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REVIEW

Guidelines for the selection of hydrogen gas inhalers
based on hydrogen explosion accidents
Yusuke Ichikawa1, 2, *, Shin-ichi Hirano1, Bunpei Sato1, 2, Haru Yamamoto3, Yoshiyasu Takefuji4, 5, Fumitake Satoh1
1 MiZ Company Limited, Kamakura, Kanagawa, Japan
2 MiZ Inc., Newark, CA, USA
3 Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA, USA
4 Keio University, Tokyo, Japan
5 Musashino University, Faculty of Data Science, Tokyo, Japan*Correspondence to: Yusuke Ichikawa, PhD, y_ichikawa@e-miz.co.jp.
orcid: 0000-0002-2526-4681 (Yusuke Ichikawa); 0000-0002-8610-8922 (Shin-ichi Hirano); 0000-0002-1826-742X (Yoshiyasu Takefuji)

Abstract
Despite the fact that we have reported on the dangers of the explosion of hydrogen gas inhalers, hydrogen gas inhalers with explosive hazards are, as a matter of fact, still being sold today. In this study, we investigated past reports of hydrogen gas inhaler explosion accidents toclarify the causes of these explosion incidents. As a result of this investigation, we found that the central cause was the leakage of hydrogengas inside the hydrogen gas inhaler. Although it is said that the explosive concentration of hydrogen is between 10% and 75%, and that thegas does not explode above 75% due to the lack of oxygen, we confirmed through a series of ignition experiments that explosions can occur
even in hydrogen gas inhalers that produce 100% hydrogen gas. Some manufacturers of such highly concentrated hydrogen gas inhalers
claim that the high concentration and purity of hydrogen is safe and that there is no risk of explosion. We believe that manufacturing or
selling such products that pose a risk of explosion or detonation is a violation of social justice. This paper presents ideas for selecting safehydrogen gas inhalers based on a survey of past accident cases.
Key words: detonation; explosion accident; explosive concentration range of hydrogen; flame arresters; hydrogen concentration; hydrogen
inhaler; hydrogen; safety
doi: 10.4103/2045-9912.344972
How to cite this article: Ichikawa Y, Hirano S, Sato B, Yamamoto H, Takefuji Y, Satoh F. Guidelines for the selection of hydrogen gas inhalers
based on hydrogen explosion accidents. Med Gas Res. 2023;13(2):43-48.

INTRODUCTION

Hydrogen (H 2) is a colorless, odorless gas at ambient
temperature and pressure.1,2 H2 causes a violent explosion
when it reacts with oxygen (O2) to form water (H2O).3
We conducted a joint research study with Keio University
and reported that hydrogen gas explodes at concentrations
above 10%.4 Especially in the concentration range of 18.3%
to 58.9%, hydrogen may cause a detonation, which is anextremely dangerous explosion accompanied by shockwaves
at a supersonic speed of Mach 5 (1700 m/s) or higher, and asubsequent flame propagation called a deflagration, which maylead to serious accidents.2 In particular, a mixed gas consistingof a 2 to 1 ratio of hydrogen and oxygen is known as an
oxyhydrogen detonating gas, which produces a loud explosive
sound (detonating sound) and a powerful explosion. The
detonating sound that accompanies hydrogen gas explosions
may destroy human auditory neurons, causing incurable lossof hearing. When it comes to the use of hydrogen as an energy
source, research on the safety of hydrogen and hydrogen
explosions has been conducted to prevent such explosions
from occurring in hydrogen facilities.5,6 On the other hand,
many commercial home-use hydrogen gas inhalers give little
consideration to safety with respect to such explosions. While
there are ongoing plans to use hydrogen in the medical field,7
explosion incidents involving hydrogen gas inhalers, whichitself poses the risk of explosion, may hinder the developmentof hydrogen medicine.

