As official investigations continue into the huge fertiliser facility blast in West, Texas, that killed 14 and injured at least 200 on 17 April, questions are being asked as to what was stored on the site, how the explosion happened and whether adequate safety precautions were in place.
"Investigators from the US Chemical Safety Board arrived on the scene last Thursday evening," Jean Gonsoulin, a spokeswoman for the board, told New Scientist.
Like 6000 other plants around the US, the facility stored, blended and distributed fertiliser - mainly anhydrous ammonia - to local farmers. Whether the plant also stored ammonium nitrate, a fertiliser doubling up as an explosive used in quarrying rock, is still unclear.
Keith Plumb, an industry consultant on explosion prevention, based at Integral Pharma Services in Cheshire, UK, says an explosion of this size may not have required the presence of ammonium nitrate. The relatively low-level blaze that broke out at the plant could have heated a tank of anhydrous ammonia, causing vapour within to expand. To prevent the tank from exploding as pressure built up, ammonia may have been vented through safety valves, gradually accumulating in a cloud above the plant.
"You could get a huge cloud of ammonia, and with a fire on at the same time, it would almost certainly ignite," says Plumb.
A mixture of natural gas and air will explode once the gas makes up as little as 5 per cent of the mixture. Ammonia levels, however, would have to reach 20 to 25 per cent for an explosion.
Plumb says that if ammonium nitrate was also stored on site, this too may then have been ignited by a larger ammonia blast.
Some of the worst chemical plant accidents in history have involved similar facilities. In 1921, 561 died after 450 tonnes of ammonium nitrate exploded in Oppau, Germany. In the US, the worst incident was in Texas City in 1947, when 581 died after ammonium nitrate exploded during loading onto a ship.
Given this history, other experts contacted by New Scientist questioned why West, a town of 2800, wasn't evacuated as soon as smoke was detected. "If not then, why not once the fire was detected prior to the explosion," says Joe Eades of the Ispahan Group, based in Singapore. Eades recently warned of complacency over safety measures in the construction of chemical plants in Asia.
As the investigations continue, it emerged that the US Environmental Protection Agency fined the same facility $2300 in 2006 for failing to have a risk management plan meeting federal standards.
The American Institute of Chemical Engineers rejects any suggestion that last week's blast was the result of slack attention to safety. "Standards only continue to become more rigorous, in the US and around the world," said Scott Berger, executive director of the institute's Center for Chemical Process Safety.
But it also emerged last week that worksite safety inspections will soon be reduced because of government cutbacks that will affect the US Occupational Safety and Health Administration. The organisation admits that it anticipates conducting 1711 fewer worksite visits in 2014 than in 2012, when it inspected 40,961 plants.