas, exposure to cold spells and PM_2.5 significantly increased the mortality risk from specific diseases among the older adults, males, and those with lower educational levels, and there was an interaction between cold spells and PM_2.5. The results underscore the importance of reducing these exposures to protect public health.

Keywords: air pollution; climate change; cold spell; fine particulate matter; mortality.

Figure 1

Location, altitude range and topography of Xining, China (C: a is Chengbei District, b is Chengxi District, c is Chengzhong District, d is Chengdong District, e is Datong County, f is Huangzhong District, and g is Huangyuan County). The average altitude of a-d is about 2500 m, and the average altitude of e-g is about 3000 m.

Figure 2

Trends in temperature and PM_2.5 over time and interim targets for daily average PM_2.5 concentrations according to the World Health Organization’s 2021 Air Quality Guidelines (interim target 4 is 25 μg/m³, 3 is 37.5 μg/m³, 2 is 50 μg/m³, 4 is 75 μg/m³, and HIGH is greater than 75 μg/m³).

Figure 3

Association of exposure to cold spells and PM_2.5 with non-accidental, cardiovascular and respiratory disease deaths. (A–C) OR (95%CI) for nonaccidental, cardiovascular, and respiratory disease deaths with exposure to cold spells, respectively. (D–F) Exposure-response curves for the association between exposure to ambient PM_2.5 and non-accidental, cardiovascular and respiratory disease deaths, respectively.

Figure 4

Additive interaction effects of exposure to cold periods and PM_2.5 on non-accidental, cardiovascular and respiratory disease mortality.

Figure 5

Excess fraction and number of excess deaths due to exposure to cold spells and high-level PM_2.5.