PROJECT SUMMARY/ABSTRACT Approximately 12% of couples in the United States seek infertility treatment, which for some is associated with treatment-related side-effects, treatment-related stressors, and a significant financial burden. Although male- factor infertility contributes to an estimated 30-50% of cases of couple infertility, risk factors, mechanisms, and potential interventions for male-factor infertility have been critically understudied. Ambient air pollution and ambient temperature extremes are ubiquitous exposures associated with systemic inflammation and oxidative stress, to which the process of spermatogenesis may be uniquely susceptible. Research has found exposure to high levels of air pollution may adversely impact semen quality; however, the effect of exposure during developmentally-sensitive windows of spermatogenesis on semen quality, impacts on sperm epigenetic factors including DNA methylation, and the cumulative impact of male partner exposure on infertility treatment outcomes have been little evaluated. This study will utilize data from the Folic Acid and Zinc Supplementation Trial (FAZST) to estimate impacts of ambient environmental factors on male fertility, adding estimates of individual exposure to ambient air pollution and temperature. Adding to sparse literature from countries with low-to-moderate levels of air pollution, this study will evaluate exposure to air pollution and high and low ambient temperatures during developmentally susceptible windows of spermatogenesis (mitosis, meiosis, spermiogenesis, and spermiation) with semen quality, leveraging four repeated semen samples from more than 1,900 participants (Aim 1). Epigenetic alterations in sperm are increasingly recognized as a potential mechanism impacting male fertility, with exposure to several oxidative stress-inducing and endocrine- disrupting environmental factors linked to differential DNA methylation in sperm. This study will conduct a novel assessment of the impact of ambient air pollution exposure on sperm DNA methylation (Aim 2). Finally, this study will utilize information on outcomes of infertility treatment cycles (in vitro fertilization and intrauterine insemination cycles) and timing of treatment-related semen samples to evaluate the impact of male partner exposure to air pollution and ambient temperature during spermatogenesis on probability of pregnancy during infertility treatment, accounting for exposure windows during embryogenesis and implantation (Aim 3). This career development award will extend the candidate’s prior training and research experience in environmental and reproductive epidemiology by providing expert mentoring and targeted training in epigenetics, air quality exposure assessment, and causal inference. Her primary mentor Dr. Richard Pilsner (epigenetics), along with members of her mentoring team Dr. Timothy Canty (air quality modeling) and Dr. Laura Balzer (causal inference), with provide the needed mentoring for the candi...