Air quality regulators on Friday killed a years-long push for stronger regulation of a dangerous acid used at two South Bay refineries that has frightened many neighbors, voting instead to accept a voluntary, oil industry pledge to enhance safety measures.

The decision by the South Coast Air Quality Management District governing board came just one week after the two refineries, in Torrance and Wilmington, offered a way to avoid tougher restrictions. They sent letters offering to install improved safety systems in the coming years if regulators ended their pursuit of a rule or agreement to reduce the risk of a catastrophic release of modified hydrofluoric acid, also referred to as MHF.

The board adopted the refineries’ plan on an 8-3 vote, handing a major victory to labor and industry groups, which have fought community groups and regulators’ attempts to restrict the highly toxic chemical following a 2015 explosion at the Torrance refinery.

Hydrofluoric acid can form a deadly, ground-hugging cloud that could drift into the densely populated communities surrounding the two refineries, and could cause mass casualties in a major leak. About 400,000 people live within three miles of either the Torrance or Valero Wilmington refineries.

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