Association of ambient particulate matter (PM2.5) with reduced sex ratio at birth after the implementation of a rapid transit system in Taipei, Taiwan.

Sex ratio at birth (SRB) is a simple, noninvasive way to monitor the reproductive health of a population. Apparently only a few studies have assessed the relationship between ambient air pollution and SRBs. The studies that investigated this relationship have yielded inconsistent results. Ambient air pollution levels, including particulate matter (PM_2.5) levels, have decreased in Taipei city after a mass rapid transit (MRT) system commenced operations in 1996. The aim of this study was to determine whether the changes in levels of PM_2.5 after the implementation of the MRT system in Taipei may have influenced the concurrent alterations noted in the SRB. In order to assess this potential association, annual numbers of male and female births were obtained from Taiwan's Department of Household Registration, Ministry of Interior Affairs from 1992 to 2023. Mean yearly SRBs were calculated for 4 time periods, 1992-1995, 1996-2004, 2005-2015, and 2016-2023. Linear logistic regression was employed to evaluate the variation in officially reported PM_2.5 levels during those periods and alterations in the ratios. The ratios for time periods 1 to 4 were 52.61%, 52.22%, 51.78%, and 51.67%, respectively. Compared to Period 1 (pre-MRT), the odds ratios for Periods 2, 3, and 4 were 0.98 (0.97-0.99), 0.97 (0.96-0.98), and 0.96 (0.95-0.98), respectively. The trend toward the decreased SRB was found to be gradual but significant.