When using an explosive gas, it is absolutely necessary
to ensure safety. Hydrogen explosions sometimes cause
tremendous damage, as in the 1937 explosion of the
Hindenburg, Poland, in which an airship used hydrogen forbuoyancy in the 1983 hydrogen explosion in an industrial
park in Stockholm, Sweden, and in hydrogen explosion at the
Fukushima nuclear power plant in 2001.8-10 Some unscrupulous
manufacturers sell hydrogen gas inhalers that generate
an explosive concentration of hydrogen while presenting
favorable hydrogen gas concentrations that indicate a low riskof explosion so as to lull purchasers into a false sense of safety,
and some doctors and medical institutions are affiliated withthese manufacturers. Hydrogen gas inhalers are mainly usedin homes or hospitals.11-16 Therefore, any risk of explosion ordetonation may lead to damages to household effects, loss ofhearing by the production of a detonating sound, fires, or insome cases, a serious accident that threatens human life.5,6,17
Development of hydrogen gas inhalers that ensure safety
requires that the developers have a strong sense of ethics andhigh level of thought towards technology.
In this study, we investigated past reports of hydrogen gas
inhaler explosion accidents and discussed the causes of these
explosions. In addition, an ignition experiment was conductedat hydrogen gas concentrations of 75% and higher, which waspreviously reported to be non-explosive, and the explosive
potential of commercially available hydrogen gas inhalers
was discussed.4-6,8-10© 2023 Medical Gas Research | Published by Wolters Kluwer - Medknow

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CASES OF HYDROGEN EXPLOSION ACCIDENTS
INVOLVING HYDROGEN GENERATORSThe following introduces cases of incidents involving hydrogen explosions. Accident cases 1–3 were reported by theConsumer Affairs Agency of Japan.18
Accident case 1: Accident involving a hydrogen-oxygen gas
inhaler
An accident report can be found in which upon lifting ahydrogen-oxygen gas inhaler, the lid flew off and an explosive
sound was produced that caused ringing in the user’s ears
(February 19, 2016).
A hydrogen-oxygen gas inhaler allows the user to inhalehydrogen and oxygen at the same time. It can be presumed
that inhalers of this kind generate an explosive concentrationof hydrogen (66%) and oxygen (34%) by electrolysis of water.
Accident case 2: Explosion accident involving a home-usehydrogen generator
Although this case did not involve a hydrogen gas inhaler,
there was an accident report in which hydrogen leakage occurred in a home-use hydrogen bubble generator. The leakedhydrogen accumulated in the body of the unit before causing
an explosion, which damaged parts of the house (the bathroomdoor) and injured the resident (report dated October 25, 2017).
Hydrogen gas inhalers of the midway dilution type, which
dilutes hydrogen gas in a location away from the source ofgeneration instead of diluting it to below the lower explosive
limit immediately at the source of generation, may allow ahigh concentration of hydrogen to leak due to deterioration
caused by aging or due to the tube and other components
falling out of place, which may in turn lead to an explosion
caused by the action of electrical energy from the electricalsystem inside the inhaler.
Accident case 3: Explosion accident involving a hydrogen server
There is an accident report dated November 11, 2015, in whicha hydrogen explosion caused a bottle to fly out of a server thatgenerates hydrogen and damaged the ceiling.
This accident was also caused by the leakage of a high concentration of hydrogen inside the product. The same productcaused another accident in which the hydrogen-generating
server burst and the broken pieces scattered and lacerated achild (report dated November 22, 2015 by the same agency).

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ate hydrogen, ignition experiments were conducted on different
concentrations of hydrogen trapped inside plastic bags and oncommercially available hydrogen gas inhalers that generate
different concentrations of hydrogen. In these experiments,
the explosive limits of hydrogen gas were verified (ignition
experiment 1), and the question of whether hydrogen gas inhalers cause hydrogen gas explosions was tested by directly
igniting the inhalers themselves.4
Ignition experiment 1
Hydrogen gas at concentrations of 4%, 10%, 15%, 20%, and100% was trapped inside plastic bags and ignited.
A hydrogen cylinder (100%) from Taiyo Nippon Sanso was
diluted to prepare these different concentrations of hydrogen,and the hydrogen concentrations were measured with a hydrogen concentration meter (XP-3140) from New Cosmos
Electric Co., Ltd. (Osaka, Japan). The experiment results areshown in Table 1.
Table 1: Ignition experiment results of different
concentrations of hydrogen
Hydrogenconcentration (%) Explosion
4
10
15
20

