Apple has confirmed that explosions last year at Chinese plants of two of its parts suppliers were caused by excessive aluminum dust.
In its annual report on its suppliers' efforts in worker safety, labor 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 dust -- specifically, aluminum dust.
Reports at the time of each explosion had identified aluminum 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."
Aluminum is more explosive than either sugar or grain dust, Edwards added, talking about two typical causes of dust explosions in the U.S.
"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 aluminum 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.
Christopher Haase, director of environmental, health and safety at Environmentally Sensitive Solutions, a Milwaukee, WI-based firm that develops industrial cleaning products, said that Chinese factories badly lagged behind American plants in their dust-explosion prevention methods, in part because government censorship prevents managers and specialists from getting the information they need to create safer workplaces.
"The last few years, the Great Firewall means they can't talk to others, or complain about the work environment," said Haase. "They're asking for help, but they have to use proxies and things like Hotmail to open a dialog with Western experts."
The "Great Firewall" is the name many have slapped on the Chinese government's attempt to control what its citizens are able to access on the Web.
In its report, Apple said that it had worked with external experts to audit all its suppliers that handled aluminum 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 company ticked off 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.
"All suppliers except one have implemented the counter-measures identified by the team of external experts," Apple said. "The one supplier that has not will remain shut down until modifications are in place."
Apple did not identify the laggard.
The Apple report can be downloaded from the company's website (download PDF).