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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).”
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."