Myth #2. Hydrogen is too dangerous, explosive, or “volatile” for common use as a fuel. The hydrogen industry has an enviable safety record spanning more than a half-century. Any fuel is hazardous and needs due care, but hydrogen’s hazards are different and generally more tractable than those of hydrocarbon fuels.34 It’s extremely buoyant — 14.4 times lighter than air (natural gas is only 1.7 times lighter than air). Hydrogen is four times more diffusive than natural gas or 12 times more than gasoline fumes, so leaking hydrogen rapidly disperses up and away from its source.35 If ignited, hydrogen burns rapidly with a nonluminous flame that can’t readily scorch you at a distance, emitting only one-tenth the radiant heat of a hydrocarbon fire and burning 7% cooler than gasoline. Although firefighters dislike hydrogen’s clear flame because they need a viewing device to see it in daylight, victims generally aren’t burned unless they’re actually in the flame, nor are they choked by smoke.
Hydrogen mixtures in air are hard to explode, requiring a constrained volume of elongated shape. In high-school chemistry experiments, hydrogen detonates with a “pop” when lit in a test tube, but if it were in free air rather than a long cylindrical enclosure, it wouldn’t detonate at all. Explosion requires at least twice as rich a mixture of hydrogen as of natural gas, though hydrogen’s explosive potential continues to a fourfold higher upper limit. Hydrogen does ignite easily, needing 14 times less energy than natural gas, but that’s of dubious relevance because even natural gas can be ignited by a static-electricity spark.36 Unlike natural gas, however, leaking hydrogen encountering an ignition source is far likelier to burn than to explode, even inside a building, because it burns at concentrations far below its lower explosive limit. Ignition also requires a fourfold higher minimum concentration of hydrogen than of gasoline vapor. In short, in the vast majority of cases, leaking hydrogen, if lit, will burn but not explode. And in the rare cases where it might explode, its theoretical explosive power per unit volume of gas is 22 times weaker than that of gasoline vapor. It is not, as has been claimed, “essentially a liquid or gaseous form of dynamite.”
Contrary to a popular misunderstanding, these safety attributes actually helped save 62 lives in the 1937 Hindenburg disaster. An investigation by NASA scientist Dr. Addison Bain found38 that the disaster would have been essentially unchanged even if the dirigible were lifted not by hydrogen but by nonflammable helium, and that probably nobody aboard was killed by a hydrogen fire. (There was no explosion.) The 35% who died were killed by jumping out, or by the burning diesel oil, canopy, and debris (the cloth canopy was coated with what nowadays would be called rocket fuel). The other 65% survived, riding the flaming dirigible to earth as the clear hydrogen flames swirled harmlessly above them. This would hardly be the case if an aircraft with only liquid hydrocarbons caught fire while aloft. It emphasizes that hydrogen is generally at least as safe as natural gas or LPG, and is arguably inherently safer than gasoline,39 although the character of their risks is not identical.