No explosion
No explosion
Explosion
Violent explosion

100

Explosion

Explosive soundNo explosive soundNo explosive soundExplosive sound produced
Ferocious explosive sound(detonation) produced
Explosive sound produced

Ignition experiment 2: Inhaler with a hydrogen concentrationof 66% (electrolysis type)
When a hydrogen gas inhaler (electrolysis type with a hydrogen concentration of 66%) from Company J was ignited,the hydrogen detonated with a ferocious explosive sound,and the inhaler’s lid and wiring parts were scattered by the
explosion’s impact (Figure 1). This hydrogen gas inhaler
with a hydrogen concentration of 66% allows users to inhale
a mixed gas of hydrogen (66%) and oxygen (34%) generated
by the electrolysis of water. A mixed gas consisting of a 2 to
1 ratio of hydrogen and oxygen is known as an oxyhydrogendetonating gas, and it causes an explosion accompanied by anespecially ferocious explosive sound.

Accident case 4: Explosion accident involving an electronic
water heater
In the epidemiological database of accidents made public by
the U.S. Consumer Product Safety Commission (https://www.
cpsc.gov/Data), there is an incident in which an electronic
water heater that generates hydrogen ignited when a flame wasbrought close to its faucet. Though this particular case did notlead to a serious accident, hydrogen leaking inside equipmentthat generates hydrogen may lead to a serious accident.

IGNITION EXPERIMENTS ON HYDROGEN AND HYDROGENGAS INHALERS

To investigate the causes of accidents in equipment that gener44

Figure 1: Ignition experiment result of hydrogen concentration of 66%,
electrolysis type.
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Ignition experiment 3: Inhaler with a hydrogen concentrationof 100% (electrolysis type)
When a hydrogen gas inhaler (electrolysis type with a hydrogen concentration of 100%) from Company I was ignited
immediately after it was turned on, transparent smoke was
emitted continuously from the hydrogen gas discharge port.
When the same hydrogen gas inhaler was ignited several
minutes after it was turned on, it exploded with a ferocious explosive sound and the container burst (Figure 2).
This experiment revealed that hydrogen gas inhalers with ahydrogen concentration of 100%, which is higher than theupper explosive limit (75%), also pose a risk of explosion.

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the lower explosive limit. There are two methods for dilutinghydrogen: the first is direct dilution, in which hydrogen isdiluted to concentrations below the lower explosive limit at
the source of generation or immediately after generation, andthe second is midway dilution, in which generated hydrogenis diluted prior to emission (Figure 5). Hydrogen gas inhalersof the direct dilution type do not allow an explosive concentration of hydrogen to accumulate or flow inside them, andthus ensure safety. On the other hand, hydrogen gas inhalersof the midway dilution type allow the generated hydrogen to
be maintained at an explosive concentration from the sourceof generation until the point where it is mixed with a dilutiongas, so they continue to pose a risk of explosion. Hydrogengas inhalers of the midway dilution type have the risk of interior hydrogen leaks due to deterioration caused by aging or
the hydrogen-sending tube falling out of place. In fact, majorhydrogen explosion accidents have been caused by hydrogenleakage inside inhalers.2,3

Figure 2: Ignition experiment result of hydrogen concentration of 100%,
electrolysis type.

Ignition experiment 4: Inhaler with a hydrogen concentrationof 100% (hydrogen-generating-agent type)
When a hydrogen gas inhaler (hydrogen-generating-agent
type with a hydrogen concentration of 100%) from Company
P was ignited, it exploded with a ferocious sound, and the top
lid burst and scattered (Figure 3). As in ignition experiment3, it was found that hydrogen gas inhalers of the hydrogengenerating-agent type with a hydrogen concentration of 100%,
which is higher than the upper explosive limit (75%), also posea risk of explosion.

