<p>Air pollution has been associated with respiratory and cardiovascular diseases; however, its effects on routinely measured blood indices remain incompletely understood. This study examined associations between short-term particulate matter with an aerodynamic diameter of 2.5&#xa0;μm or less (PM_2.5) and hematological and biochemical indices in middle-aged and older adults. A study involving 989 adults aged 40 years and older, recruited from the community in Taiwan from 2018 to 2022, assessed hematological (erythrocyte, leukocyte, and thrombocyte) and biochemical (electrolyte levels, renal function, glycemic control, hepatobiliary function, lipid profile, and iron status) indices. PM_2.5 exposure over 1-day, 7-day, and 1-month periods was estimated to evaluate both immediate and lagged effects. Associations between PM_2.5 and blood indices were explored employing linear regression models, adjusted by age, sex, body mass index, smoking status, vegetarian diet, relative humidity, temperature, work status, exercise habits, and comorbidities. Higher PM_2.5 exposure was associated with changes in several hematological and biochemical indices, including lower hematocrit, mean platelet volume, sodium, calcium, magnesium, creatinine, albumin, and iron, and higher mean corpuscular hemoglobin concentration, red cell distribution width, monocytes, and total bilirubin. The magnitude of the effects increased as the exposure period lengthened. Associations were generally more frequent among participants without baseline anemia. Although these modest changes should be interpreted as subclinical biomarker alterations rather than clinical endpoints, they may help characterize early systemic responses to short-term PM_2.5 exposure in community settings.</p> Graphical Abstract <p></p>