## 1 Introduction  
Cardiovascular disease (CVD) is the leading cause of mortality worldwide. Exposure to particulate matter 2.5 (PM2.5), which is an inhalable particle that is 2.5 \(\upmu\)m or even smaller, independently increases CVD incidence and mortality [1, 2, 3]. Additionally, increased PM2.5 concentrations even below the United States (US) standard for PM2.5 increased the risk of CVD [2]. PM2.5 causes systemic inflammation that occurs and transfers from the respiratory system and alters the autonomic nervous system by activating alveoli sensory receptors [4, 5]. Moreover, PM2.5 translocates to the bloodstream due to the size of fine particles, thereby directly damaging the cardiovascular system [4, 5]. Thus, PM2.5-induced pathophysiological changes in respiratory and cardiovascular systems can negatively influence vascular structure and endothelial function.  
The endothelium is an important anatomical structure that maintains vascular tone by releasing various vasoactive substances. Endothelial dysfunction is an early marker of atherosclerosis and a predictor for future CVD events [6, 7, 8]. Brachial artery flow-mediated dilation (FMD) is a clinical and noninvasive measure for assessing vascular endothelial function through endothelium-dependent vasodilation in response to the physiological stimulus. Exercise training is a well-known behavioral intervention to improve FMD, and even acute aerobic exercise may increase FMD [9; 10; 11]. However, the benefits of exercise on vascular endothelial function may differ with PM\({}_{2.5}\) levels. Generally, increased minute ventilation during exercise increases PM\({}_{2.5}\) inhalation compared to resting, which is a non-active condition. A higher frequency of mouth breathing during exercise circumvents the filtration in the nasal cavity [12; 13; 14]. Additionally, an exercise-induced increase in airflow velocity facilitates the movement of inhaled PM\({}_{2.5}\) into the deeper region of the respiratory system [14]. Only a few studies have demonstrated the effect of exercise on endothelial function in high PM\({}_{2.5}\) (HPM\({}_{2.5}\)) concentrations and reported inconsistent findings [15; 16; 17; 18]. Moreover, previous studies were conducted using diesel exhaust in an indoor exposure chamber or an outdoor ambient environment. Understanding the influence of exercise in an indoor space with higher PM\({}_{2.5}\) levels compared with lower PM\({}_{2.5}\) conditions on vascular endothelial function is important because outdoor PM\({}_{2.5}\) rapidly infiltrates into indoor spaces and influences indoor air quality [19]. Therefore, our study controlled the indoor PM\({}_{2.5}\) concentrations with penetrated outdoor ambient PM\({}_{2.5}\) to emphasize the importance of managing PM\({}_{2.5}\) levels in indoor exercise spaces. This study aimed to examine and compare the influence of low and high PM\({}_{2.5}\) levels during acute moderate-intensity aerobic exercise on brachial artery FMD in healthy young males. We hypothesized that the effect of exercise on brachial artery FMD would be limited in HPM\({}_{2.5}\) levels compared to lower PM\({}_{2.5}\) (LPM\({}_{2.5}\)) levels due to its negative influence on the cardiovascular system.