Figure 4: Most commercially available hydrogen gas inhalers pose risks of
explosion, detonation, and deflagration.

Figure 5: Direct dilution type inhaler (safe) and Midway dilution type inhaler(dangerous).

Figure 3: Ignition experiment result of hydrogen concentration of 100%,hydrogen-generating-agent type.

SAFE HYDROGEN GAS INHALERS AND DANGEROUS
HYDROGEN GAS INHALERS

Most commercially available hydrogen gas inhalers emit hydrogen that is generated by the electrolysis of water directly
through the inhalation port, which poses a risk of explosion(Figure 4).
Generated hydrogen should be diluted with air or anotherdilution gas in order to ensure that the concentration is below
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FLAME ARRESTERS ARE INSUFFICIENT TO PREVENTEXPLOSIONS

Some hydrogen gas inhalers are equipped with a flame arrester
to prevent flame backflow in the piping used to carry hydrogengas.19,20 However, even if flames can be extinguished by theplacement of a flame arrester, this does not prevent the detonating sound that accompanies hydrogen explosions. Flamearresters are not intended to prevent hydrogen explosions whenhydrogen between the source of hydrogen generation and theflame arrester leaks inside the inhaler, and there is every reason to believe that installing a flame arrester is insufficient toprevent explosions nor to ensure explosion safety (Figure 6).
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SUMMARY

Figure 6: Flame arrester valve type inhaler (dangerous).

FIVE POINTS TO CHECK WHEN SELECTING HYDROGENGAS INHALERS

Hydrogen has the properties described above. Thus, whenpurchasing a hydrogen gas inhaler, it is important to select a
product that gives due consideration to safety. The requirements that hydrogen gas inhalers should meet are listed below(Figure 7).

LIST OF COMMERCIALLY AVAILABLE HYDROGEN GASINHALERS THAT GENERATE A HIGH CONCENTRATION OF
HYDROGEN

Table 2 lists the hydrogen gas inhalers on the market that
produce high concentrations of hydrogen.

It has been reported that the explosive concentration range ofhydrogen is from 10% to 75%.21,22 Considering this range, it
is often assumed that hydrogen gas with a concentration of
100% does not explode. However, in our ignition experiments,hydrogen gas inhalers that generate hydrogen with a concentration of 100% either exploded or caused a deflagration (refer
to ignition experiments 1 and 4).
The results of these ignition experiments show that hydrogenwith a concentration of 100% may be diluted by the surrounding air to reach a concentration within the explosive range of
10% to 75%. This hydrogen, diluted to an explosive concentration, can then explode in the presence of static electricity
or another ignition source. Consequently, even hydrogen gas
inhalers that generate hydrogen with a concentration of 100%can still pose a risk of explosion (Figure 8).
Some commercially available hydrogen gas inhalers generate hydrogen with a concentration in the range of 66% to 67%.
This is an oxyhydrogen detonating gas, which is a mixed gas
consisting of hydrogen and oxygen at a ratio of 2 to 1 generated by the electrolysis of water, and it may explode with anextremely ferocious detonating sound. This detonating soundhas the risk of damaging the auditory nerve (Figure 8).
The explosive concentrations of flammable gases such as
methane and propane are 5% to 15% and 2% to 10%, respectively, which are much narrower than that of hydrogen (10%Figure 7: Five points to checkwhen selecting safe hydrogen gas
inhalers.

Figure 8: Hydrogen concentrations
and the explosive properties ofhydrogen.

