TORRANCE (CBSLA.com) — The explosion at ExxonMobil’s South Bay facility last week was caused by overpressurization in an air-pollution-control system, according to a report released Monday.

The blast occurred just before 9 a.m. Wednesday at a processing unit in the 3700 block o f West 190th Street in Torrance.

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The explosion, which injured four workers, was triggered by pressure in an electrostatic precipitator, which controls particulate matter emissions from a fluid catalytic cracking unit, according to the South Coast Air Quality Management District.

The AQMD noted in a report that the pollution-control mechanism is “a relatively new unit, permitted in December 2008 and has been in operation since 2009/2010.” The catalytic cracking unit was not in operation at the time of the blast and “was not being vented to the ESP at the time of the incident,” according to the report.

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“The cause of overpressure and explosion of the ESP is not yet provided and is under investigation,” the report stated.

Torrance residents reported ash falling from the sky that day which was later determined to be spent catalyst. Tests of the substance found trace amounts of chromium in addition to fiberglass and glass wool, the AQMD said. No asbestos was found.

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