Lebanese authorities said 2,750 tonnes of ammonium nitrate had been stored for years at the Beirut port without safety measures. That stockpile exploded on Tuesday, killing more than 150 people, injuring thousands and causing widespread damage.
Ammonium nitrate is an industrial chemical commonly used in fertilisers and as an explosive for quarrying and mining.
It is 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 explode.
Storing the chemical near large fuel tanks, in bulk and in a poorly ventilated facility could cause a massive blast. The larger the quantity, the greater the risk it will detonate.