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Fine particulate matter pollution (PM 2.5) is widely considered to be a top-ranked risk factor for premature mortality and years of life lost (YLL). However, evidence regarding the effect of daily air quality improvement on life expectancy is scarce, especially in the Middle East such as Iran. This study aimed to investigate the potential benefits in life expectancy at concentrations meeting the daily PM 2.5 standards during 2012–2016 in Tehran, Iran. We collected daily non-accidental mortality and data on air pollutants and weather conditions from Tehran, Iran, 2012–2016. A quasi-Poisson or Gaussian time-series regression was employed to fit the associations between ambient PM 2.5 and mortality or YLL. Potential gains in life expectancy (PGLE) and attributable fraction (AF) were estimated by assuming that daily PM 2.5 concentrations attained the World Health Organization air quality guidelines (WHO AQG) 2005 (25 μg/m3) and 2021 (15 μg/m3). During the study period, a total of 221,231 non-accidental deaths were recorded in Tehran, resulting in 3.6 million YLL. The mean concentration of ambient PM 2.5 was 34.7 μg/m3 (standard deviation: 15.3 μg/m3). For a 10-μg/m3 rise in 4-day moving average (lag 03-day) in PM 2.5 concentration, non-accidental mortality and YLL increased by 1.12% (95% confidence interval: 0.60, 1.65) and 20.73 (7.08, 34.39) person years, respectively. A relatively higher effect was observed in males and young adults aged 18–64 years. We estimated that 39830 [AF = 1.1%] and 74284 [AF = 2.1%] YLL could potentially be avoided if daily PM 2.5 concentrations attained the WHO AQG 2005 and 2021, respectively, which corresponded to potential gains in life expectancy of 0.18 (0.06, 0.30) and 0.34 (0.11, 0.56) years for each deceased person. PM 2.5 -associated PGLE estimates were largely robust when performing sensitivity analyses. Our findings indicated that short-term exposure to PM 2.5 is associated with increased non-accidental YLL and mortality. Prolonged life expectancy could be achieved if the particulate matter air pollution level were kept under a stricter standard. • The average concentration of PM 2.5 was 34.7 μg/m3 during the study period, and about 350 days per year exceeded the daily standard of WHO air quality guidelines 2021 (15 μg/m3). • Life expectancy might increase by 0.34 years for each death by attaining WHO air quality guidelines 2021. • Up to 2.1% of YLL could be attributed to PM 2.5 exposure higher than WHO air quality guidelines 2021. [ABSTRACT FROM AUTHOR]