Risk Verification of High-Concentration Hydrogen Inhalation from a Safety Engineering Perspective: The Worst-Case Scenario of Human Body Hydrogen Explosions Based on "Heinrich's Law"

MiZ Co., Ltd. (Headquarters: Kamakura City, Kanagawa Prefecture) has analyzed the human body hydrogen explosion accidents reported in connection with high-concentration hydrogen inhalers from the perspective of safety engineering. In this analysis, the structural occurrence of accidents was systematically verified using "Heinrich's Law" as a framework.

As a result of the verification, it became clear that explosion accidents caused by high-concentration hydrogen inhalers have already reached the "major accident" stage under "Heinrich's Law," and that multiple major accidents have been confirmed. From a safety engineering standpoint, unless the identical risk factors are removed, the possibility of even more serious accidents occurring can be foreseen.

## Background: Academic Papers on the Explosion Risk of High-Concentration Hydrogen Inhalers
MiZ Co., Ltd. and a research group involving Keio University and others have published multiple peer-reviewed academic papers regarding the danger of hydrogen explosion accidents caused by high-concentration hydrogen inhalers and the development of low-concentration hydrogen generation technology to prevent them. Through these papers, the research group has repeatedly sounded the alarm about the explosion danger of high-concentration hydrogen inhalers (Figure 1).

## (Paper 1) 2015: Paper on Technology to Dilute Hydrogen to a Safe Concentration Where Explosions Do Not Occur
Title: "Convenient methods for ingestion of molecular hydrogen: drinking, injection, and inhalation" Med Gas Res. 2015 Oct 26;5:13

## (Paper 2) 2019: Paper on Safe Hydrogen Inhalation Technology
Title: "Explosion prevention of hydrogen gas inhalers" Med Gas Res. 2019 Jul-Sep;9(3):160-162.

## (Paper 3) 2023: Paper on Explosion Accidents of High-Concentration Hydrogen and Guidelines for Selecting Inhalers
Title: "Guidelines for selecting hydrogen gas inhalers based on hydrogen explosion accidents" Med Gas Res. 2023 Apr-Jun;13(2):43-48

## (Paper 4) 2026: Paper on Human Body Hydrogen Explosions and Low-Concentration Hydrogen Therapy
Title: "Preventable human body hydrogen explosions caused by high-concentration hydrogen inhalers occurring in Japan - Proposing a shift to safe low-concentration hydrogen therapy" Int J Risk Saf Med. 2026 Jan 5:9246479251414573.

## The Worsening Explosion Accidents of High-Concentration Hydrogen Inhalation
In 2015, based on a close examination of existing literature and empirical studies assuming an inhalation environment, MiZ Co., Ltd. announced that there is a risk of explosion if the hydrogen concentration exceeds 10% under daily environments. This is an empirical value assuming an inhalation environment, distinct from the lower explosive limit of hydrogen defined under ideal conditions. As will be described later, explosion accidents involving high-concentration hydrogen inhalers had already been reported to the Consumer Affairs Agency around this time.

As of 2019, the penetration rate of hydrogen gas inhalers was still at a low level. However, accidents ranging from minor to severe, such as "hearing loss" and "tinnitus" caused by the explosion of high-concentration hydrogen inhalers, were on an upward trend, and MiZ Co., Ltd. foresaw that explosion accidents caused by high-concentration hydrogen inhalers would worsen.

In 2022, with the spread of high-concentration hydrogen inhalers, they warned once again that in addition to fires and damage to household goods caused by the explosion of the inhaler units themselves, multiple personal injury accidents were actually occurring.

Subsequently, up to the year 2026, the high-concentration hydrogen gas inhalation...