Physiological equivalent temperature and the risk of preterm birth in Tehran, Iran.

This study was conducted to investigate the effects of climatic parameters-particularly the Physiological Equivalent Temperature (PET) index-on the risk of preterm birth (PB) in the city of Tehran. The present research is an ecological time-series study with daily data from March 2018 to March 2024. Data on preterm births, meteorological parameters from four synoptic stations, and air pollutants (PM₂.₅, PM₁₀, NO₂, SO₂, CO, O₃) from 13 air quality monitoring stations were collected. To analyze non-linear relationships and delayed effects, Distributed Lag Non-linear Models (DLNM) were used, controlling for air pollutants, long-term trends, and seasonality. Results showed the mean daily number of preterm births was 37.04. The mean PET was 19.25 °C, ranging from - 7.6 °C to 41.6 °C. Exposure to extreme cold (1st percentile PET = -1.7 °C) over a 0-30 day lag period significantly increased the cumulative relative risk of preterm birth (RR = 1.25; 95% CI: 1.02-1.52), with this effect primarily observed at lags of 20 to 30 days. In contrast, extreme heat (99th percentile PET = 39.1 °C) had no significant effect (RR = 0.94; 95% CI: 0.81-1.10). A significant negative correlation was found between PET and combustion-related pollutants, indicating the accumulation of these pollutants under cold conditions. In the semi-arid climate and polluted air of Tehran, extreme cold-not extreme heat-is a significant environmental risk factor for preterm birth. This finding highlights the necessity of developing early warning systems for cold waves, providing seasonal guidance to pregnant women, and ensuring equitable access to heating facilities.