Short-term exposure to ambient fine particulate matter and out-of-hospital cardiac arrest: a nationwide case-crossover study in Japan

Background: PM_2·5 is an important but modifiable environmental risk factor, not only for pulmonary diseases and cancers, but for cardiovascular health. However, the evidence regarding the association between air pollution and acute cardiac events, such as out-of-hospital cardiac arrest (OHCA), is inconsistent, especially at concentrations lower than the WHO daily guideline (25 μg/m³). This study aimed to determine the associations between exposure to ambient air pollution and the incidence of OHCA.

Methods: In this nationwide case-crossover study, we linked prospectively collected population-based registry data for OHCA in Japan from Jan 1, 2014, to Dec 31, 2015, with daily PM_2·5, carbon monoxide (CO), nitrogen dioxide (NO₂), photochemical oxidants (O_x), and sulphur dioxide (SO₂) exposure on the day of the arrest (lag 0) or 1-3 days before the arrest (lags 1-3), as well as the moving average across days 0-1 and days 0-3. Daily exposure was calculated by averaging the measurements from all PM_2·5 monitoring stations in the same prefecture. The effect of PM_2·5 on risk of all-cause or cardiac OHCA was estimated using a time-stratified case-crossover design coupled with conditional logistic regression analysis, adjusted for daily temperature and relative humidity. Single-pollutant models were also investigated for the individual gaseous pollutants (CO, NO₂, O_x, and SO₂), as well as two-pollutant models for PM_2·5 with these gaseous pollutants. Subgroup analyses were done by sex and age.

Findings: Over the 2 years, 249 372 OHCAs were identified, with 149 838 (60·1%) presumed of cardiac origin. The median daily PM_2·5 was 11·98 μg/m³ (IQR 8·13-17·44). Each 10 μg/m³ increase in PM_2·5 was associated with increased risk of all-cause OHCA on the same day (odds ratio [OR] 1·016, 95% CI 1·009-1·023) and at lags of up to 3 days, ranging from OR 1·015 (1·008-1·022) at lag 1 to 1·033 (1·023-1·043) at lag 0-3. Results for cardiac OHCA were similar (ORs ranging from 1·016 [1·007-1·025] at lags 1 and 2 to 1·034 [1·021-1·047] at lag 0-3). Patients older than 65 years were more susceptible to PM_2·5 exposure than younger age groups but no sex differences were identified. CO, O_x, and SO₂ were also positively associated with OHCA while NO₂ was not. However, in two-pollutant models of PM_2·5 and gaseous pollutants, only PM_2·5 (positive association) and NO₂ (negative association) were independently associated with increased risk of OHCA.

Interpretation: Short-term exposure to PM_2·5 was associated with an increased risk of OHCA even at relatively low concentrations. Regulatory standards and targets need to incorporate the potential health gains from continual air quality improvement even in locations already meeting WHO standards.