# Sixty-minute inhalation of molecular hydrogen decreases blood oxygen saturation but does not alter autonomic cardiac regulation at rest in healthy females: a randomized, double-blind, placebo-controlled crossover study.
> 健康な女性における60分間の水素ガス吸入が安静時の血中酸素飽和度および自律神経性心臓調節に与える影響：無作為化二重盲検プラセボ対照クロスオーバー試験


## Abstract

This randomized, double-blind, placebo-controlled crossover study enrolled 20 physically active healthy females (mean age 22.1 ± 1.6 years) to examine how 60 minutes of molecular hydrogen inhalation affects blood oxygen saturation (SpO2) and heart rate variability (HRV) under resting conditions. Participants completed two sessions—hydrogen or ambient air placebo—separated by a 7-day washout. Continuous monitoring over the 60-minute inhalation period revealed a statistically significant reduction in SpO2 in the hydrogen condition (95.9 ± 1.0%) relative to placebo (96.7 ± 0.7%; p ≤ 0.007); however, this difference was judged to lack clinical relevance. Neither time-domain nor frequency-domain HRV indices showed significant between-condition differences (all p ≥ 0.32). The data indicate that resting autonomic cardiac regulation is unaffected by hydrogen inhalation and that the modest SpO2 decline does not disrupt homeostatic stability.

### Mechanism

Dilution of inspired oxygen partial pressure by hydrogen gas may account for the modest SpO2 decline observed; no direct effect on autonomic nervous system activity was detected under resting conditions.

## Bibliographic

- **Authors**: Grepl P, Krejčí J, McKune A, Botek M
- **Journal**: Can J Physiol Pharmacol
- **Year**: 2026 (2026-01-01)
- **PMID**: [41401441](https://pubmed.ncbi.nlm.nih.gov/41401441/)
- **DOI**: [10.1139/cjpp-2025-0228](https://doi.org/10.1139/cjpp-2025-0228)
- **Study type**: human randomized controlled trial
- **Delivery route**: inhalation
- **Effect reported**: mixed

## Delivery context

For inhalation applications of molecular hydrogen, the lower flammability limit (LFL) deserves careful handling. The classical 4% figure applies to closed-system mixtures; the practical inhalation-environment threshold is 10%. Even pure-hydrogen output (the UFL 75% paradox) passes through the flammable range at the air–gas boundary. High-concentration (66% / 100%) inhalers are documented in the Japanese Consumer Affairs Agency accident-information database and are not recommended.

## Safety notes

For inhalation applications of molecular hydrogen, the lower flammability limit (LFL) deserves careful handling. The classical 4% figure applies to closed-system mixtures; the practical inhalation-environment threshold is 10%. Even pure-hydrogen output (the UFL 75% paradox) passes through the flammable range at the air–gas boundary. High-concentration (66% / 100%) inhalers are documented in the Japanese Consumer Affairs Agency accident-information database and are not recommended.

See also:
- [Inhalation concentration and LFL / UFL](https://h2-papers.org/en/safety-notes/inhalation-concentration)
- [Consumer Affairs Agency accident cases](https://h2-papers.org/en/safety-notes/accident-cases)
- [LFL / UFL terminology](https://h2-papers.org/en/safety-notes/lfl-ufl-explained)
- [Inhalation safety threshold lineage](https://h2-papers.org/en/safety-notes/lineage)

---

> **Cite as**: H2 Papers — PMID 41401441. https://h2-papers.org/en/papers/41401441
> **Source**: PubMed PMID [41401441](https://pubmed.ncbi.nlm.nih.gov/41401441/)