Timeline of leaks since early 2017, according to a South Coast Air Quality Management District report. (SCAQMD briefing document)

Updated Feb. 3

The chemical is modified hydrofluoric acid and it's been used for decades at Torrance Refinery and Valero's Wilmington Refinery.

The deadly acid known as MHF is now under scrutiny by Southern California air quality regulators considering whether it should be banned or if the refineries should be required to install additional safety measures.

On Friday, the AQMD board voted 9-4 to hold off on giving guidance to staffers about whether it should pursue a ban of MHF or permit the refineries to add more safety measures. Instead, they told staffers to do further work with refineries and community stakeholders over the next 90 days and present a recommendation to the board's Refinery Committee.

Concern about the use of MHF has been heightened since an explosion that ripped through the Torrance Refinery in February 2015. It blew a 40-ton chunk of metal into the air that landed only five feet from tanks of MHF. Federal investigators dubbed it a close call that could have had deadly consequences.

The incident motivated neighbors, including some scientists with Ph.Ds, to start delving into research about what risk modified hydrofluoric acid represented to their families.

They looked specifically at whether its use is substantially less dangerous than its super-toxic predecessor, hydrofluoric acid. Turns out, it is not significantly different, according to conclusions of researchers at the South Coast Air Quality Management District.

Here are some basics:

WHAT IS MODIFIED HYDROFLUORIC ACID?

Modified hydrofluoric acid is a strong and potentially lethal acid used in the manufacture of high-octane gasoline. It is used at only two California refineries and several others nationwide. Torrance Refinery keeps 25,000 gallons on site, and Valero refinery in Wilmington keeps 55,000 gallons.

This form of hydrofluoric acid used by the two refineries has been "modified" with an additive intended (but not fully proven AQMD researchers say) to make the acid less likely to form a toxic traveling cloud if it spills.

Hydrofluoric acid that has not been modified is still used in far smaller amounts in various industrial settings like glass etching, metal cleaning, and electronics manufacturing. On occasion workers have been badly injured by coming into contact with even small amounts of the acid.

An explosion at what was then known as the ExxonMobile refinery in Torrance caused four minor injuries on February 18, 2015. (Photo by Daniella Segura/KPCC)

WHY IS IT CONSIDERED DANGEROUS?

Hydrofluoric acid, when exposed to the air, can create a toxic ground-hugging cloud that can move on the wind and injure or kill those in its path, causing damage to bones and tissue.

Here's one harrowing scenario: A golfball-sized hole in a tank could lose 1,000 gallons within two minutes, quickly becoming a vapor. Lethal concentrations of the acid could travel in a cloud for a few miles, according to a study. About 400,000 people live within three miles of the Wilmington and Torrance refineries.

After a public outcry about hydrofluoric acid at the refineries in the 1980s, the AQMD banned it in 1991. But the agency made some procedural errors in how it circulated state environmental quality documentation, and that caused the ban to be invalidated.

WHAT HAPPENED NEXT?

With the ban thrown out, the industry came up with a modified form of the acid to address safety concerns. Modified hydrofluoric acid has an additive intended (but not fully proven) to reduce the propensity of spilled acid to create a dangerous fog. Instead of forming a fog, the vapor would fall to the ground in droplets, at least in theory.

The first version of MHF had 30 percent additive, but that made the chemical unstable. Then the modification was later lowered to 7 percent, leading MHF opponents to dismiss it as being no safer than straight hydrofluoric acid.

WHAT SOLUTIONS HAVE BEEN PROPOSED?

The South Coast Air Quality Management District's scientific staff has been working with the industry, residents and environmentalists for the past two years to craft a new rule regarding MHF. They are looking at a phase-out over several years, or an increase in safety measures required for its continued use.

Staff is asking the AQMD governing board at its Feb. 1 meeting for guidance as to which option they prefer.

Banning MHF from two local refineries would be costly. The refineries claim it could cost them up to $900 million to convert refineries to new processes using sulfuric acid or some newer and less tested method.

The refineries and their unions have packed local hearings with employees to remind policy makers that hundreds of good-paying local jobs could be at stake if the chemical is banned. They say the supply of gasoline and jet fuel could be interrupted and the costs increased. Of course, the Torrance Refinery has been shut down before. It was offline for more than a year after the February 2015 explosion.

Meanwhile, residents who live near the refineries, environmentalists and others have also packed the hearings calling for a safer form of manufacturing gasoline, now that aging refineries are surrounded by thousands of residents.

BAN VERSUS SAFETY MEASURES

The safety measures under consideration could include heavy water showers to dissolve any escaping chemical cloud, tall barriers to contain a cloud, and backup safety systems like extra power systems to operate water pumps.

The refineries, their workers' unions, the Western States Petroleum Association, local chambers of commerce and a few charities that get donations from refineries support continued use of Modified Hydrofluoric Acid under an memorandum of understanding (MOU) that would add more safety measures. Those groups voiced concern over the potential loss of refinery jobs during a closure or retool.

Groups of Torrance and Wilmington residents, and environmental justice groups want a rapid phase-out because they don't believe MHF is significantly safer than Hydrofluoric Acid or that the chemical can be contained within the refinery walls if a serious release occurred.

Los Angeles County Department of Public Health also favors a rapid phase-out of MHF because its emergency response systems could be overwhelmed in a larger release, said toxicologist Katie Butler.

"The hard truth is I'm not sure our department would be prepared to respond to a large-scale disaster that has been forecast in some of the models," Butler said.

UPDATES:

Feb. 3, 10:30 a.m.: This article was updated with details about the AQMD vote, as well as more information about how either a ban or increased safety measures would work.

This article was originally published on Feb. 1 at 6:30 p.m..