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Table 2: List of commercially available hydrogen gas inhalers that generate a high concentration of hydrogen
Product nameSupplier

H2 gas concentration (%)

MethodH2JI1
Cielserein
Mintech MT-A100
H2MULTI POD
Pure+Cube
LOURDESHYDROFIX
Hydrogen Inhaler
Suiso Choju CH-100
PHG-150TA
Suiso Care
La Briller Luxe
La Briller Elan
SS-100HydroRich
Hydrogen Generator
WL-959Doctors Man Co., Ltd. (Kanagawa, Japan)
Inter Christine K.K. (Kanagawa, Japan)
Mintech K.K. (Tokyo, Japan)
I•TEC INTERNATIONAL (Tokyo, Japan)
Dr's Choice K.K. (Tokyo, Japan)
H2FACTORY Inc. (Aichi, Japan)
Kenko Shien Center Co., Ltd. (Aichi, Japan)
Eco Higashinippon Co., Ltd. (Fukushima, Japan)
Eco Higashinippon Co., Ltd. (Fukushima, Japan)
Kenko Co., Ltd. (Gifu, Japan)
ISMZ Co., Ltd. (Osaka, Japan)
ISMZ Co., Ltd. (Osaka, Japan)
SUISO JAPAN (Aichi, Japan)
Pal Corporation (Kanagawa, Japan)
Kanon Co., Ltd. (Osaka, Japan)
Guangzhou Wei Lai Health Management Technology Co., Ltd.
(Guangzhou, China)
Shanghai Asclepius Meditec Co., Ltd. (Shanghai, China)
Shanghai Asclepius Meditec Co., Ltd. (Shanghai, China)
Guangzhou Zhongquing Energy Technology Co., Ltd.
(Guangzhou, China)
Aqua Bank Co. Ltd. (Osaka, Japan)
Asklepios Medical Co., Ltd. (Tokyo, Japan)
Helix Japan Co., Ltd. (Tokyo, Japan)
HPJ K.K. (Hokkaido, Japan)
Three-E SCIENCE Co., Ltd. (Kyoto, Japan)
Brand: select air HOME (Japan)
WCJ Co., Ltd. (Osaka, Japan)
Bio Logic Health Inc. (Tokyo, Japan)

99.99999.99599.99599.999
99.99
99.5
99
99
99
99
99
99
99
99
99
99

Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Chemical reaction
Chemical reaction
Electrolysis

66
66
66

Electrolysis
Electrolysis
Electrolysis

66
66
66
66
64
66
3 (midway dilution)
3 (midway dilution)

Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis
Electrolysis

AMS-H-03
BYT-JP-H03
QM-1500
KENCOSBYT-JP series
Hycellvator
HydroPower
Beautyfly
Select air HOME
Lita Air
H2 Aroma Generator

to 75%). In addition, the minimum ignition energy required forhydrogen is much lower than that of methane and propane.23-26
The broader explosive concentration and lower ignition energy
suggest that hydrogen gas poses a much greater risk of explosion than these other flammable gases.
When choosing a hydrogen inhaler, it is important to searchfor one with minimal risk of explosion. Given that mostcommercially available hydrogen inhalers still pose a risk of
explosion, the following summary provides guidelines thatminimize this risk of explosion. Consulting such guidelines
will maximize the safety of the user upon selection of a hydrogen gas inhaler.

build upon the work non-commercially, as long as appropriate credit
is given and the new creations are licensed under the identical terms.

References
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Acknowledgements
We are grateful to Ms. Yoko Satoh, Dr. Guannan Shi, Mr. Susumu
Mine and Mr. Yoshihiro Mitekura (MiZ Company Limited) for their
excellent advice on writing this manuscript.
Author contributions
YI and FS designed and wrote the manuscript; SH, BS, HY and YT
supported this study by giving advice and revised the manuscript. All
authors read and approved the final manuscript.
Conflicts of interest
YI, SH, BS and FS are employees of MiZ Inc.
Open access statement
This is an open access journal, and articles are distributed under
the terms of the Creative Commons AttributionNonCommercialShareAlike 4.0 License, which allows others to remix, tweak, andMedical Gas Research ¦ June ¦ Volume 13 ¦ Issue 2

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Date of submission: February 8, 2021
Date of decision: May 1, 2021
Date of acceptance: April 19, 2021
Date of web publication: May 12, 2022

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