In its final report on the explosion, the U.S. Chemical Safety Board (CSB) said ongoing releases of sugar from poorly designed and maintained dust collection equipment, conveyors, and sugar handling equipment led to the blast.

Inadequate housekeeping allowed highly combustible sugar dust to build up throughout the plant’s packing buildings.

A conveyor had been enclosed, creating a confined, unventilated space where sugar dust could accumulate to an explosive concentration. It’s likely the dust was ignited by an overheated bearing.

The initial explosion caused a cascade of secondary dust explosions in adjacent packing buildings.

On top of these problems, the CSB said Imperial hadn’t conducted evacuation drills for its employees and that the explosions and fires disabled most of the emergency lighting, making it difficult for workers to escape.

A 2006 CSB study identified 281 combustible dust fires and explosions between 1980 and 2005 that killed 119 workers and injured 718, and extensively damaged industrial facilities. In April, OSHA started development of a combustible dust standard. The National Fire Protection Association (NFPA) has recommended practices for preventing dust fires and explosions.