PARAMOUNT (CBSLA.com) — A metal finishing facility in Paramount was ordered by air quality regulators to shut down all processing equipment and operations that could emit a toxic compound, the fourth time it has been ordered to do so this year.

Aerocraft Heat Treating Co., Inc. at 15701 Minnesota Ave., was ordered by the South Coast Air Quality Management District to curtail such operations by midnight Thursday because of elevated levels of hexavalent chromium in the air.

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“The threshold, agreed to by Aerocraft, was specified in an administrative order adopted by the independent SCAQMD Hearing Board on Dec. 16, 2016,” Sam Atwood, the SCAQMD’s media relations manager, said.

“This is the fourth time since the order was adopted that Aerocraft has had to curtail operations due to exceeding the threshold in the order. Prior curtailments occurred from Jan. 19 to Jan. 27; Feb. 14 to Feb. 21; and March 2 to March 7.”

The company won’t be permitted “to resume operations of any equipment with the potential to emit hexavalent chromium until SCAQMD determines that the average of the three most recent samples, collected over about one week, are below 1.0 nanograms per cubic meter,” Atwood said.

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Along with the California Air Resources Board, SCAQMD also has, since Dec. 23, monitored air quality at six Paramount schools, where recorded average levels have been below 0.3 nanograms per cubic meter.

The SCAQMD found last fall that Aerocraft and another Paramount metal processing company — Anaplex Corp. at 15547 Garfield Ave. — “were the source of high levels of hexavalent chromium through exhaustive and fast-paced investigation” that included intensive monitoring in an industrial area of Paramount and emissions testing of equipment inside facilities.”

Hexavalent chromium is associated with lung cancer when inhaled over long periods, “typically years to decades,” according to Atwood.

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(© Copyright 2017 CBS Broadcasting Inc. All Rights Reserved. City News Service contributed to this report.)