The Torrance refinery explosion released material in the air that could irritate nearby residents' skin, eyes and throats.
Exxon Mobil officials say fine dust from the large blast contained inorganic material used in the refinery's processes. The light dust contained some metal oxides and amorphous silica, a clear or gray odorless silicon dioxide powder, Exxon Mobil spokesman Todd Spitler said.
Exxon Mobil and the Torrance Fire Department tested the dust and determined it was not toxic but warned about potential symptoms.
"The material is not expected to be hazardous to people or animals under the conditions it was released," he said. "However, it may cause irritation to the skin, eyes, and throat."
A portion of the refinery remains closed following Wednesday's explosion that injured four contractors, rattled nearby homes and forced students at 14 schools to remain indoors. The contractors suffered minor injuries and have been released from the hospital.
Still, the blast was felt by the industry and consumers as it immediately resulted in a surge in wholesale and retail gas prices.
"We are diligently working to ensure continued supply to our customers," Spitler said.
The blast prompted the California Division of Occupational Safety and Health to order an immediate shutdown of the affected unit. Officials say the electrostatic precipitator -- a filtration device that removes fine particulates – exploded.
The refinery has accumulated more than $100,000 in penalties for equipment and safety violations over five years of continuous inspections, according to the Cal/OSHA records released Thursday.
State inspectors cited the refinery in November for 25 safety and equipment violations, according to the records.
The citations stem from 15 routine visits state inspectors made from May through November and centered on the facility's Crude Unit and Coker Unit, where a worker was killed in 2009.
The units are not believed to be involved in Wednesday's blast.
State records show that the citations issued in November range from minor incidents -- such as failing to notify a crane operator about potential hazards -- to serious ones, including failing to inform employees about the presence of asbestos or establishing emergency procedures related to respirators.
Spitler said that after an extensive review of the initial findings, Cal/OSHA cited the refinery for two serious violations — failing to maintain piping insulation and missing a guard on a cable winch — and 12 general violations.
He said the refinery had addressed all of the serious citations.
A general citation is defined as a violation that affects the safety or health of employees, whereas a serious citation is defined as one that could result in serious injury or death.
In one incident dated September 2011, the company was cited for not tracking accidents that may lead to major accidents. In one case, the company did not report an accident until five months after it happened.
The following year, a coker drum operator was injured when a 760-pound steel guide plate dropped on his left foot during a rigging operation. The worker was unqualified to perform rigging operations.
The company was fined $47,000 in that incident.
The company appealed all of the citations, according to state records. The status of those appeals is unknown.
Cal/OSHA spokeswoman Erika Monterroza said the number of violations is sometimes a reflection of the refinery's size.
"The quantity of citations over a five-year period for this type of facility is not out of the norm," Monterroza said. "Refineries are large facilities with many operating and interconnected units."
None of the citations issued since 2010 appear to be related to the plant's electrostatic precipitators where state officials believe Wednesday's explosion may have occurred.
Standing 12 stories high and weighing more than 1,000 tons, the electrostatic precipitators reduce ammonia and particulate emissions.
The massive devices were installed to comply with environmental regulations imposed by the South Coast Air Quality Management District, according to company documents.
Although the sprawling facility remains operational, Cal/OSHA has shut down the fluid catalytic cracker, the larger unit that houses the affected equipment, citing hazardous conditions.
Spitler said the company is cooperating fully with the appropriate agencies to assess the impact, determine the cause of the incident and develop repair plans.
"We learn from all incidents and will use information from this incident to reinforce our commitment to continued improvement," he said.