Introduction: Nowadays, air pollution is one of the main health and environmental problems in developing cities, which is a result of the increasing use of fossil fuels, heat generators and the activities of industrial districts and industries. Materials and methods: In order to conduct this descriptive cross-sectional study, the sampling site of GARMSAR industrial district in Semnan province was selected. For air sampling, a large sample sampling pump, a cyclone holder filter and a fiberglass filter were used. The air sampling operation lasted from June to September, 2021. Sampling was performed in three days a week at a speed of 0. 5 m/s. Therefore, 15 samples were taken per month and 60 samples were taken in each study period to measure each group of particles. Results: Based on the results of the evaluation of air pollutants, among the measured pollutants, only the amount of suspended particles and sulfur dioxide in some sampling days is higher than the maximum allowable concentration of the standard ambient air. The Air Quality Index (AQI) for Particulate Matter less than 10 µ, m (PM10 ) in August exceeded the allowable level (100), and for Sulfur dioxide (SO2 ) in September. Conclusion: Since the concentration of particulate matter and sulfur dioxide and consequently the amount of AQI in some sampling days is higher than the maximum allowable concentration of ambient air standard, continuous monitoring of emission sources, determination and control of emission sources, and creating local and industrial guidelines in this area is essential.