Ammonium nitrate was at the heart of the massive Beirut explosion

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

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.

AMMONIUM NITRATE: DEADLY WHEN CONTAMINATED 

Ammonium nitrate contains two groups: 

Ammonia (NH4+) - a nitrogen and four hydrogens, which provide the fuel. Nitrate that comprises of a nitrogen and three oxygens (NO3-) that provide the oxygen necessary for combustion.

It contains both groups required for a fire and if heated then the three components of the fire-triangle are present - that is fuel, oxygen and heat. 

An explosion occurs when a large amount of an energetic substance detonates, producing a large volume of confined, hot gases that expand and cause a shock wave. 

The video footage of the incident show initial white/grey smoke followed by an explosion that released a large cloud of red/brown smoke and a large white 'mushroom cloud'. 

These indicate that the gasses released are white ammonium nitrate fumes, toxic, red/brown nitrous oxide and water. 

SOURCE: Stewart Walker, ammonium nitrate expert from Flinders University 

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'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

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