Hydrogen is highly flammable (2H2 + O2 → 2H2O). It can ignite in combination with as little as 23% air (≈5% oxygen). By comparison, propane needs a mix of almost 90% air to ignite. This means that even small amounts of air in lines or storage tanks are potentially dangerous. In addition, the friction caused by high pressure gas passing through a narrow valve could theoretically create enough heat to ignite hydrogen. Therefore, it is vital that no air be in lines or storage tanks used for hydrogen. This comes down to the simple matter of purging with nitrogen (see How does Purge Gas Work).
Never smoke near a place where hydrogen is generated or being used. Remove all possible sources of flame and sparks. Hydrogen should only be generated and used in a well ventilated out door area. Precautions must be taken to remove all possibilities of fire or explosion.
Dangerous heat and chemical reaction may result from incorrect operation of hydrogen generators. Removing a hydrogen generator from a water tank while in operation will produce excessive heat and may cause a dangerous chemical reaction. Never remove a generator from the water while in operation until the generation of hydrogen gas has stopped.
Hydrogen also burns with an invisible flame, which creates the risk of discovering a fire by walking into it. Again, awareness and simple precautions can address the risk.
Hydrogen is highly reactive, and combines readily with metals, leading to corrosion and fatigue (hydrogen embrittlement). Thus all pipes and fittings have to be of stainless steel. This raises the price of installations, but not the difficulty.
Hydrogen gas is a small, high energy molecule, prone to escape. Leaks can be tenacious, more so than with other gases. Plumbing can thus be a more fussy business.
On the plus side, hydrogen is so light that given an escape route it will dissipate very quickly, greatly reducing the danger of accumulated gases exploding. Compare propane, which is heavier than air and can thus collect in low spots.
Hydrogen gas generators offer several safety features that free-standing high-pressure gas cylinders do not.