Anhydrous ammonia, the substance that exploded catastrophically Wednesday at a company in West, Tex., near Waco has been used as a cheap, if somewhat hazardous, fertilizer since the 1940s.

The chemical provides nitrogen to growing plants. That element is an essential constituent of proteins, which are the workhorse compounds of cells. The trouble is that while nitrogen gas makes up more than three-quarters of the Earth’s atmosphere, most living things can’t easily extract it from the air. They have to get it in a more accessible form.

Anhydrous ammonia--one nitrogen and three hydrogen atoms, designated NH3--is a way of efficiently delivering the element to plants. It is injected directly into the ground by devices hauled behind tractors. About 30 percent of the “agricultural nitrogen” used in the United States comes in the form of anhydrous ammonia. More than 110 million tons of it are used around the world each year.

The pungent chemical is the same one used in many household cleaners, except that it has been purified to contain no water (which is what “anhydrous” means). To remain in liquid form it must be stored under high pressure. White cylindrical tanks holding anhydrous ammonia are part of the landscape of rural America.

Ammonia burns, but not easily. It requires a high temperature (1204 degrees Fahrenheit) and the right ammonia-to-air-mixture (about 1-to-4) to catch fire. It’s not like propane or methane, which burn at lower temperatures and concentrations, making them far more dangerous.

The hazards of anhydrous ammonia, however, aren’t small. Sprayed on skin or eyes, it causes severe burns. Inhaled as a gas, it can fatally damage the lungs. And stored in liquid form, it can explode under the right conditions.

The explosions of liquids released from pressurized containers are called “boiling liquid expanding vapor explosions”, or BLEVEs. Perhaps the most famous one in modern times was the explosion of a tank of liquid propane during in a fire in Kingman, Arizona, on July 5, 1973. It killed 11 firefighters and one railroad worker.

A BLEVE doesn’t have to be triggered by a fire, although it usually is. The rapid evaporation of a chemical when the pressure keeping it in liquid form is released causes an explosion in itself. When that is supplemented by the near-instantaneous burning of the vapor, the result can be catastrophic.

Neal Langerman, a chemist with the San Diego-based consulting company called Advanced Chemical Safety, speculated the explosion in West happened this way:

Anhydrous ammonia tanks are never more than 85 percent filled with the liquid chemical. The other 15 percent of the space is taken up by ammonia in gas form. When the fire got to the tanks it heated the part of the tank over the gas bubble to the point where it melted.

At that point, Langerman said, the gas would have escaped and been ignited by the fire, which was well over the 1204 F ignition temperature. The liquid ammonia in the tank would then have vaporized, escaped and caught fire too--all in the fraction of a second.

“The explosive release of the ammonia--both gas and liquid--would have very quickly transitioned to an actual detonation,” said Langerman, who has also served as an official of the American Chemical Society.

In fires that threaten stores of anhydrous ammonia the strategy is to spray water on the tanks to keep them cool enough so that the metal doesn’t melt. Rising pressure inside the tanks blows out relief valves, which allows the contents to vent and burn in a controlled way. All that, however, must be done from a safe distance.

“What the standard says is: You back off, up to a mile if necessary, and you only apply water remotely. You get your people out of there,” Langerman said.

The three key questions at this point are: Where did the fire start? Was West Fertilizer operating the depot according to standards? Did the firefighters recognize the particular risk of stored anhydrous ammonia?