Background: Preterm birth is a major perinatal health problem but factors leading to it are still not completely understood. Objectives: To identify the relation between acute increase in ambient air pollution in a few hours before onset of labour and the risk of preterm birth. Methods: We collected registered birth outcome data and hourly ambient air pollution measurements during 2009 â€’ 2013 in Brisbane, Australia. Using the time-stratified case-crossover design and conditional logistic regression models with natural cubic splines, we assessed the shape of air pollution â€’ preterm birth curve, after controlling for potential confounders. We also examined the effect modification of other factors. Results: The association between air pollution [nitrogen dioxide (NO2), sulphur dioxide (SO2), and carbon monoxide (CO)] and preterm birth was non-linear. Threshold concentrations for the mean of 0 â€’ 24 hours NO2, 24 â€’ 48 hours SO2, and 24 â€’ 48 hours CO before onset of labour were 7.6 parts per billion (ppb), 3.8 ppb, and 162.5 ppb, respectively. Increases in air pollution concentrations above thresholds were associated with increased risks of preterm birth. The ORs of preterm birth at 95th percentile of NO2, SO2 and CO against the thresholds were 1.17 (1.08, 1.27), 1.01 (0.99, 1.04), and 1.18 (1.06, 1.32), respectively. The associations were modified by demographic factors, for example, maternal smoking and socioeconomic status (SES). Conclusion: Acute increases in ambient air pollution concentrations above certain levels before onset of labour may stimulate preterm birth. Keywords: air pollution, child health, preterm birth Key Messages: 1. This is the first study to examine the very short-term impacts of air pollution on preterm birth. 2. This study provides very important informtion to prevent preterm birth from expsoure to high concentrations of air pollutants. 3. This is the first study to report the threhold effects of air pollution on preterm birth.