What is ammonium nitrate and why did it explode?

It has also been used as a key component in improvised explosives, notably in the 1995 Oklahoma City bombing, in the 2002 bombings in Bali, Indonesia, and by far-right Norwegian terrorist Anders Behring Breivik in his 2011 shooting and bombing attack.

It's not yet clear what caused the stockpile in Beirut's port to ignite, with such deadly results, on Tuesday evening.

"Ammonium nitrate is ... relatively safe by itself, although a strong oxidant, but highly dangerous when contaminated by any kind of fuel, such as oil or organic material, even in just a few per cent," Roger W. Read, Honorary Associate Professor at the University of New South Wales' School of Chemistry, told the Science Media Centre.

"In the presence of heat, such a mixture can easily lead to catastrophic outcomes," Read added.

Ammonium nitrate is not flammable in itself, Associate Professor Stewart Walker, from the school of Forensic, Environmental and Analytical Chemistry at Flinders University in Adelaide, Australia, told CNN.

"In this instance, it appears that there was a fire and that fire has caused the ammonium nitrate that had been stockpiled to combust, and when it's in a confined space, it releases a lot of hot gas," he said.

"Because the gas takes up a higher volume than the solid, there's a build-up of pressure and because of the heat released, the hot gas is higher in volume, so you get to the point that when it's confined it will suddenly explode and will release that pressure in a shockwave," Walker added.

Video footage of Tuesday's explosion shows that "the fire starts with a grey-white cloud and then, at the time of the explosion, there is a large column of reddy-orange-brown smoke and a large white 'mushroom cloud,' which is the shockwave," said Walker.

The red-orange-brown smoke is characteristic of nitrous oxide, a toxic gas released from the ammonium nitrate, he said.

Has this happened before?

It's not the first time that ammonium nitrate, which is reasonably cheap to manufacture, has been implicated in deadly industrial explosions.

Perhaps the closest comparison, in terms of scale, is a blast in Texas City in 1947. The fire caused an explosion and additional fires that damaged more than 1,000 buildings and killed nearly 400 people, according to the website of the Texas Historical Association.

For perspective, that explosion was triggered by 2,300 US tons (about 2,087 metric tons) of ammonium nitrate, according to US Homeland Security.

"Poorly stored ammonium nitrate is notorious for explosions -- for example in Oppau, Germany; in Galveston Bay, Texas; and more recently at West in Waco, Texas; and Tianjin in China," Andrea Sella, Professor of Inorganic Chemistry at University College London, told the Science Media Centre.

"This is a catastrophic regulatory failure because regulations on the storage of ammonium nitrate are typically very clear. The idea that such a quantity would have been left unattended for six years beggars belief and was an accident waiting to happen."

More than 100 people were killed in the port city of Tianjin in 2015 when a warehouse containing several hundred tons of dangerous chemicals, including ammonium nitrate, went up in a series of massive explosions.

The blast at a fertilizer plant in the small town of West, Texas, in 2013 killed 15 people, including 12 first responders, and destroyed 500 homes. It was so powerful it caused a magnitude-2.1 earthquake.

Were other materials involved in Beirut?

The size of the blast, as well as Lebanon's troubled history, has prompted speculation that military explosives may have been involved.

US President Donald Trump added to the confusion when he referred to the incident at an "attack," though US Defense Department officials told CNN afterward there was no indication that was the case. Lebanese officials also raised concerns with US diplomats about the use of the phrase "attack," two State Department officials said.

Former British army major Chris Hunter, an expert in fertilizer bombs and explosives disposal, said he was "almost certain" that the main detonation was caused by an "industrial accident."

Hunter, who continues to work in Iraq and Syria training bomb disposal crews and was involved in operations to prevent a fertilizer bomb plot in London a few years ago, has watched numerous videos of the explosions.

"You can see an intense fire at the early stages, plus what may be fireworks or ammunition exploding. This is the sort of heat source that would be needed to cause a large amount of ammonium nitrate to explode. It would normally burn but if ignited by intense fire, then the white cloud that characterized this blast is what we'd expect from ammonium nitrate," he said.

"If the reports of 2,700 tonnes of the chemical are accurate then this is also consistent with the scale of a blast and the damage done," he added.

How should ammonium nitrate be stored?

Previous disasters have led to improved regulations for the safe storage of ammonium nitrate -- a valuable inorganic fertilizer which has helped to feed the world's growing population -- Walker said; such rules mean it tends to be kept away from population centers.

"Both of these things will be questioned in the investigation into the Beirut explosion, because they had such a large amount of ammonium nitrate, which may not have been stored appropriately, and in an area where there is a large number of people," he said.

Safe storage would mean keeping the ammonium nitrate away from any source of ignition, keeping it away from anything combustible and keeping it in small enough containers, far enough away from one another, that if one were to ignite it would not spread to the others, he said.

Given its potentially hazardous nature, governments typically restrict access to ammonium nitrate, and require people to have a license to buy it. Breivik, the Norwegian terrorist, was able to acquire and stockpile it because he had a small farm, Walker explained.

As a result of these precautions, accidents more often occur when the chemical is in transit, either in trucks or by ship, Walker added.

Two such incidents have occurred in Queensland, Australia, he said. In 2014, a truck carrying ammonium nitrate exploded in Wyandra after rolling, destroying a bridge. And in 1972, three people were killed in Taroom when a truck transporting ammonium nitrate exploded after experiencing an electrical fault and fire.

Who produces it?

Russia is the world's largest producer of ammonium nitrate, according to the Food and Agriculture Organization of the United Nations.

But the country has faced obstacles in their attempts to sell the fertilizer to Europe. For decades, the European Union has placed anti-dumping tariffs on their import to prevent Russian producers from undercutting EU-based suppliers.

"It's a question of having a level playing field between European producers and Russian producers," Antoine Hoxha, technical director of the Fertilizers Europe organization, told CNN.

"Dumping is not a fair trade practice: there are international agreements on this. If there is a practice of dumping and it is verified, then they take measures to restore a playing field," he said. The EU has applied the measures on a rolling basis, most recently reducing them slightly in 2018.

The production of ammonium nitrate is subject to strict testing measures in Europe, which Hoxha says were ramped up after a 2001 explosion in Toulouse, France that killed dozens.

"You physically abuse the product by heating and cooling it, to genuinely alter its substance and to simulate it being mistreated or mis-stored or heated and cooled, which might happen in extreme storage conditions," Jo Gilbertson, sector head for fertilizers at the UK's Agricultural Industries Confederation, told CNN.

"We deliberately abuse it, and then try to blow it up," he said, adding that the same standards may not be applied outside the EU.

"It's an industry where you must never be complacent," Gilbertson added. "The product we're dealing with is incredibly safe when you look after it. But if you don't look after it, it's very easy to make a mistake."