Fine particles (PM(2.5)) are implicated as an important risk to cardiovascular health. N95 respirators had been widely used to provide protection by filtering particles. Yet the practical effects of wearing respirators have not been fully understood. This study aimed to evaluate the cardiovascular effects of respirator wearing against PM(2.5) and underpin the understanding of the mechanisms of cardiovascular responses triggered by PM(2.5). We conducted a randomized, double-blind crossover trial among 52 healthy adults in Beijing, China. Participants were exposed to outdoor PM(2.5) for 2 h in alterations wearing true respirators (with membranes) or sham ones (without membranes). We measured ambient PM(2.5) and tested the filtration efficiency of the respirators. We compared the heart rate variability (HRV), blood pressure and arterial stiffness indicators between the true respirator group and the sham respirator group. Concentrations of ambient PM(2.5) during the 2-h exposure ranged from 4.9 to 255.0 mug/m(3). The filtration efficiency of true respirators was 90.1 % and that of sham ones was 18.7 %. Between-group differences varied by pollution levels. On less polluted days (PM(2.5)< 75 mug/m(3)), participants wearing true respirators showed lower levels of HRV and higher levels of heart rate compared with those wearing sham respirators. These between-group differences were inconspicuous on heavily polluted days (PM(2.5)>/= 75 mug/m(3)). We found that a 10 mug/m(3) increase in PM(2.5) was associated with a 2.2 % to 6.4 % decrease in HRV, prominent at 1 h after the start of exposure. N95 respirators have good performance in reducing PM(2.5) exposure. Short-term exposure to PM(2.5) can induce very acute responses in autonomic nervous function. However, the overall effects of wearing respirators might be not always favorable to human health in terms of their inherent adverse effects, which seem dependent on the levels of air pollution. Precise individual protection recommendations warrant to be developed.