Mathew AV, et al. - Whether daily exposures to higher levels of ambient fine particulate matter (PM_2.5) and colder outdoor temperatures would impair high-density lipoprotein (HDL) functionality, was investigated in healthy adults (age: 32.1 ± 9.6 years) living in southeast Michigan. Using mixed models, researchers associated measured lipoproteins, serum cholesterol efflux capacity (CEC) and HDL oxidation markers with ambient and personal-level exposures. For the first time till date, an evidence suggesting a possible impact of ambient PM_2.5 (even at low levels) and outdoor temperatures on serum CEC, a critical anti-atherosclerotic HDL function, was provided via this investigation. Higher ambient PM_2.5 exposures (per 10 μg/m³) were found to be associated with significant increases in the total cholesterol-to-HDL-C ratio as well as reductions in CEC despite low prior 7-day mean outdoor temperature and PM_2.5 levels.