June 21 (Reuters) – Massive explosions that engulfed a Philadelphia refinery in flames on Friday has renewed concerns about the oil industry’s use of a highly toxic chemical to make high-octane gasoline at plants in densely populated areas.

Aerial video of the scene at Philadelphia Energy Solutions Inc’s refinery, the largest in the northeast, in Philadelphia on local television showed significant damage and the massive complex nearly engulfed in flames.

One of the explosions took place in a hydrofluoric acid alkylation unit a chemical processing unit that has been involved in three near-misses of releases into cities in California, Texas and Wisconsin, according to safety officials. Hydrofluoric acid (HF) can form a toxic cloud at room temperature and exposure can lead to severe health problems and even death.

Around the Philadelphia refinery, air quality has not yet been cited as a problem on Friday morning.

There have been campaigns by residents who live near refineries, environmental groups and the nation’s largest industrial labor union to end the use of HF in units that produce octane-boosting components for gasoline.

Fifty, or more than a third, of 135 U.S. refineries operate HF alkylation units, according to the Chemical Safety Board (CSB), which investigates industrial fires and explosions.

In April, the CSB called on the U.S. Environmental Protection Agency to revisit a study on HF’s use and the potential to replace it with another process after hearing from worried residents during its probe of a 2018 refinery fire.

“Refinery workers and surrounding community residents are rightly concerned about the adequacy of the risk management for the use of hazardous chemicals like HF,” the board said in that letter, which grew out of the CSB’s probes of fires at refineries in Wisconsin, California and Texas.

The United Steelworkers union (USW), whose members work at major refineries, launched a campaign in 2010 to end the oil industry’s use of HF. In 2013, the USW found 26 million people in the U.S. were at risk of HF exposure from a refinery accident. A USW spokesperson was not immediately available to comment on Friday.

The oil industry has rejected calls to convert HF alkylation units to sulfuric acid, which does not form a low-hanging cloud that can drift into neighborhoods, saying the cost would be too high and alternatives not well developed.

HF is risky because it forms dense vapor clouds low to the ground that can easily move into the communities around refineries. Because it destroys nerve tissue, burns may be initially painless. In high enough concentrations it can cause cardiac arrest.

Most of the 27,000 residents of Superior, Wisconsin, were evacuated in 2018 due to the risk of a release of HF following an explosion at Husky Energy Inc’s refinery, according to news reports at the time.

A 2015 explosion at a Los Angeles-area refinery, then owned by Exxon Mobil Corp, raised the risk that a storage tank containing HF could have been ruptured, according to the CSB.

The USW campaign began following a 2009 explosion on an HF alkylation unit at the Citgo Petroleum Corp refinery in Corpus Christi, Texas, that the CSB found likely released up to 4,000 lbs (1,814 kg) of HF. Much more than that was contained by water spray cannons around the burning unit.

Before the Corpus Christi fire burned out, the water supply system was nearly exhausted and hoses bringing additional supplies ruptured but were quickly replaced or repaired, the CSB found.

As of Friday morning, the CSB had not decided whether to send investigators to the Philadelphia fire.

(Reporting By Erwin Seba in Houston and Jessica Resnick-Ault in New York Editing by Marguerita Choy)

*Original article online at https://www.cnbc.com/2019/06/21/reuters-america-explainer-philadelphia-refinery-blast-puts-new-spotlight-on-toxic-chemical.html