<p>In the first stage of this study, the number of days exceeding the limit value recommended by the World Health Organization (WHO) for 24-h average (daily) PM2.5 concentration, the monthly average PM2.5 concentration, annual average PM2.5 concentration and seasonal variation of PM2.5 at air quality measurement stations in Kocaeli, one of the heavily industrialized cities of Türkiye, during 2021 − 2023 were investigated. In the second stage of this study, premature deaths in adults aged ≥ 30&#xa0;years due to long-term exposure to PM2.5 in Kocaeli city for 2021, 2022 and 2023 were estimated using AirQ + software. The average annual PM2.5 concentration in Kocaeli city was 19.17 ± 2.46, 19.12 ± 1.52 and 19.63 ± 1.98&#xa0;µg/m³ in 2021, 2022 and 2023, and these values were 3.83, 3.82 and 3.93 times higher than the recommended limit value by WHO, respectively. AirQ + model results showed that 1,059 (705 − 1,376), 1,051 (699 − 1,369) and 1,174 (780 − 1,530) all natural cause deaths in Kocaeli city in 2023, 2022 and 2021, respectively, were associated with exposure to ambient PM2.5 pollution. Moreover, 11.71% (7.8 − 15.22%), 10.67% (7.09 − 13.89%) and 10.25% (6.81 − 13.35%) of natural cause deaths in Kocaeli city were associated with long-term exposure to ambient PM2.5 pollution in 2023, 2022 and 2021, respectively. An average of 3,284 premature deaths associated with long-term exposure to PM2.5 in three years could have been prevented if PM2.5 concentrations in Kocaeli city had not exceeded the WHO recommended limit value.</p> Graphical Abstract <p></p>