Gaseous hydrogen has some outstanding specifications compared to other fuel types, as can be seen in table 1.
Lower explosion limit (%, air)
Upper explosion limit (%, air)
Flash point ͦC
Lowest ignition energy mJ
Density (20 ͦC, 1 bar)
Boiling point ͦC
Critical temperature ͦC
Critical pressure bar
Diffusion coefficient cm2/s
Table 1: Fuel specifications .
As can be seen in Table 1, hydrogen has a very wide flammability range (lower and upper explosion limit) compared to other fuels, at between 4% and 75%. The optimal combustion condition is a 29% hydrogen-to-air volume ratio. Detection sensors are almost always installed in hydrogen systems to quickly identify any leak and minimize the potential for undetected flames.
As mentioned above, hydrogen is the smallest known molecule. It has a low viscosity, which is why it is prone to leakage. In a confined space, leaking hydrogen can accumulate and reach flammable concentrations. Any gas other than oxygen is an asphyxiator in sufficient concentrations. In a closed environment, leaks of any size are a concern, as hydrogen is impossible for human senses to detect and can ignite in a wide range of concentrations in air. However, proper ventilation and the use of detection sensors can mitigate these hazards.
Hydrogen has the smallest ignition energy, much lower than that required for other common fuels. This means that small sparks can easily ignite it.
Hydrogen has high energy content by weight (density) but not by volume, which is a challenge for storage. In order to store sufficient quantities of hydrogen gas, it is compressed and stored at high pressures. As can be seen in Table 1, the critical pressure for gaseous hydrogen is 13 bar. For comparison, hydrogen is compressed to 350-700 bar in storage tanks in FCEVs. For safety, hydrogen tanks are equipped with pressure relief devices that prevent the pressure in the tanks from becoming too high .
The easiest way to decrease the volume of a gas, at constant temperatures, is to increase its pressure. Thus, at 700 bar, hydrogen has a density of 42 kg/m3, compared to 0.089 kg/m3 under normal pressure and temperature conditions. At this pressure, 5 kg of hydrogen can be stored in a 125 liter tank .
As can be seen in Figure 1, the density of hydrogen highly depends on the temperature and pressure.
Figure 1: Hydrogen density at different temperatures and pressures. 
Due to its weight, hydrogen has a high diffusion rate, which results in rapid dispersion. This means that if a hydrogen cloud comes into contact with an ignition source in an open space with no confinement, flames will propagate through a flammable hydrogen-air cloud at several meters per second, and even more rapidly if the cloud is above ambient temperature .
Hydrogen can, nevertheless, be used as safely as other common fuels when simple guidelines are followed. This will be dealt with in the sub-section: Standards and regulations.