Hydrogen is the most abundant element in the universe; however, here on the surface of the Earth, pure hydrogen gas is relatively rare. That's because hydrogen gas -- which is usually found in molecular form, with two hydrogen atoms bound together to form H2 -- is so light that, if not contained, it will rise rapidly to the top of the Earth's atmosphere and escape into space. Most hydrogen on the Earth's surface is bound together with other types of atoms as molecules that form various substances. For example, H2O, better known as water, and CH4, also known as methane, both contain hydrogen molecules. Before it can be used as a fuel, the hydrogen must first be extracted from these substances and then contained, usually in highly compressed liquid form.
Are there dangers associated with pure hydrogen? To put it simply, yes. When liquid hydrogen is stored in tanks, it's relatively safe, but if it escapes there are associated hazards.
Topping the list of concerns is hydrogen burns. In the presence of an oxidizer -- oxygen is a good one -- hydrogen can catch fire, sometimes explosively, and it burns more easily than gasoline does. According to the American National Standards Institute, hydrogen requires only one 10th as much energy to ignite as gasoline does. A spark of static electricity from a person's finger is enough to set it off. Ideally, no oxygen should be present in the liquid hydrogen tanks in a fuel cell vehicle, but trace amounts of air may contaminate the hydrogen supply. If the hydrogen should escape, it will immediately come into contact with the oxygen in air.
Another concern is that hydrogen flames are nearly invisible. When hydrogen catches fire, the flames are so dim and hard to see that they're both hard to avoid and hard to fight.
Next, there's the potential for hydrogen to asphyxiate people. Hydrogen isn't poisonous, but if you should breathe pure hydrogen you could die of asphyxiation simply because you'll be deprived of oxygen. Worse, you won't necessarily know that you're breathing hydrogen because it's invisible, odorless and flavorless -- much like oxygen.
The final concern that we want to mention here is that liquid hydrogen is cold. Because it's highly compressed, liquid hydrogen is extremely cold. If it should escape from its tank and come in contact with skin it can cause severe frostbite.
How seriously should you take these dangers? Could this list of dangers make hydrogen look more hazardous than it really is? That's possible, as you'll find out on the next page.