Ammonium nitrate facts: why is it so dangerous

Beirut was rocked by a huge explosion on Tuesday afternoon, when, according to the Lebanese president, 2,750 tonnes of ammonium nitrate stored at the docks exploded. The blast destroyed much of the port, damaged buildings up to 10 km away and could be heard 150km away on Cyprus.

The Lebanese government confimred on Wednesday afternoon that the explosion had killed 135 people and injured 5,000. However, with many people missing, some of whom are thought to be buried in the rubble the death toll is expected to rise.

Follow live updates and breaking news of the aftermath of the Beirut explosion.

Ammonium nitrate: what is it and why is it so dangerous

The blast was caused by 2,750 tonnes of ammonium nitrate, which had been dangerously stored at the docks since, according to local media, they were impounded from the ship, the Rhosus, which had been allegedly abandoned by its owner in 2013.

Here's an explainer on ammonium nitrate:

Ammonium nitrate is an industrial chemical commonly used in fertilisers and as an explosive for quarrying and mining. It is an oxidiser considered relatively safe if uncontaminated and stored properly. But it is extremely dangerous if contaminated, mixed with fuel or stored unsafely.

A large quantity of ammonium nitrate exposed to intense heat can trigger an explosion. Storing the chemical near large fuel tanks, in bulk in large quantities and in a poorly-ventilated facility could cause a massive blast. The larger the quantity, the more risk it will detonate.

Ammonium nitrate: "explosion scaled down from a nuclear bomb"

"On a scale, this explosion is scaled down from a nuclear bomb rather than up from a conventional bomb," said Roland Alford, managing director of Alford Technologies, a British company that specialises in disposal of explosive ordnance.

"This is probably up there among the biggest non-nuclear explosions of all time."

Experts have noted the colour of the smoke and "mushroom cloud" seen in footage of Tuesday's blast as characteristic of ammonium nitrate explosions.

"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," said Stewart Walker from the school of Forensic, Environmental and Analytical Chemistry at Flinders University.

"If you make ammonium nitrate explosive, you shouldn’t get that brown plume. That tells me the oxygen balance was not correct - so it wasn’t mixed as an explosive," he said. "The Beirut blast looks like an accident, unless it was arson."

Past ammonium nitrate accidents

Some of the world's deadliest industrial accidents were caused by ammonium nitrate explosions:

* In 1921, an explosion of ammonium sulphate and nitrate fertiliser at the Oppau plant in Germany killed 565 people.

* In 1947, a fire detonated around 2,300 tonnes of the chemical aboard a vessel in the U.S. port of Texas City, causing a tidal wave. At least 567 people were killed and more than 5,000 injured.

* In Toulouse, France in 2001, an explosion at an ammonium nitrate depot killed 31 and injured 2,500.

* Ammonium nitrate stored at a Texas fertiliser plant detonated in an explosion that killed 14 and injured about 200 in 2013.

* In 2015, explosions at a warehouse storing ammonium nitrate and other chemicals in the Chinese port of Tianjin killed at least 116 people.

* Andrea Sella, professor of Inorganic Chemistry at the University of California, Los Angeles, said of the Beirut blast: "The idea that such a quantity would have been left unattended for six years beggars belief, and was an accident waiting to happen."

Use of ammonium nitrate in bombings

Ammonium nitrate can be mixed with other substances to make a bomb. It was used in Irish Republican Army (IRA) bombings in London in the 1990s, the 1995 explosion that blew up a federal building in Oklahoma City, killing 168 people, and the 2002 blasts in Bali nightclubs in which more than 200 died.

Many of the homemade bombs that were used against U.S. troops in Afghanistan contained ammonium nitrate.