What is ammonium nitrate, the chemical in the deadly Beirut explosion?

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The colossal explosion in Lebanon's capital this week was apparently caused when more than 2,700 tons of ammonium nitrate, a highly volatile chemical, detonated.

The stockpile had been left for years at the city's port in a densely populated neighborhood, according to government officials, who are still investigating. Ammonium nitrate is commercially available and is often used in fertilizers and explosives.

Although some port officials have been placed under house arrest, it was still unclear Thursday what exactly triggered a blast that flattened the area and sent a cloud of orange smoke mushrooming across the city, blowing out windows and decimating homes. The explosion was so loud it was heard nearly 140 miles away in Cyprus. At least 137 people were killed and more than 5,000 injured as of Thursday.

Prime Minister Hassan Diab said it was "unacceptable" that that amount of ammonium nitrate had been in a warehouse for six years without "preventive measures" to protect it.

What is ammonium nitrate and how dangerous is it?

"It's a very common chemical that anybody that has used fertilizer has dealt with routinely and doesn't think anything about it," said Nathan Lewis, a chemistry professor at the California Institute of Technology. "Just give it a little fuel and you're asking for trouble, and that's what apparently happened."

Lewis said the chemical can be safely stored in its pure form by itself.

A Pakistani dealer holds fertilizer containing ammonium nitrate in Multan, Pakistan, on Aug. 25, 2011.Khalid Tanveer / AP file

"On the other hand, either inadvertently or deliberately, if you mix a fuel with the ammonium nitrate, then it's a very dangerous explosive," he said. "There are protocols for storing it safely: Don't let it get near fuel, don't let it be in a confined space."

Karthish Manthiram, an assistant professor of chemical engineering at the Massachusetts Institute of Technology, said that what causes such a dangerous explosion is the rapid expansion of ammonium nitrate from a solid to a gas.

"This amount of ammonium nitrate is about enough to fill half an Olympic-size pool, and that amount of ammonium nitrate expanded would then be enough to fill half of AT&T Stadium or half of the Dallas Cowboys' Stadium," he said. "So, it's just an enormous increase in volume."

How should ammonium nitrate be stored?

Manthiram said the United States has quite a few regulations for storing ammonium nitrate in certain quantities.

The Occupational Safety and Health Administration and the Department of Homeland Security both regulate the substance, Manthiram said. OSHA stipulates that if you have more than 1,000 pounds of fertilizer-grade ammonium nitrate, it has to be stored in a one-story building with adequate ventilation in case of a fire and a sufficient water supply and fire hydrants to fight a potential fire, he said.

Then there are additional regulations from DHS, the Chemical Facility Anti-Terrorism Standards, which applies to hundreds of chemicals that could be misused under the context of terrorism, he said.

"This is certainly an enormous safety threat that would have been dealt with quite differently" in the U.S., Manthiram said of the amount of material stored in Beirut.

Jimmie Oxley, a chemistry professor at the University of Rhode Island, said that some 200 factories produce the chemical around the world and that it is "a very valuable commodity, both as a fertilizer and as an explosive."

"If you're doing mining of any type, you're going to be using this material to make your explosive," she said.

Oxley said that no manufacturer would have stored the material the way it was held at the port in Beirut.

Generally such storage sites are near mines and thus not close to large population centers, she said. There are only two plants in the U.S. that make fertilizer-grade ammonium nitrate, both in the South and neither in the middle of large residential areas, Oxley said.

Have there been previous incidents involving ammonium nitrate?

The chemical has been involved both in terror attacks and in accidents leading to hundreds of deaths in the past.

"The U.S. faced certain disasters long ago of this sort," Manthiram said.

In 1947, at least 581 people were killed when more than 2,000 tons of the chemical exploded on a cargo ship that had docked at a port in Texas City, Texas.

That same year, in Brest, France, a Norwegian ship that contained about 3,000 tons of ammonium nitrate exploded, causing 29 deaths.

"These were among a number of early incidents that led to the emergence of regulations," he said. "With 70 more years of experience at handling chemicals of this sort, I think there's really no good reason for something like this to happen."

Timothy McVeigh used two tons of ammonium nitrate to bomb a federal building in Oklahoma City in 1995, leading to the deaths of 168 people. It was also used in a 1970 bombing on the University of Wisconsin-Madison campus that led to one death and several injuries.

"Virtually every day can go by seeming as if this is an innocent molecule, because it's a fertilizer, which makes it sound like it's harmless in a way," Manthiram said, "and then one incident of this sort can completely change one's perspective."

What about the origins of the 2,750 tons of chemical in Beirut?

The ammonium nitrate arrived in Beirut in the fall of 2013 on a Russian-owned cargo ship, the Rhosus, the ship's then-captain, Boris Prokoshev, told NBC News. It was en route from the Georgian Black Sea port of Batumi to Mozambique, where it was to be used as fertilizer, he said.

It stopped off in Beirut but was impounded for safety reasons because it was overloaded and listing, Prokoshev said.

"The ship couldn't take it," he said of the weight.

The ship's Russian owner, Igor Grechushkin, abandoned the ship, refusing to pay docking fees, fines, and even salaries and food for the crew, according to the captain and Natalia Sokolova, a representative for the Seafarers Union of Russia, which represented the crew during its dispute with the owner.

NBC News tried but was not able to reach Grechushkin for comment.

"The Beirut port authority would not give them permission to abandon a ship carrying this type of cargo," Sokolova said. "In the end, a court seized the vessel to sell it as a means to pay off the ship owner's debts, and a port agent found locals to unload the cargo and the crew went home."

The governor of Beirut, Marwan Abboud, backed up this account, telling the country's LBCI television station that the chemical was kept in the port under "judicial order" and that "there was nobody who took the responsibility to make a decision to remove it."

"Those responsible will be held accountable whomever and wherever he is," Interior Minister Mohammed Fahmi told reporters Wednesday after he inspected the port, adding that the investigation will take a maximum of five days.