In its annual report on its suppliers’ efforts in worker safety, labour rights and environmental impact, Apple said the explosions at facilities operated by Hon Hai Precision Industry -- better known by its trade name, Foxconn -- and Pegatron last year were traced to small airborne particles.

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

Other reports had put the injured tally at the latter explosion at 61. According to the Apple report, both explosions involved combustible aluminium dust.

Reports at the time of each explosion had identified aluminium dust -- produced while milling and polishing the casings of such Apple products as its iMac, MacBook and iPad -- as the likely culprit.

Interviews with air-quality experts last May after the Foxconn explosion also pointed to combustible dust.

Very fine particles of all kinds of materials can trigger an explosion, said Brian Edwards, director of engineering at Atlanta-based Professional Engineering, in an interview at the time.

"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, using a more technical term for an explosion. "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 dust explosions in the US

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

Edwards speculated 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.

Such explosions can be massive, he noted. "In a dust explosion, you typically see structural damage," Edwards said.

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. "We have established new requirements for handling combustible dust throughout our supply chain," said Apple.

The Apple report can be viewed from the company's website