## 4 Discussion  
The current study investigated the difference in the response of vascular endothelial function to aerobic treadmill exercise between HPM\({}_{2.5}\) and LPM\({}_{2.5}\) levels. Consistent with our study hypothesis, the acute improvement of vascular endothelial function by aerobic exercise was demonstrated to be nullified in HPM\({}_{2.5}\) conditions. Additionally, the baseline diameter of the brachial artery was increased only after exercising in HPM\({}_{2.5}\) conditions. Baseline and peak diameters were smaller, and TTP was delayed in HPM\({}_{2.5}\) conditions compared to LPM\({}_{2.5}\) conditions before exercise. To our knowledge, this is the first study that examined the influence of inhaled fine particulate matter on vascular endothelial function while exercising under indoor HPM\({}_{2.5}\) and LPM\({}_{2.5}\) conditions, naturally created by outdoor ambient PM\({}_{2.5}\). Previous studies explored the effect of aerobic exercise in HPM\({}_{2.5}\) concentrations on vascular function, but findings were inconsistent, and exercise interventions were conducted with different PM\({}_{2.5}\) sources [15; 16; 17; 18; 23; 24]. Studies involving outdoor cycling in a high-traffic route did not reveal any relationship between vascular function and PM\({}_{2.5}\) concentrations [15; 17]. The high-traffic route PM\({}_{2.5}\) concentrations were lower than the US EPA 24 h PM\({}_{2.5}\) standard, which might not negatively affect the cardiovascular system in healthy adults, although these studies were conducted with similar PM\({}_{2.5}\) sources to our study using ambient PM\({}_{2.5}\). Additionally, stationary cycle exercise in an exposure chamber with 300 ug/m\({}^{3}\) of PM\({}_{2.5}\) created by diesel exhaust did not influence brachial artery FMD in healthy young males [16]. However, our study used ambient PM\({}_{2.5}\) to form the HPM\({}_{2.5}\) condition, which can explain the discrepancy of findings with studies that only used diesel exhaust. Previous studies reported FMD improvement in response to exercise [25; 26]. These findings support our result in LPM\({}_{2.5}\) conditions and suggest exercise as beneficial to enhance vascular endothelial function. However, high ambient PM environment exposure can negate the positive effect of exercise on FMD. We found that the increased scaled FMD in LPM\({}_{2.5}\) conditions disappeared when the same exercise was performed in HPM\({}_{2.5}\) conditions. Similarly, FMD was significantly decreased after 30 min of running in a high ambient PM\({}_{1}\) condition in young adults [27]. PM exposure generally increases reactive oxygen species (ROS) production [28; 29]. Oxidative stress induces the reduction in nitric oxide (NO) production and availability, which are key factors for vasodilation in response to reactive hyperemia [30]. Additionally, PM inhalation may facilitate leukotriene synthesis by ROS, which engenders endothelial dysfunction [31]. Furthermore, sympathetic nervous system activation by PM exposure might be associated with decreased FMD [32; 33]. Collectively, increased PM inhalation during exercise may cause vascular endothelial dysfunction by oxidative stress, inflammation, and sympathetic nervous system activation. The present study revealed lower pre-exercise baseline and peak diameters of the brachial artery in HPM\({}_{2.5}\) conditions than in LPM\({}_{2.5}\) conditions. We assume that vasoconstriction occurred due to PM\({}_{2.5}\) inhalation on the way from the participants' residence to the laboratory. Unfortunately, we were unable to accurately track the PM\({}_{2.5}\) exposure of study participants before starting pre-exercise measures. Participants spent 30 min to 2 h on the road or on public transportation to visit our laboratory. The brachial artery could have been constricted in response to the increased PM\({}_{2.5}\) inhalation before the pre-exercise procedures because participants were likely to be exposed to a high outdoor PM\({}_{2.5}\) environment in the HPM\({}_{2.5}\) visit.