Relative to conventional fuels, hydrogen has a wider flammability range in air (4–75%), higher permeability through some materials, and a lower ignition energy (0.02 mJ). These characteristics make it vital to provide adequate ventilation, prevent leaks, and eliminate ignition sources in any hydrogen system design and operation. System monitoring and detection is important to ensure that all safety precautions are active and operating as intended.
The very rapid 20 m/s rise rate of gaseous hydrogen in air under ambient conditions greatly aids with ventilation and dilution. Hydrogen also has an auto-ignition temperature of 585°C, which is higher than most fuels. Hydrogen is colorless, odorless, and not toxic to breathe. However, low oxygen detection is needed anywhere hydrogen may accumulate near personnel since asphyxiation is possible if insufficient oxygen is available.
When combusted, hydrogen produces no smoke or soot, which eliminates associated inhalation risks common with fossil and other hydrocarbon fuels. The resulting flame also produces much less radiant energy compared to hydrocarbon fires, thereby reducing the zone of potential heat damage or burns. A hydrogen flame is nearly invisible under daylight conditions, requiring infrared sensors or cameras for detection. At night, the flame is pale blue in appearance. For all hydrogen systems, emergency and fire response planning and coordination is a critical consideration.
The use of LH2 introduces additional safety concerns beyond gaseous hydrogen due to the temperature extremes and phase change characteristics inherent in cryogenic fluid systems. Personnel training, appropriate protective clothing, human interface designs, and safe operations are key to mitigating frostbite and other physiological risks. Appropriate equipment design that eliminates the possibility of human contact with cryogenic surfaces is preferable whenever possible. Exclusion zones, caution and warning systems, safety sensors, and approved operational procedures further mitigate risks to personnel.
The large temperature ranges in cryogenic systems require careful materials selection and design to accommodate differences in thermal expansion and contraction. Phase change from liquid to vapor (and sometime the reverse) occurs throughout a LH2 system, resulting in potential rapid pressure changes in isolated volumes. This must be addressed with appropriate design, operations, and pressure relief devices. If maximum vent relief flow rates are high enough, flaring may be required.
Mitigation of ice buildup is necessary where it may cause key components to not operate properly or create other hazards. Likewise, prevention of oxygen condensing out of the air is addressed with proper insulation on any surfaces that may reach low enough cryogenic temperatures. Any LH2 spills will begin to immediately vaporize and rise as the vapor warms. However, the initially cold hydrogen vapor will be denser than air and can result in temporary regions of high concentration near the ground.
Material selection for hydrogen service must address the design and operational requirements for strength, ductility, fatigue, permeability, and other material properties. Approved cleaning processes must be followed to ensure that unacceptable contaminants are not introduced to the system from materials. Purity levels of the hydrogen are generally dictated by the fuel cell specifications or other feed requirements. Purging and inerting of the assembled system is required for various operations to prevent the introduction of air or other contaminant fluids.
[1] Image source: Liquid Hydrogen Systems Course, 2025.
[2] Text source: Decarbonizing Mobility with Liquid Hydrogen, SAE Research Report EPR2024015, 2024.
[2] Text source: Decarbonizing Mobility with Liquid Hydrogen, SAE Research Report EPR2024015, 2024.
