According to following URL, the Foxconn Chengdu manufacturing plant
explosion yesterday was "due to a special dust, which is used to clean
the screens of the iPads. It is flammable. And someone was smoking."

http://micgadget.com/12651/breaking-foxconn-chengdu-manufacture-plant-explosion-video/

The first video in above link (also in following link) shows lots of
people running around at the plant, which employs about 100000
people and is Foxconn's 2nd largest. Some webpages refer to
magnesium dust catching fire, and others to aluminum dust being
common in the air at the factory.

http://gizmodo.com/5803963/would-the-foxconn-factory-explosion-further-delay-ipad-shipments
http://www.examiner.com/technology-in-national/sacom-report-presaged-chengdu-blast-health-and-safety-issues-alarming

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jiw