Safety Blind Spots of High-Concentration Hydrogen Inhalation: Major Product Accidents Caused by 'Normalization of Deviance' and the Need for Intrinsically Safe Design

In January 2026, MiZ Co., Ltd. (Kamakura City, Kanagawa Prefecture) and a research group including Keio University reported in the peer-reviewed U.S. international medical journal 'International Journal of Risk and Safety in Medicine' the fact that 'human body hydrogen explosion accidents' involving facial fractures and internal organ damage are occurring one after another behind the performance competition of high-concentration hydrogen inhalers (Ichikawa et al., 2026). This press release examines the possibility that the dangerous phenomenon in safety engineering, 'Normalization of Deviance,' exists in the background of why serious risks continue to be overlooked, and advocates for a shift to low-concentration hydrogen inhalation from the perspective of 'Intrinsically Safe Design' that fundamentally eliminates explosion risks.

Summary of this research
- Multiple serious human body explosion accidents, including complex facial fractures, internal organ tissue rupture, bronchial lacerations, and hearing loss, have been reported in the Consumer Affairs Agency Accident Information Data Bank involving high-concentration hydrogen inhalers with device output concentrations of 67-100% by volume.
- The 'normalization of deviance' (Pinto, 2014), where safety neglect such as 'it was fine before' or 'hydrogen disperses quickly' is accepted as 'normal,' is pointed out as the background to these serious accidents.
- 'Normalization of deviance' is a structure common to major accidents such as the Challenger disaster and nuclear power plant accidents.
- According to 'Heinrich's Law,' the reported major accidents are at a stage that foreshadows the next fatal accident.
- A shift to low-concentration hydrogen inhalation, which keeps the device output concentration at or below the empirically verified value of 10% by volume for the inhalation environment, is a fundamental solution from the perspective of 'Intrinsically Safe Design.'

Background: The escalating 'high-concentration competition' and worsening human body explosion accidents
While hydrogen inhalation is rapidly becoming popular, a performance-oriented competition to achieve 'higher concentrations' and 'higher generation volumes' is intensifying in some parts of the market without sufficient safety verification. Hydrogen is a flammable gas that can explode instantly even with a tiny static spark when its concentration in the air exceeds 10% by volume. In a January 2026 academic paper, MiZ Co., Ltd. and the research group including Keio University revealed the fact that multiple life-threatening serious human body hydrogen explosion accidents, including complex facial fractures, internal organ tissue rupture (requiring ICU admission), massive bleeding from bronchial lacerations, and hearing loss, have been reported in the Consumer Affairs Agency Accident Information Data Bank (Ichikawa et al., 2026).

In 2015, MiZ Co., Ltd. announced that there is a risk of explosion when the hydrogen concentration exceeds 10% by volume in everyday environments, based on a review of existing literature and empirical studies simulating inhalation environments. The value of 10% by volume is an empirically verified value for the inhalation environment, distinct from the lower flammability limit of hydrogen defined under ideal conditions (Ichikawa et al., 2026).

Definition of Terms
Normalization of Deviance: A phenomenon where dangerous abnormal conditions that should not be tolerated are gradually treated as 'normal (no problem)' because they have not led to accidents in the past (Pinto, 2014). This concept became widely known through the analysis of the Challenger disaster (1986).

Intrinsically Safe Design: A design philosophy that ensures safety by eliminating hazardous factors themselves at the design stage, without relying on operational, maintenance, or human countermeasures. For hydrogen inhalation devices, this corresponds to a design that suppresses the device output concentration to 10% by volume or less, the empirically verified value for the inhalation environment, thereby eliminating the 'combustible material' element.

Heinrich's Law: An empirical rule from industrial accident statistics (Heinrich, 1931) stating that behind one major accident, there are 29 minor injury accidents and 300 near-miss incidents.

Hydrogen Inhaler: A device that uses water electrolysis to generate hydrogen gas (H₂) for intake into the body via the respiratory system. The choice of device output concentration is a design variable that determines safety. MiZ Co., Ltd. advocates for a design that keeps the device output concentration at or below the empirically verified value of 10% by volume for the inhalation environment (Ichikawa et al., 2026).

Empirically Verified Value for Inhalation Environment (10% by volume): The empirically verified threshold for explosion risk in a hydrogen inhalation environment (above 10% by volume). This value was announced by MiZ Co., Ltd. in 2015 based on a review of existing literature and empirical studies simulating inhalation environments, taking into account inhalation-specific conditions such as the device outlet, exhalation pathway, human body, and device design (Ichikawa et al., 2026).

Classical Lower Flammability Limit (LFL) 4% by volume: The value reported by Coward & Jones (1952) in U.S. Bureau of Mines Bulletin 503. It is the theoretical minimum concentration measured as the lowest concentration at which an upward flame propagation can be sustained when hydrogen and air are premixed in a closed vertical tube at 1 atm and room temperature, ignited in a static state. This primarily targets closed system scenarios such as containers, piping, and mines.

Relationship between LFL 4% and the Empirically Verified Value 10%: The hydrogen inhalation environment is an open system where hydrogen gas generated by water electrolysis at atmospheric pressure is continuously released into the atmosphere, continuously diffuses and dilutes with room air, and is supplied as a flowing gas to the inhalation pathway. This is fundamentally different from the measurement conditions of the classical LFL, which assumes a premixed static gas in a container or pipe, in three aspects: spatial conditions, mixing state, and flow state. The two are different indicators targeting different physical conditions, and it is appropriate to use the empirically verified value of 10% by volume as the standard for safety evaluation of hydrogen inhalation devices.

The Lesson of the Space Shuttle Accidents: What is 'Normalization of Deviance'?
'Normalization of deviance' became widely known from the analysis of the 1986 Challenger explosion. Signs of gas leakage from the O-rings had been repeatedly observed in previous launches, but the successful experience of 'it was fine before' was prioritized, leading to the loss of all seven crew members in the mid-air disintegration (Figure 1). In the 2003 Columbia mid-air disintegration, the peeling of external insulation foam was considered a minor problem, resulting in a fatal accident. The fact that an accident has not surfaced or appears minor does not mean it is inherently safe. The accumulation of the mistaken belief that 'it was fine before' can suddenly surface as an irreversible major accident.

Figure 1: 'Normalization of deviance' is a phenomenon where, even if there are dangers or abnormalities that should not be tolerated, if they do not immediately lead to an accident, they gradually become established within the organization as 'something not a problem.' 'Normalization of deviance' has been confirmed not only in the Challenger accident but also in the Columbia accident, the Chernobyl nuclear power plant accident, the Fukushima Daiichi nuclear power plant accident, chemical plant accidents, and many other major accidents.

Cases of 'Normalization of Deviance' in the Proliferation of High-Concentration Hydrogen Inhalers and the Shift to Safe Hydrogen Inhalation
Six Cases of 'Normalization of Deviance' in the Proliferation of High-Concentration Hydrogen Inhalers
During the proliferation process of high-concentration hydrogen inhalers, perceptions that downplay the risk of explosion have been spread. Representative cases of 'normalization of deviance' are organized below. ① 'Higher hydrogen concentration and generation volume are better' (directly increases the scale of an explosion). ② 'Hydrogen is safe because it disperses quickly' (localized high-concentration areas form inside the cannula and in the breathing zone before dispersion). ③ 'It is hard to ignite unless in a special environment' (the minimum ignition energy is low, and it can be ignited by everyday static electricity). ④ '