A fertilizer plant in the community of West, Texas that exploded on Wednesday to deadly effect was known to produce and store a volatile and potentially dangerous form of nitrogen-based fertilizer known as anhydrous ammonia.
Many fertilizer plants either produce or use anhydrous ammonia—a gas that is one part nitrogen and three parts hydrogen—as a base for creating different fertilizer types, said Kurt Steinke, a soil scientist at Michigan State University (MSU).
"Anhydrous ammonia can be combined with different compounds, such as nitric acid, sulfuric acid, or even carbon dioxide from the atmosphere to produce the different kinds of fertilizers that we use today," he said.
Anhydrous ammonia—often referred to as simply ammonia—can be cheaply manufactured and is an effective fertilizer in its own right. But producing it requires intense heat and it must be stored at high pressures.
"When used in agriculture anhydrous ammonia is compressed into a liquid and must be stored under high pressure in specially designed tanks. When the air temperature around the tank increases the temperature of the liquid inside the tank increases causing the liquid to expand thus increasing the internal tank pressure," Steinke said. "If you have a leak in the ammonia tank ... the liquid can quickly convert to a gas rapidly combining with body moisture to cause severe dehydration and chemical burns."
The West, Texas Plant
According to news reports, the West Fertilizer plant—which is located close to schools and residences—was storing as much as 54,000 pounds of the dangerous gas at the site as recently as 2007.
According to a report filed by Texas regulators and reviewed by The Dallas Morning News, the stored fertilizer posed no fire or explosive risks. The worst possible scenario, the report said, would be a 10-minute release of ammonia gas that would kill or injure no one.
Wednesday's explosion, which happened around 7:50 p.m., suggested otherwise. Beyond the death toll, which authorities said was unclear, the blast injured dozens of people and was compared to a "nuclear bomb" by some witnesses.
The ultimate cause of the explosion, which decimated a four-block area around the factory and shook the ground as far as 50 miles away, is still under investigation.
Sgt. William Patrick Swanton of the nearby Waco police department said authorities are not sure if the fire and the subsequent explosion was an accident or the result of an intentional act, according to ABC News.
"We are not indicating that it is a crime, but we don't know," Swanton said. "What that means to us is that until we know that it is an industrial accident, we will work it as a crime scene."
History of Fertilizer Fire
The West explosion is the latest in a long string of fatal incidents involving anhydrous ammonia and its close chemical cousin, ammonium nitrate.
Since 1921, at least 17 unintended fatal explosions involving this class of chemical compounds have been recorded, according to The Guardian.
The most deadly was a 1947 cargo ship explosion in the port of Texas City, Texas which killed at least 550 people and injured 3,500, according to the Guardian. It remains the deadliest industrial accident in the United States history, Salon reports.
In 1994, ammonium nitrate was used to make the bomb that killed 168 people at a federal office building in Oklahoma City.
As a result of that event, the sale of ammonium nitrate in the United States is now heavily regulated and agricultural usage has sharply decreased.
"Ever since the Oklahoma City bombing back in the 1990s, most ammonium nitrate has ceased production and is no longer produced, so it's not as readily available," said MSU's Steinke.