Ozone and possible heat effects on out-of-hospital cardiac arrest: a time-series study

Out-of-hospital cardiac arrest (OHCA) is a time-critical emergency with high mortality. Short-term environmental exposures, including temperature and air pollution, may trigger acute cardiovascular events, but their independent and lagged effects in northern Chinese cities remain unclear.

We conducted a time-series study of 7,996 OHCA cases recorded by Tianjin's prehospital emergency system from July 2023 to June 2024. Daily OHCA counts were linked with meteorological and air pollution data. A distributed lag non-linear model with a quasi-Poisson link estimated non-linear and lagged effects of daily mean temperature (lag 0-14). Six pollutants (PM₂.5, PM10, NO₂, SO₂, CO, O3) were examined in single- and two-pollutant models using lag0-1 exposures, adjusting for meteorological and temporal covariates.

Daily OHCA averaged 21.9 cases. Temperature showed a monotonic increase in cumulative risk, with a minimum risk temperature of -1.9 °C. High temperatures showed a possible short-term association" peaking at lag 1 (RR ≈1.57 at the 95th percentile), whereas low temperatures showed no significant effect. Confidence intervals included unity, indicating limited statistical certainty. Ozone was the only pollutant consistently associated with OHCA. Each IQR increase in lag0-1 O3 corresponded to a 15.3% higher risk. In sex-stratified analyses, the effect was comparable in men (RR = 1.22, 95% CI: 1.08-1.39) and women (RR = 1.22, 95% CI: 1.04-1.43). Heat-related risks appeared stronger in women (RR = 3.16) than in men (RR = 1.58), though estimates were imprecise. Age-specific analyses were limited by sparse data, but in the ≥75-year group ozone showed a positive trend. Interaction analyses suggested ozone may amplify heat effects, with a significant interaction (p = 0.043). Lag-response analyses confirmed a short-term peak at lag 1 day in the high-ozone group.

Elevated ozone concentrations are important short-term triggers of OHCA in Tianjin, while high ambient temperatures showed a possible short-term association with limited statistical certainty. Ozone effects were robust across sexes, while heat-related risks appeared stronger in women. Age-specific analyses were inclusinve, underscoring the need for larger cohorts. Exploratory analyses suggested a possible ozone-temperature interaction, but given imprecision and lack of multiple-testing adjustment, this finding should be interpreted cautiously.