Short and long term impacts of outdoor air pollution on semen quality Introduction: Ulaanbaatar is one of the most polluted capital cities1. In winter, the average daily PM2.5 level of pollution is 687 micrograms per cubic meter, which is 80 times higher than the WHO recommended safe level2. Air pollution contributes to a variety of diseases, including cardiovascular disease, chronic obstructive pulmonary disease, and neurological disorders and infertility3. According to the World Health Organization, 48 million couples and 186 million people worldwide are infertile4, 5. Factors affecting semen quality include chronic diseases, urogenital infections, environmental pollution, lifestyles, and geographical location of nations12,13 . Fine particles and toxins (PM10, SO2, NO2, PM2.5) in ambient air pollution affect the maturation of male gametes and activate DNA damage, cell apoptosis, oxidation, and inflammation14,15. According to researchers, foreign Ambient air pollution affects semen quality, but there is little evidence. Therefore, more research is needed in this regard. Materials and methods: The survey was conducted from January 1, 2020 to April 31, 2021 based on the retrospective longitudinal survey model based on the NСМCH-Infertility Reproductive Center. To measure sperm quality, the sample was diluted for 15 to 60 minutes, stored at 20 to 37 degrees Celsius, and viewed under a microscope at 200x magnification using a Makler Chamber instrument for 1 hour to determine sperm motility and concentration. Count 200 sperm in the field of view and evaluate and classify sperm motility. Measurement of air pollutants: The average daily amount of air pollutants in Ulaanbaatar is determined by the average of 6 guard stations of the Meteorological and Environmental Research Agency. The study used 24-hour measurement data of 4 air pollutants (SO2, NO2, PM2.5, PM10) at 1-hour intervals. Total measurement results from October 1, 2019 to December 31, 2020 are included. Statistical analysis of the survey was performed using STATA 16.0 software. Statistical analysis of the survey was performed using STATA 16.0 software. After clearing and sorting the survey data, if the missing value is 10 percent or less of the total data, the variable is deleted and analyzed. Results: High levels of air pollution have a negative effect on semen volume and sperm count. For example, an increase in particulate matter (PM2.5) in the ambient air by 1 μg / m3 reduces the amount of semen by 0.002 ml and the number of sperm by 17.0 thousand. However, when the amount of PM10 particles increases by 10 μg / m3, the concentration in the semen decreases by 1.14 million. Outdoor air pollution is likely to have a long-term negative impact on semen quality. For example, when the amount of PM2.5 particles in the ambient air increases by 1 μg / m3, the number of sperm in the semen decreases by 81.0 thousand after 3 months. However, if the average amount of SO2 in the ambient air increases by 1 μg / m3, the number of sperm in the semen will decrease by 1.49 million after 3 months. Conclusions: 1. The use of toxic habits in the two groups of men from Ulaanbaatar, who were treated at the andrology laboratory of the National Center for Obstetrics and Gynecology, did not differ statistically. 2. PM10 levels are 129.111.6 μg / m3 higher than the average daily levels of air pollutants in Ulaanbaatar, which indicates a level of adverse health effects. 3. Long- and short-term exposure to ambient air pollutants can adversely affect semen volume and sperm count.