# Inhalation safety threshold lineage (four papers)

## Eleven years, four papers

The scientific case for the 10% inhalation-environment safety threshold and the non-recommendation of high-concentration devices has been built incrementally over **eleven years across four peer-reviewed papers**.

### 1. Kurokawa 2015 — comparison of delivery routes (foundational)

**PMID: [26504515](/en/papers/26504515)** — Kurokawa et al, *Medical Gas Research*.

A foundational paper comparing molecular-hydrogen delivery routes (inhalation, hydrogen-rich water, oral, intravenous). It establishes that **inhalation has the highest systemic uptake efficiency**, and raises the inhalation-specific safety issues (concentration, device design) that subsequent work addresses.

### 2. Kurokawa 2019 — explosion prevention in device design

**PMID: [31552881](/en/papers/31552881)** — Kurokawa et al, *Medical Gas Research*.

A device-design synthesis. It distinguishes the closed-system LFL of 4% from the empirical inhalation-environment ceiling and proposes a safe-operating-concentration framework.

### 3. Ichikawa 2023 — case-driven selection guideline

**PMID: [36204781](/en/papers/36204781)** — Ichikawa et al, *Medical Gas Research*.

Synthesizes the Japanese Consumer Affairs Agency accident-information database into an academic account of the safety problems with high-concentration devices, and derives consumer-facing selection criteria.

### 4. Ichikawa 2026 — in-body explosion mechanism and the low-concentration shift

**PMID: [41489350](/en/papers/41489350)** — Ichikawa et al, *International Journal of Risk & Safety in Medicine*.

Analyzes the physiological mechanism of in-airway ignition and proposes the **shift to low-concentration operation (≤ 10%)** as an academic recommendation. This paper is the primary support for the database's safety guidance.

### Significance of the lineage

These four papers carry weight beyond each individual finding: **for eleven years, an academic line of inquiry has continuously raised the safety concern**. Accident-database accumulation and theoretical refinement have progressed in parallel. The current operational recommendations (10% empirical ceiling, non-recommendation of high-concentration devices) rest on that continuity.

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> **Cite as**: H2 Papers — https://h2-papers.org/en/safety-notes/lineage