When Torrance city officials agreed to let Torrance Refinery increase the strength of a toxic chemical used to manufacture gasoline, neither party informed the region’s air quality regulators.

That omission, in the late 1990s, and the work of local activists to dig up details of the deal and the hazard the chemical poses is partly why the South Coast Air Quality Management District will consider banning the toxin, said Philip Fine, deputy executive officer for planning and rule development.

It could take months for the AQMD board to vote on new rules that could eventually bar the use of modified hydrofluoric acid. Also known as MHF, it is used at two refineries in California: PBF Energy’s Torrance Refining Co. and at Valero’s Wilmington refinery.

At one point, the Torrance refinery was under a court order to dilute the chemical by 30 percent with an additive that would make it less dangerous to the surrounding community. Since that time, though, the refinery has pulled back to using only 10 percent of the additive with sign-off from the city of Torrance.

Torrance residents, unnerved by an explosion at the refinery in 2015,  were motivated by the near-disaster to research chemicals used there. They cheered AQMD’s decision to consider the ban on MHF.

“Most battles can’t be won by a small group alone,” said Sally Hayti, president of the Torrance Refinery Action Alliance.  “It’s not easy for a small group of volunteers with no funding to battle a wealthy multinational corporation and its supporters.”

The group  plans a Feb. 18 rally to support the end of MHF use in Torrance.

Research by Hayati and other local activists pushed the AQMD to consider a phase-out, Fine said.

“They brought to our attention that through a series of events that the district wasn’t previously aware of (that) the percentage of the additive to the MHF had been decreased over time, and they claim that did not afford the level of protection” Fine said. “And they have also provided evidence that even at higher levels of additive it may not provide a significant amount of protection.”

Fine said the material provided by the group, plus an independent study of MHF and its alternatives commissioned by the AQMD and industry information on the chemical would all be evaluated.

Two factors led the AQMD to propose the phase-out, Fine said. One was the February 2015 explosion at the Torrance Refinery during which an 80,000-pound chunk of equipment landed perilously near a tank of MHF. The U.S. Chemical Safety Board described that incident as a near-miss that could have caused a disastrous release of the MHF. Second, the plant was taken over in July by new owner PBF Energy, a New Jersey company, which  has had to deal with some start-up glitches such as plant shut-downs, a fire and flaring at the refinery.

“That made us start thinking about some of the potential safety issues that may occur if there is going to be all these repeated events,” Fine said.

The change in formulation occurred in the late 1990s when the refinery was owned by Mobil Oil Corp. and being sued by Torrance over the hazard posted by undiluted hydrofluoric acid, which can form a vapor cloud that could travel into the surrounding community killing and injuring those who come in contact with it.

In a consent decree the company agreed to dilute the hydrofluoric acid with an additive that it said would keep the chemical from forming a vapor cloud if it escaped the plant.

Initially, the additive was supposed to be 30 percent of the chemical mix, but in the late 1990s the city agreed to a company request to lessen the diluted formula to only 10 percent of additive.  The decision was made in a closed session of the Torrance City Council and approved by a unanimous vote, said Mayor Pat Furey in a January interview. He said the city has declined to make documents from the closed meeting public.

Years later, with modified hydrofluoric acid once more stoking local concerns, the AQMD looked back into its documentation and found no record of having signed off on the change, Fine said.

Modified hydrofluoric acid contains an additive (the contents of which are a trade secret) to suppress the volatility of hydrofluoric acid. Four refineries in the United States use MHF, according to a report commissioned by the AQMD.

PBF Energy will likely push back on any move to bar MHF, said Western Region President Jeffrey Dill.

“We believe hydrofluoric acid is really actually the best technology for us to be using, not only now but going forward,” Dill said. He described the refinery’s alkylation unit where the chemical is used as the best and most environmentally responsible technology for the refinery to be using.

Despite the refinery’s shutdowns, flaring and even a fire since PBF Energy took ownership in July, Dill said there had been no “offsite or no community impact from the MHF acid unit for certainly as long as we’re aware of.”

The AQMD should not be surprised that the more concentrated form of MHF is being used, said PBF Energy spokeswoman Gesunia Paras in an email statement Friday.

“It is our understanding based on comments we have heard that the South Coast Air Quality Management District signed a Memorandum of Understanding with Valero, agreeing to 6 percent at their facility,” Paras said. “This demonstrates that SCAQMD has approved use of the additive at rates lower than 30 percent in the past.”

A spokeswoman for Valero’s Wilmington Refinery did not respond to a request for information about the concentration of additive in the MHF used there.