STOCKHOLM - The storage of ammonium nitrate, the substance behind the deadly explosion in the port of Beirut on Tuesday, has come under increased regulation in recent years to avoid involvement of the ubiquitous product in accidents which, although rare, can be devastating.

WIDELY USED AND PRODUCED

The world's annual production of ammonium nitrate is over 20 million tons, which means the amount that exploded in Beirut (2,700 tonnes) is made almost every hour.

Storing hundreds or even thousands of tons in the same place is frequent, and a single farmer can easily use several tonnes a year.

Analyst firm IHS says just over three-quarters of the world's supply goes to agriculture as a high-nitrogen fertilizer for crops.

The rest, in a higher and more volatile concentration, is used for explosives, particularly in mining and construction.

The substance can be found in its natural state, particularly in Chile, where it used to be mined.

But it has been synthesized since the early 20th century and is now almost exclusively produced in factories.

ACCIDENTS RARE, BUT TERRIBLE

There have been a few dozen accidents over the past century, with appalling consequences.

One of the earliest, at a BASF plant in Oppau, Germany, resulted in 561 deaths in 1921.

In 1947, in Texas City, an explosion of two ships in port carrying 3,500 tons killed 581 people.

According to a European Commission memo, accidents have been recorded at production sites, warehouses and during transport.

"Even small storages of ammonium nitrate fertilizers, defined as low as 10 tons in some legislations, may place the population at high risk if proper safety measures and procedures are not fully in place," the memo said.

Industry representatives say however that the risk is minimal when safety protocols are respected and point out that the substance needs to reach a temperature of just under 200 degrees Celsius before it can combust.

Ammonium nitrate is "not toxic to handle and cannot burn or explode spontaneously," said a spokeswoman for Yara, one of the world's leading producers along with Russia's Eurochem, CF Industries of the US and Chile's Enaex.

According to the UN's Food and Agriculture Organization (FAO), Russia is the world's leading producer.

"Insensitive to shocks and friction, ammonium nitrate is a mediocre explosive unless it is mixed with fuels such as hydrocarbons, or if it is melted and put under pressure during, for example, a violent fire," the French chemistry organization Societe Chimique de France said.

STRICTER REGULATION

According to the IHS, "there is continuous pressure around the world to regulate use and trade of ammonium nitrate because of its potential for misuse as an explosive in terrorism or for accidental detonation."

The organization also said that countries including Afghanistan, China, Colombia, the Philippines, and Turkey "have banned the sale of ammonium nitrate as a fertilizer".

In Europe, stockpiles are regulated by the Seveso 3 directive, which was strengthened following an accident at the AZF plant in Toulouse, France, in 2001.

"The EU regulation does not regulate nor impose limits on the storage of ammonium nitrate. Instead, the Seveso directive has in place a tiered approach to the level of controls: the larger the quantities of dangerous substances present within an establishment, the stricter the rules," Lukasz Pasterski, spokesman for Fertilizers Europe, told AFP.

By contrast in the US, it is forbidden to store more than 2,500 tonnes in a building not equipped with automatic sprinklers.

China tightened security measures after an explosion in a warehouse storing ammonium nitrate, among other chemicals, in the city of Tianjin, killed at least 165 people.

Ammonium nitrate, which is widely available and cheap, has often been used in bomb attacks.

Two tonnes of the product, combined with gasoline, was enough for American extremist Timothy McVeigh to damage several housing blocks in Oklahoma City in 1995, and kill 168 people.

Using the same modus operandi, Norwegian far-right extremist Anders Behring Breivik damaged Oslo's government district in 2011.

As a result of increased regulation farmers have started switching to another fertilizer, urea, according to analysts.