Different effects of PM10 exposure on preterm birth by gestational period estimated from time-dependent survival analyses

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We conducted this study to determine if the preterm risks due to PM(10) exposure vary with the exposure periods during pregnancy. This study was also conducted to estimate the different effects of PM(10) exposure on preterm birth by exposure periods using the extended Cox model with PM(10) exposure as a time-dependent covariate. We studied birth data obtained from the Korea National Statistical office for 374,167 subjects who were delivered between 1998 and 2000 in Seoul, South Korea. We used PM(10) data that was measured hourly to give 24-h averages at 27 monitoring stations in Seoul. The extended Cox model with time-dependent exposure was used to determine if the risk of preterm delivery could be associated with PM(10) exposures for each trimester during pregnancy. Effect of PM10 exposure prior to the 37 weeks of gestational period was stronger on the risk of premature birth than that posterior to the 37 weeks of gestational weeks. This trend was consistent for each trimester; however, the hazard ratios for preterm delivery associated with PM(10) exposure in the first and third trimester were slightly higher than those of the second trimester. The risk of preterm birth associated with exposure to PM(10) differed with the exposure period of the neonates. Therefore, when studying the impact of air pollution exposure during pregnancy, the exposure period during pregnancy should be considered.