Reduction in Fractional Exhaled Nitric Oxide after Moderate Exercise Among Adult Open Water Swimmers

Background: We recently showed that elevated Fraction Exhaled Nitric Oxide (FeNO) is associated with an increased risk of developing pulmonary edema or barotrauma of descent during freediving. It was also noted that FeNO decreased slightly (within 60min) after freediving among symptomatic and asymptomatic divers. Aerobic exercise in sedentary asthmatics has also been shown to reduce FeNO, while resistance exercise in healthy adults produced no change. Based on these findings, we hypothesized that reduction in FeNO is a physiological change related to both immersion and exertional respiratory processes. Here, we test this hypothesis among open water swimmers participating in submersed aerobic exercise. Methods: We measured FeNO, (NIOX VERO™), heart rate (HR), and peripheral oxygen saturation (SpO 2 , MightySAT™, Masimo), in open-water swimmers before and after an open water swim. Water temperature, swim distance, and duration were recorded, and demographic data (age, sex) were collected via questionnaire. Statistical analysis included paired T-tests and linear regression. Results: 46 participants (mean ± SD, age 49.1 ± 14.1 years, 63% female) swam an average of 1981 ± 895 m in open ocean ranging from 17.8–21.6°C. FeNO decreased significantly from 23.0 ± 13.8 to 19.4 ± 12.1 ppb (p < 0.001). HR increased from 69.9 ± 15.1 to 86.4 ± 20.1 bpm (p < 0.001), SpO 2 did not change significantly (98.1 ± 1.6% to 97.6 ± 2.7%, p = 0.2). Delta FeNO did not correlate to Delta HR (R 2 = 0.002) or baseline HR (R 2 = 0.031) Discussion Open water ocean swimming was associated with a significant decrease in FeNO when measured within 30 minutes. Change in FeNO was not correlated with change in HR. This suggests that immersion and/or exercise caused an immediate effect on FeNO. Whether it depends on turnover, diffusion capacity or blood volume changes remains to be studied. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.