Hydrogen is a flammable gas, which means flammability must be addressed openly and accurately when discussing hydrogen inhalation. At the same time, flammability risk is often misunderstood or understated. Understanding when hydrogen is flammable, and how systems are designed to manage that risk, is essential for responsible education.
When Is Hydrogen Flammable?
Hydrogen gas is flammable only within a specific concentration range in air, approximately 4% to 75% by volume. Below the lower flammability limit (~4%), hydrogen will not ignite. Above this threshold, ignition is possible, even with something as small as static electricity.
This means that:
- Flammability depends on concentration, mixing with air, and ignition conditions
- Hydrogen is not inherently dangerous at all concentrations
Importantly, when dissolved in water, hydrogen gas is not flammable. Similar to dissolving gun powder in water. When it comes to hydrogen inhalation, it is important to weigh the explosion risk with the benefits. MHI does not believe the safety risk is worth the benefits for equipment that operates above the lower flammability limit. Reports have been made of devices exploding, though it is possible this is often underreported. As a result, responsible inhalation systems are designed to operate below flammability thresholds, control hydrogen concentration and dilution, prevent H2 gas accumulation in enclosed spaces, and avoid ignition sources within the delivery pathway. Flammability risk is therefore a systems-level consideration, not simply a property of hydrogen itself.
This is explored in detail in Course 207 of the Level 2 Certification courses.
Flammability risk vs. biological oxygen effects
Safety discussions around hydrogen inhalation often conflate two separate issues: flammability risk and biological oxygen effects. Flammability risk is a physical and engineering concern, determined by gas concentration, mixing with air, ignition sources, ventilation, and system design. It is governed by well-defined combustion thresholds and managed through concentration limits and proper engineering controls. Biological oxygen effects are a physiological consideration. Breathing oxygen above normal atmospheric levels (~21% FiO2), especially over time, can increase oxidative stress and alter redox signaling, independent of fire or explosion risk. These oxygen-related effects are distinct from hydrogen’s safety profile and should be evaluated separately when hydrogen–oxygen mixtures are used.
A quick note on oxyhydrogen (Hydrogen–Oxygen) Devices
Some hydrogen inhalation systems generate or deliver oxyhydrogen, a mixture of hydrogen and oxygen gases. While these gases are commonly used in research and clinical settings independently, combining hydrogen and oxygen at the given ratio (66% H2 and 33% O2) significantly increases the flammability profile.
When hydrogen and oxygen are present together, the mixture can fall directly within flammable or explosive ranges, depending on concentration, flow rate, and system design. Additionally, many may want a higher flow rate (especially if using a nasal cannula) to increase their Fraction of Inspired Hydrogen (FiH2), but higher flow rates increase the flammability risk. Lower flow rates reduce the flammability risk, but if the concentration of hydrogen the device produces is above 4%, then the risk is not completely mitigated. Therefore, these systems should only be used by individuals who fully understand the associated risks, operating limits, and safety protocols. Improper use, modification, or misunderstanding of these devices can substantially increase flammability risk.
Common Misconceptions
Several misconceptions contribute to unnecessary concern:
- “Hydrogen inhalation benefits outweigh the risks”
If hydrogen is above 4%, there is a flammability risk. Even devices that deliver 100% hydrogen (which is above the Flammability Limit, once the gas meets the ambient air, it quickly dilutes into the flammability range. This flammable gas can be ignited with something as little as static electricity. - “Water-based hydrogen products pose the same risk”
Hydrogen dissolved in water is not flammable, as the gas is no longer present in a combustible air mixture.
Read more:
- Concentration vs. Flow vs. Duration
- What Is FiH₂?
- Nasal Cannula Considerations for Hydrogen Inhalation
