On the evening of 4 August, Pierre Khoueiry was making birthday plans with his wife and two-year-old daughter when a blast shattered the windows of the family’s apartment in Beirut. About 2.5 kilometres away, in the city’s port, a powerful explosion had sent a huge orange fireball into the sky, followed by a massive shock wave that overturned cars, damaged buildings and shook the ground across the Lebanese capital. “It was a moment of great fear,” says Khoueiry, a genomics researcher at the American University of Beirut.

Lebanese authorities say that the explosion, which killed at least 220 people, injured more than 5,000 and left an estimated 300,000 people homeless, was caused by 2,750 tonnes of ammonium nitrate, a chemical compound commonly used as an agricultural fertilizer, which had been stored for 6 years at a port warehouse. An investigation into what triggered the explosion is under way, and early reports suggest that it was probably a nearby fire.

The blast is one of the largest accidental ammonium nitrate explosions ever recorded (see ‘Explosive chemical’) — so powerful that it was heard more than 200 kilometres away in Cyprus. The sheer amount of ammonium nitrate involved is “nuts, insane, huge”, says Andrea Sella, a chemist at University College London.

The chemical has caused serious industrial disasters in the past. In 1921, an explosion at an ammonium nitrate factory in Oppau, Germany, killed 561 people and could be heard hundreds of kilometres away. And in 2015, the detonation of around 800 tonnes of ammonium nitrate in the port of Tianjin, China, killed 173 people.

Manufactured as little beads that resemble cooking salt, ammonium nitrate is cheap to buy and usually safe to handle, but storing it can be a problem. Over time, the compound absorbs moisture, which can make the beads stick together into a huge rock, says Sella. When such a large quantity of compacted ammonium nitrate is exposed to intense heat — if, say, an accidental fire breaks out — it can trigger an explosion. The shock wave following a such a blast can be deadly. The explosion produces an area of high pressure that travels faster than the speed of sound, shattering glass and injuring people.

In Beirut, the disaster has had such tragic consequences for reasons unrelated to the explosion itself. Efforts to treat injured people have been hampered by damage to hospitals near the blast site, many of which were already under strain from the coronavirus pandemic. People who weren’t able to get medical assistance quickly enough might end up with lifelong consequences of their injuries, says Paul Gardner-Stephen, who studies disaster-mitigation technology at Flinders University in Adelaide, Australia. “Lives and livelihoods will be lost, because there’s a limited capacity to respond,” he says.

Lebanon was also already reeling from an economic crisis that triggered anti-government protests starting in October 2019, when the country’s currency began to lose its value against the dollar, says Charlotte Karam, a social scientist at the American University of Beirut. Since then, food prices have gone up and about one in three people in the country are unemployed.

Now, with the destruction of Beirut’s port and the country’s main grain silo — which was close to the warehouse storing the ammonium nitrate — Karam says the consequences of the blast will be felt across the nation. “This is a crisis layered upon multiple crises — an economic crisis, a political crisis, a health crisis,” she says. “We need to work together to rebuild Lebanon.”


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