Abstract Body (Do not enter title and authors here): Background
Respirator masks are widely used in East Asia for air pollution and, since COVID-19, in public health. However, their physiological impact during exercise remains poorly understood. We evaluates cardiopulmonary responses to exercise under three conditions: no mask, KF80 (Korean Filter; ≥80% filtration of 0.6 μm particles), and KF94 (≥94% filtration of 0.4 μm particles; functionally equivalent to N95).

Methods
Forty healthy adults (mean age 33.3 ± 7.2 years, 50% female) underwent a randomized crossover trial, performing cardiopulmonary exercise testing (CPET) using a modified Bruce protocol under no mask, KF80, and KF94. Each session was separated by ≥48 hours. Gas exchange and physiological variables were measured in real time. Outcomes included peak oxygen uptake (VO₂max), metabolic equivalents (METs), heart rate (HR), tidal volume (VT), minute ventilation (VE), respiratory rate (RR), respiratory exchange ratio (RER), oxygen saturation (SpO₂), and systolic blood pressure (SBP). Repeated-measures ANOVA with Bonferroni correction was applied.

Results
VO₂max and METs both declined with greater filtration: VO₂max was 35.6 ± 8.7 ml/kg/min (No Mask), 31.0 ± 6.5 (KF80), and 29.9 ± 8.0 (KF94; p < 0.001); METs were 7.8 ± 1.3, 6.8 ± 1.2, and 6.2 ± 1.4, respectively (p < 0.001). Maximal HR decreased from 181.9 ± 15.7 bpm (No Mask) to 176.6 ± 15.9 (KF80) and 171.0 ± 15.3 (KF94; p < 0.001). At 3-min post-exercise, HR remained lower under KF94 (104.2 ± 20.8 bpm) than No Mask (112.6 ± 21.5; p = 0.026), suggesting delayed autonomic recovery. SpO₂ at VO₂max declined under KF94 (91.8 ± 4.3%) relative to No Mask (94.1 ± 3.0%) and KF80 (93.0 ± 4.2%; p = 0.012); 9 of 40 subjects experienced SpO₂ < 80%. All ventilatory measures declined with increased mask filtration: VE was 66.4 ± 24.3 (No Mask), 43.0 ± 16.8 (KF80), 35.2 ± 15.6 L/min (KF94; p < 0.001); VT was 1.9 ± 0.6, 1.4 ± 0.6, and 1.2 ± 0.4 L (p < 0.001); and RR was 35.6 ± 8.7, 31.0 ± 6.5, and 29.9 ± 8.0 bpm (p < 0.01). RER remained unchanged across conditions (p = 0.24), suggesting consistent metabolic substrate utilization and comparable anaerobic thresholds.

Conclusion
KF94 respirators reduce aerobic capacity, maximal HR, and ventilatory efficiency in healthy adults without causing hemodynamic instability. Reduced VO₂max, blunted HR response, and episodes of desaturation underscore performance trade-offs. These findings support cautious mask use during exertion within evolving post-pandemic health policy.