Slightly soluble in water (0.17 mg/100 mL)

Critical temperature is - 239.9 °C; noncondensible above this temperature

Highly flammable gas; explosion hazard in the presence of heat, flame, or oxidizing agents.

Hydrogen is practically nontoxic. In high concentrations this gas is a simple asphyxiant, and ultimate loss of consciousness may occur when oxygen concentrations fall below 18%. Skin contact with liquid hydrogen can cause frostbite.

Hydrogen is a highly flammable gas that burns with an almost invisible flame and low heat radiation. Hydrogen forms explosive mixtures with air from 4 to 75% by volume. These explosive mixtures of hydrogen with air (or oxygen) can be ignited by a number of finely divided metals (such as common hydrogenation catalysts). In the event of fire, shut off the flow of gas and extinguish with carbon dioxide, dry chemical, or halon extinguishers. Warming of liquid hydrogen contained in an enclosed vessel to above its critical temperature can cause bursting of that container.

Hydrogen is a reducing agent and reacts explosively with strong oxidizers such as halogens (fluorine, chlorine, bromine, iodine) and interhalogen compounds.

Because of its flammable and gaseous nature, hydrogen should be handled using the "basic prudent practices" of Chapter 5.C, supplemented by the additional precautions for work with flammable compounds (Chapter 5.F) and for work at high pressure (Chapter 5.H). In particular, hydrogen cylinders should be clamped or otherwise supported in place and used only in areas free of ignition sources and separate from oxidizers. Expansion of hydrogen released rapidly from a compressed cylinder will cause evolution of heat due to its negative Joule-Thompson coefficient.