Gaseous hydrogen has some outstanding specifications compared to other fuel types, as seen in table 1.
Lower explotion limit (%,air)
Upper explotion 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. 
Hydrogen is a fuel that must be handled properly, like gasoline/petrol or natural gas. It is colorless, odorless, and non-toxic. Furthermore, it is the lightest and smallest element and can due to its molecule size embrittle some metals. Methane is also odorless, but industry adds sulfur-containing odorant so it can be detected. Nevertheless, this cannot be used in combination with hydrogen, since there are no known odorants light enough to travel with hydrogen at the same dispersion rate. Current odorants also contaminate fuel cells, which is an important application for hydrogen. Hydrogen is 14 times lighter than air. Thus, if released in an open environment, it will rise and disperse rapidly leading to a safety advantage in an outside environment.
As seen in Table 1 hydrogen has a very wide flammability range (lower and upper explosion limit) compared to other fuels, between 4 % and 75 %. The optimal combustion condition is a 29 % hydrogen-to-air volume ratio. Detection sensors are almost always installed with hydrogen systems to quickly identify any leak and minimize the potential for undetected flames.
As mentioned before hydrogen is the smallest existing molecule. It has a low viscosity, which is why it prone to leakage. In a confined space, leaking hydrogen can accumulate and reach a flammable concentration. Any gas other than oxygen is an asphyxiator in sufficient concentrations. In a closed environment, leaks of any size are a concern, since hydrogen is impossible to detect for human senses and can ignite over a wide range of concentrations in air. Proper ventilation and the use of detection sensors can mitigate these hazards.
Hydrogen has the smallest ignition energy, much lower than required for other common fuels. This means that small sparks will ignite hydrogen.
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 seen in Table 1 the critical pressure for gaseous hydrogen is at 13 bars. For comparison, hydrogen is compressed to 350 and 700 bars in storage tanks in FCEV. For safety, hydrogen tanks are equipped with pressure relief devices that will 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. So at 700 bars 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 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 a rapid dispersion. This means that if a hydrogen cloud comes in contact with an ignition source in open spaces 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 safely as other common fuels when simple guidelines are followed. This will be dealt with in the subsection: Standards and regulations.