Ammonium nitrate, which Lebanese authorities have said caused the devastating Beirut blast, is an odourless crystalline substance commonly used as a fertiliser that has been the cause of numerous industrial explosions over the decades.
Notable among these was an explosion at a fertiliser plant in the US state of Texas 2013 that killed 15 and was ruled deliberate, and another at a chemical plant in Toulouse, France, in 2001 that killed 31 people but was accidental.
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Two tonnes of it was used to create the bomb in the 1995 Oklahoma City attack that destroyed a federal building, leaving 168 people dead.
Lebanese Prime Minister Hassan Diab said 2,750 tonnes of ammonium nitrate that had been stored for years in a Beirut Port hangar had blown up, killing dozens of people and causing widespread damage across the Lebanese capital.
In agriculture, ammonium nitrate fertiliser is applied in granule form and quickly dissolves when exposed to moisture, allowing nitrogen – which is key to plant growth – to be released into the soil.
However, 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 news agency.
“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.”
Ammonium nitrate is an oxidiser – it intensifies combustion and allows other substances to ignite more readily, but is not itself very combustible.
That is why there are generally very strict rules about where it can be stored: For example, it must be kept away from fuels and sources of heat.
In fact, many countries in the European Union require that calcium carbonate to be added to ammonium nitrate to create calcium ammonium nitrate, which is safer.
In the United States, regulations were tightened significantly after the Oklahoma City attack.
Under the Chemical Facility Anti-Terrorism Standards, for example, facilities that store more than 900kg (2,000 pounds) of ammonium nitrate are subject to inspections.
Despite its dangers, Oxley says legitimate uses of ammonium nitrate in agriculture and construction have 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.”