Ammonium nitrate – identified as the cause of the deadly explosion in Beirut – is an odourless crystalline substance used as a fertiliser that has been behind many industrial explosions and terrorist attacks over the decades.
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, and it has been widely used by the Taliban in improvised devices.
Experts say a fire in Beirut started after a spark from a welder likely ignited the highly reactive chemical, causing a blast the equivalent to three million kilotons of TNT, killing at least 100 people and leaving thousands more injured.
There were 2,750 tonnes of the hazardous chemical held in the warehouse at the time of the explosion – which measured as the equivalent of a 3.5 earthquake.
Survivors of the blast which devastated Beirut overnight were sifting through the ruins of the city on Wednesday for bodies as the death toll rose to 100 with more than 4,000 wounded, and hospitals struggling to cope
Death and injury from the explosion would have come in a number of phases, according to Dr David Caldicott from the Australian National University.
‘Primary injuries are blast-related, as a consequence of the overpressure wave interacting with the hollow space in victims; lung injuries are often survived, but subsequently fatal, and bowel injuries are common.
‘Secondary injuries are caused by flying debris; effectively environmental shrapnel.
‘Tertiary injuries are as a consequence of being thrown by the blast, and quaternary injuries by other features such as inhalation.’
When combined with fuel oils, ammonium nitrate creates a potent explosive widely used in the construction industry, but its deadly power has also been harnessed by insurgent groups to create bombs.
As well as the Oklahoma City bomb in the US, it has been used in a number of IRA attacks on the UK.
These include the Bishopsgate attack in April 1993 that left 40 injured and a 40ft wide crater.
A 3,300lb bomb in Manchester in June 1996 planted by the IRA left 2,000 injured but no deaths due to a phone warning an hour before the blast.
Other fertiliser bomb attacks include one on the Baltic Exchange building in the City of London that was hit by a one-tonne bomb planted by the IRA in 1992. It killed three people.
In agriculture, ammonium nitrate fertiliser is applied in granule form and quickly dissolves under moisture, allowing nitrogen 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, said.
‘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’s because ammonium nitrate is an oxidiser – it intensifies combustion and allows other substances to ignite more readily, but is not itself very combustible.
For these reasons, 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, with inspections required if more than 2,000lbs of it are stored in one place.
Despite its dangers, Oxley said 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 fertilizer,’ she said.
‘We need ammonium nitrate, we just need to pay good attention to what we’re doing with it.’
That doesn’t change how dangerous it can be. For example, an explosion at a Texas fertilizer plant in 2013 killed 15 and was ruled deliberate.
Another at a chemical plant in Toulouse, France in 2001 that killed 31 people but was accidental also involved ammonium nitrate.
Many of those killed or injured by the explosion in Beirut would have been hit by the shock wave or fire – but shrapnel from destroyed buildings would also have had a devastating impact.
The ammonium nitrate had remained unsecured in the warehouse for six years, according to Lebanon prime minister Hassan Diab, who said it had been taken from a ship and placed in the warehouse.
A general view of the harbour area with smoke billowing from an area of a large explosion, with damage and debris after a large explosion rocked the harbour area of Beirut
According to experts it actually takes very specific circumstances for ammonium nitrate to explode as it isn’t an explosive in its own right – it is an oxidiser that draws oxygen to a fire and makes it more intense.
If the chemical becomes contaminated with something like oil then it can explode and at that point becomes highly explosive.
Speaking on this morning’s BBC Today Programme, Philip Ingram, an expert on chemical weapons, said it was appalling so much of the chemical was stored in one place.
‘It has been responsible for some of the largest accidental explosions we’ve seen since it has been used,’ Ingram said.
‘In its pure, well stored, basic form it is relatively safe, but when it is poorly stored, in a confined space, over time it gets contaminated and something can spark it off.
‘It generates its own heat and once that is started it continues to generate it and over time it can lead to a high order explosion like the one we’ve seen in Beirut.’
Two tonnes of Ammonium Nitrate was used to create the bomb in the 1995 Oklahoma City (pictured) attack that destroyed a federal building, leaving 168 people dead
Because of this highly reactive state when contaminated and the fact it is cheap and readily available, ammonium nitrate has become a chemical of choice for terrorists.
In these improvised explosives the detonator goes off first and the energy causes the ammonium nitrate to vaporise and become a gas.
This forms large amounts of oxygen as the molecules breakdown and drives the explosion to become bigger.
Other incidents have occurred where ammonium nitrate in storage or transit has caused an explosion, with widespread destruction to the surrounding area.
Incidents in Australia involving transportation include a truck carrying ammonium nitrate that experienced an electrical fault and a fire and exploded, killing three people in Taroom, Queensland on August 30 1972.
Another incident in Wyandra, Queensland on September 6 2014 saw a truck carrying ammonium nitrate explode after rolling – destroying a bridge.