Hydrogen is flammable at concentrations between 4% and 75% in air, which is a very wide range compared to other common fuels. It also burns with a pale blue flame that is almost invisible during daylight hours, so fires are almost impossible to see with the naked eye. Hydrogen fires have low radiant heat, so you can't sense the presence of a flame until you are very close to it (or even in it).

Source: Hydrogen Tools