Combined effects of apparent low temperature and PM2.5 pollution on circulatory emergency ambulance calls: a time-series analysis in Shijiazhuang, China.

BACKGROUND

The health impacts of compound environmental exposures, particularly the joint effects of low temperature and air pollution, remain insufficiently understood.

OBJECTIVE

This study aimed to evaluate the independent and joint effects of apparent low temperature (AT) and PM2.5 on cardiovascular emergency ambulance calls (EACs).

Methods

Daily cardiovascular emergency ambulance calls (EACs), meteorological, and air pollution data in Shijiazhuang, China (2014-2023) were analyzed. AT was calculated using the Steadman formulation. Distributed lag non-linear models (DLNM) were applied to assess lagged effects, while joint exposure models and relative excess risk due to interaction (RERI) were used to evaluate joint effects and interactions.

RESULTS

A total of 82,714 EACs were included. Apparent low temperature was significantly associated with increased risk of cardiovascular EACs, with delayed effects. Most joint exposure scenarios further elevated risks, with the strongest cumulative effect observed under the P1T3 scenario (RR = 1.146, 95% CI: 1.088-1.207). Subgroup analyses showed higher susceptibility among males, older adults, and patients with hypertension. Both synergistic and antagonistic interactions were identified.

CONCLUSION

Combined exposure to apparent low temperature and PM2.5 significantly increases the risk of cardiovascular emergency ambulance calls, with complex lagged and heterogeneous effects. These findings provide important epidemiological evidence for understanding compound environmental risks and underscore the need to incorporate multi-factor environmental exposures into public health early warning and intervention strategies.