Abstract： Objective:To investigate the association between exposure to major air pollutants (including PM 2.5, PM 10, NO 2, SO 2, O 3 and CO) and 28-day all-cause mortality in patients admitted to the intensive care unit (ICU). Methods:The electronic medical records of critically ill patients admitted to the ICU in Hubei Province General Hospital and Jingzhou Central Hospital were collected from August 2018 to August 2019 and from May 2021 to May 2022. Patients' exposure to air pollutants was assessed based on the average concentrations at their place of residence during the previous two months. A generalized linear regression model was used to estimate the association between air pollutant exposure and 28-day all-cause mortality in critically ill patients. Subgroup analysis was then conducted to examine the impact of individual air pollutant exposure on 28-day mortality, which served as the primary outcome in this study. The effect size and confidence interval were adjusted for patient characteristics including age, gender, smoking or drinking habits, length of hospital stay, and SOFA score. Additionally, mediation analysis was conducted using the mediation package (Bruce R) in R software. The direct effect represented the association between exposure to air pollutants and risk of mortality, while the indirect effect aimed to assess whether neutrophil-lymphocyte ratio (NLR), platelet-lymphocyte ratio (PLR), and monocyte- lymphocyte ratio (MLR) served as mediating variables in the correlation between exposure to air pollutants and mortality risk.Results:The study included a total of 3 772 critically ill patients admitted to the ICU, with a 28-day all-cause mortality rate of 40.0% (1 509/3 772). A significant positive correlation was observed between an incremental increase of 1 μg/m 3 in air pollutants (specifically PM 2.5, NO 2 and CO) and an elevated risk of mortality within 28 days among critically ill patients. Conversely, there is an inverse relationship between O 3 levels and mortality risk. Additionally, male gender and SOFA rating >3 scores were identified as independent risk factors for all-cause mortality in ICU patients exposed to NO 2 or CO. The Neutrophil/lymphocyte ratio (NLR) played a mediating role in the correlation between PM 2.5 or CO exposure and mortality risk, accounting for 9.09% or 4.71% of the correlations, respectively. Conclusions:The exposure to air pollutants (PM 2.5, NO 2, CO) significantly associate with a high risk of 28-day all-cause mortality in ICU patients. Even at low levels of air pollution, NO 2, CO remains positively correlate with the mortality risk in critically ill patients, who belong to a vulnerable population.