A time-stratified case-crossover analysis of the association between short-term ozone exposure and emergency ambulance dispatches in Dezhou city

Objective To analyze the association between short-term ozone (O₃) exposure and emergency ambulance dispatches (EAD) in Dezhou city, Shandong province, and to provide scientific evidence for O₃ pollution control and public health interventions from a health perspective.

Methods Daily all-cause EAD data recorded by the Dezhou 120 Emergency Dispatch Command Center from January 1, 2020, to December 31, 2022, as well as meteorological data and pollutant data from the China Meteorological Data Network and the China High Air Pollutants dataset during the same period, were collected and matched according to the calling address. A time-stratified case-crossover design was adopted to analyze the impact of short-term O₃ exposure on EAD in Dezhou city by integrating a distributed lag non-linear model (DLNM) with a conditional logistic regression model. Subgroup analyses were conducted to explore vulnerable populations.

Results The number of EAD in Dezhou city from January 1, 2020, to December 31, 2022, was 344 287, with 1 096 case days and 3 728 matched control days. After excluding the influence of average temperature and relative humidity, the time-stratified case-crossover analysis showed a positive correlation between short-term O₃ exposure lagged 0–7 days and the cumulative risk of EAD. For every interquartile range (IQR) increase in the daily maximum 8-hour average O₃ concentration, the cumulative risk of EAD increased by 8% (OR = 1.08, 95%CI: 1.04–1.11). The single-day lag effect analysis showed that the cumulative effect of short-term O₃ exposure on EAD risk was greatest on the day of exposure (OR = 1.06, 95%CI: 1.05–1.08), decreased rapidly within 2 days, and a potential harvesting effect appeared on days 2–4. Subgroup analyses showed that for every IQR increase in the daily maximum 8-hour average O₃ concentration, the cumulative risk of EAD increased by 9% (OR = 1.09, 95%CI: 1.04–1.13) in males, 6% (OR = 1.06, 95%CI: 1.01–1.11) in females, 6% (OR = 1.06, 95%CI: 0.99–1.13) in the < 35 age group, 6% (OR = 1.06, 95%CI: 1.01–1.11) in the 35–65 age group, 10% (OR = 1.10, 95%CI: 1.05–1.15) in the > 65 age group, 4% (OR = 1.04, 95%CI: 1.01–1.07) in the cold season, and 7% (OR = 1.07, 95%CI: 1.04–1.11) in the warm season. Sensitivity analyses showed that with 8-day and 9-day lags in short-term O₃ exposure, the cumulative risk of EAD increased by 8% (OR = 1.08, 95%CI: 1.04–1.11) and 8% (OR = 1.08, 95%CI: 1.05–1.12), respectively, for every IQR increase in the daily maximum 8-hour average O₃ concentration. When the degrees of freedom for the lag dimension in the model were 5 and 6, the cumulative risk of EAD increased by 7% (OR = 1.07, 95%CI: 1.04–1.11) for every IQR increase in the daily maximum 8-hour average O₃ concentration. After additionally adjusting for gaseous pollutants NO₂ and SO₂ in the model, the cumulative risk of EAD increased by 9% (OR = 1.09, 95%CI: 1.06–1.13) and 8% (OR = 1.08, 95%CI: 1.05–1.12), respectively, for every IQR increase in the daily maximum 8-hour average O₃ concentration. Changing the maximum lag days of O₃ and the degrees of freedom of the lag dimension in the model, as well as adjusting for gaseous pollutants NO₂ and SO₂ in the model, did not result in statistically significant differences from the main model results (all P > 0.05), indicating the robustness of the model.

Conclusions Short-term O₃ exposure can increase the risk of EAD in Dezhou city, with greater impacts on males, individuals over 65 years of age, and EAD during the warm season.