Two explosions at facilities in China operated by Hon Hai Precision Industry – (Foxconn) and Pegatron last year were traced to small airborne particles of aluminium.

The Foxconn plant explosion in May 2011 in Chengdu, China, killed four and injured 18 others. Another explosion in December at a Shanghai factory run by RiTeng Computer Accessory, a subsidiary of Pegatron, injured 59.

Both companies were producing components for Apple Inc.

According to a report by Apple both explosions most likely involved combustible aluminium dust produced while milling and polishing the casings of such Apple products as its iMac, MacBook and iPad.

Very fine particles of a wide range of materials can trigger an explosion, said Brian Edwards, director of engineering at Atlanta-based Professional Engineering. "By creating fine particles or dust powder, you increase the surface area of the material, which with a spark, can cause a rapid exothermic reaction," said Edwards, "That decreases the minimum energy needed for ignition and speeds up the reaction."

Aluminium is more explosive than either sugar or grain dust, Edwards added, talking about two typical causes of past dust explosions.

"You have to the have the right concentrations of dust in the air, you need sufficient oxygen, and you need an ignition source.", said Edwards.

Edwards suggests that the cause of the Foxconn explosion was aluminium dust that had collected in the ductwork that drew off dust from polishing or grinding equipment, which in turn ignited dust in the actual workspace.

In its report, Apple said that it had worked with external experts to audit all its suppliers that handled aluminium dust, and with one exception, put new measures into place. The company has ensured several action items, including ventilation requirements, regular inspections of ductwork, and banning the use of compressed air for cleaning, a practice that makes more dust airborne. The company that has not yet complied will remain closed until it does so.