Ambient temperature and air pollution, and the risk of preterm birth in Tehran, Iran: a time series study

The aim of the present study was to determine the relationship between temperature and air pollution, and preterm birth in Tehran, Iran.

In this time series study, the daily data of preterm births, air pollution, and maximum, minimum and mean temperature from March 2015 to March 2018 were used. To evaluate the effect of air pollution and temperature with and without adjustment of their mutual effects on preterm birth in lags (days) 0-21, the Distributed Lag Non-linear Models (DLNM) was used. The relative risk (RR) was estimated for extreme, moderate and mild heat (99th, 95th, 75th percentile) and cold (1st, 5th, 25th percentile) compared with the median, and for each 10-unit increase in PM_2.5, NO₂, and O₃, 5-unit increase in SO₂, and 1-unit increase in CO.

The highest RR was seen in extreme (26.9 °C) and moderate (24.8 °C) heat of minimum temperature on lag 0 (RR = 1.17; 1.05-1.31, Adjusted RR = 1.16; 1.04-1.29, RR = 1.15; 1.05-1.26, Adjusted RR = 1.14; 1.03-1.25, respectively). In regard of cold, the only significant effect was for maximum temperature on lags 7-9 (RR = 1.02; 1.00-1.04). Each 10-unit increase in PM2.5 in Lag 0 (RR = 1.008; 1.001-1.014) and lag 1 (RR = 1.004; 1.001-1.007) and in NO₂ in lag 0 (RR = 1.006; 1.000-1.012) had significant effects.

Maternal exposure to a minimum daily temperature of 26.9 and 24.8 °C compared to 13.2 °C increased the risk of preterm birth by 17 and 15% on the same day, respectively. This risk increased by 0.8 and 0.6%, on the same day for each 10-unit increase in PM_2.5 and NO₂, respectively.