What is ammonium nitrate, the everyday fertiliser behind the Beirut explosion?

The substance is widely used in construction and agriculture but can also be used by terrorists. What is ammonium nitrate and why is it so dangerous?

Damaged cars are pictured in front of billowing smoke behind the grain silos at the port of Beirut. AFP

A drone picture shows smoke from the scene of an explosion at the seaport of Beirut. AP Photo

Lebanon's President Michel Aoun wears a protective face mask as he visits the scene of Tuesday's explosion in Beirut. AFP

An emergency command vehicle of the Lebanese Red Cross is pictured in the aftermath of yesterday's blast. AFP

A man inspects the damage of yesterday's blast. AFP

A survivor is taken out of the rubble after a massive explosion in Beirut. AP Photo

A damaged hospital is seen after a massive explosion in Beirut. AP Photo

Lebanese soldiers search for survivors after a massive explosion in Beirut.AP Photo

An ambulance drives near the site of Tuesday's blast in Beirut's port area. Reuters

Lebanese national flags fly at half-mast outside the presidential palace in Baabda, following Tuesday's blast in Beirut's port area. Reuters

A woman is evacuated from the partially destroyed Beirut neighbourhood of Mar Mikhael. AFP

An injured man sits next to a restaurant in the trendy partially destroyed Beirut neighbourhood of Mar Mikhael. AFP

A man walks past damaged building and vehicles near the site of Tuesday's blast in Beirut's port area. Reuters

A man wearing a protective face mask walks past damaged buildings and vehicles near the site of Tuesday's blast in Beirut's port area. Reuters

A woman sweeps at a damaged hospital following Tuesday's blast, in Beirut. Reuters

A man wearing a face mask moves a gurney at a damaged hospital following Tuesday's blast in Beirut. Reuters

The wreckage of a ship is seen following yesterday's blast at the port of Lebanon's capital Beirut. AFP

A view shows the aftermath at the site of Tuesday's blast in Beirut's port area. Reuters

Lebanese soldiers and people gather outside American University of Beirut medical centre following the explosion in Beirut. Reuters

An injured man sits outside American University of Beirut medical centre following an explosion in Beirut. Reuters

The explosion caused damage to Lebanon's PM Hassan Diab's office

The damage at Lebanon's PM Hassan Diab's office following the blast.

Pictures of the damage at Lebanon's PM Hassan Diab's office

People gather outside American University of Beirut medical centre following the explosion in Beirut. Reuters

Lebanese soldiers stand outside American University of Beirut medical centre following the explosion in Beirut. Reuters

Damaged vehicle and buildings near the scene of Tuesday's blast in Beirut's port area. Reuters

A man walks by an overturned car and destroyed buildings. Getty Images

A view shows the damaged facade of a building following Tuesday's blast in Beirut's port area. Reuters

A view shows the aftermath of yesterday's blast at the port of Beirut. AFP

A man pushes a buggy with a child on Wednesday past a damaged vehicle near the scene of overnight blast in Beirut's port area. Reuters

In this drone picture, the destroyed silo sits in rubble and debris. AP Photo

People inspect the damage near the scene of Tuesday's blast in Beirut's port area. Reuters

People inspect the scene of Tuesday's blast in Beirut's port area. Reuters

Shattered glass lies in front of a building following a blast in the Lebanese capital Beirut. AFP

Lebanese army troops carry a wounded man evacuated from a ship at Beirut's port. AFP

This picture shows damage at Beirut-Rafic Hariri International Airport. Courtesy Lebanese Plane Spotters / Facebook

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Ammonium nitrate, which Lebanese authorities say caused the deadly blast in Beirut on August 4 2020, is a fertiliser that has been favoured in the past by extremist groups such as Al Qaeda and the Taliban, which have been drawn to its explosive nature.

An odourless crystalline substance, ammonium nitrate must be stored with great care because of how dangerous it can be, experts say. It has been the cause of numerous industrial explosions over the decades.

What is ammonium nitrate?

Former Lebanese Prime Minister Hassan Diab said 2,750 tonnes of ammonium nitrate that had been stored for years in a Beirut port-side warehouse had blown up, killing dozens of people and causing extensive damage to the Lebanese capital.

Chemical weapons expert Hamish de Bretton-Gordon said it was not normal for such a large amount of the fertiliser to be stored in one place.

"Ammonium nitrate … because of its volatility and its explosiveness, that is why terrorists and other people have used it and of course it's very easy to get hold of," he told The National.

“The storage of it is very, very tightly controlled because it is so volatile.”

Normally strict rules would dictate where substances such as ammonium nitrate would be kept, he said.

“Because it is so volatile, you must store it somewhere where there is no possibility that it could be ignited by flame or any other explosion,” Mr de Bretton-Gordon said.

Ammonium nitrate has been behind several major incidents including, notably, a blast at a Texas fertiliser plant in 2013 that killed 15 people and was ruled deliberate and another at a chemical plant in Toulouse, France, in 2001 that killed 31 people but was accidental.

Another worry was the potential spread of toxic gases emitted by the explosion, Mr de Bretton-Gordon said.

“Ammonium and nitrogen oxide, which is also produced by this explosion – they’re both very non-persistent, so they would disperse very, very quickly.

“But, around the time, it would be a concern and if the fires are still burning, ammonium and nitrogen oxide would still be given off. So the downwind hazard, as we call it, hopefully everyone in the downwind hazard has been cleared out of the area as they certainly shouldn’t be breathing in these toxic fumes without wearing PPE (personal protective equipment).”

Firefighters try to extinguish flames after a large explosion rocked the harbour area of Beirut, Lebanon, on August 4, 2020. EPA

In agriculture, ammonium nitrate fertiliser is applied in granule form and quickly dissolves when wet, allowing nitrogen – which is central to plant growth – to be released into the soil.

When combined with fuel oils, ammonium nitrate creates a potent explosive widely used by the construction industry, but also by hard-line groups such as the Taliban for improvised explosives.

Under normal storage conditions and without very high heat, it is difficult to ignite ammonium nitrate, Jimmie Oxley, a chemistry professor at the University of Rhode Island, told AFP.

"If you look at the video [of the Beirut explosion], you saw the black smoke, you saw the red smoke, that was an incomplete reaction," she said.

"I am assuming that there was a small explosion that instigated the reaction of the ammonium nitrate – whether that small explosion was an accident or something on purpose I haven't heard yet."

That is because ammonium nitrate is an oxidiser – it intensifies combustion and allows other substances to ignite more readily, but is not itself particularly combustible.

For these reasons, there are generally strict rules about where it can be stored: for example, it must be kept away from fuels and sources of heat.

Many countries in the European Union require that calcium carbonate be added to ammonium nitrate to create calcium ammonium nitrate, which is safer.

In the United States, regulations were tightened significantly after it was used as a component in the 1995 Oklahoma City bombing.

Under the Chemical Facility Anti-Terrorism Standards, for example, places that store more than 900 kilograms of ammonium nitrate are subject to inspections.

Despite its dangers, Ms Oxley said legitimate uses of ammonium nitrate in agriculture and construction had made it indispensable.

"We wouldn't have this modern world without explosives, and we wouldn't feed the population we have today without ammonium nitrate fertiliser," she said.

"We need ammonium nitrate, we just need to pay good attention to what we're doing with it."