['Air pollution, a risk factor for cardiovascular diseases, can exert its effects through the microcirculation. Retinal blood vessel width is considered a marker for microvascular health and is associated with short-term PM10 exposure. microRNAs are key regulators of complex biological processes in cardiovascular health and disease and miRNA expression can be affected by air pollution exposure. Studies investigating the effect of ambient air pollution exposure on miRNA expression in combination with an assessment of the microvasculature do not exist.', '50 healthy adults (50% women, 23-58 years old) were examined once a month from December 2014 until April 2015 in Flanders (Belgium). Fundus photos and venous blood samples were collected during the study visits. PM10 data were obtained from a nearby monitoring station. Image analysis was used to calculate the width of retinal blood vessels, represented as the Central Retinal Arteriolar/Venular Equivalent (CRAE/CRVE). Total miRNA was isolated from blood and the expression of miR-21, -146a and, -222 were measured using quantitative real-time PCR. Mixed models were used for statistical analysis.', 'Each short-term increase of 10µg/m(3) PM10 during the 24h preceding the study visit was associated with a 0.58µm decrease (95% CI: -1.16, -0.0005; p=0.056) in CRAE, a 0.99µm increase (95% CI: 0.18, 1.80; p=0.021) in CRVE, a 6.6% decrease (95% CI: -11.07, -2.17; p=0.0038) in miR-21 expression and a 6.7% decrease (95% CI: -10.70, -2.75; p=0.0012) in miR-222 expression. Each 10% increase in miR-21 was associated with a 0.14µm increase (95% CI: 0.0060, 0.24; p=0.046) in CRAE whereas a similar increase in miR-222 expression was associated with a 0.28µm decrease (95% CI: -0.50, -0.062; p=0.016) in CRVE. These associations were also found in exposure windows ranging from 2h to 1 week. Finally, we observed that the association between PM10 exposure and CRAE was mediated by miRNA-21 expression.'] Are miRNA expression profiles and retinal blood vessel calibers associated with short-term particulate matter air pollution exposure?