Updated

The largest U.S. oil refinery violated federal air pollution laws thousands of times during the last five years, releasing 10 million pounds of illegal pollution, including cancer-causing toxins, without facing proper fines or being forced to fix equipment, environmental groups claim.

Exxon Mobil Corp., which owns the refinery, is the latest target of Sierra Club and Environment Texas, which recently forced Shell into a $5.8 million settlement over its Clean Air Act violations and has filed a lawsuit against Chevron Phillips.

The environmental groups have not yet sued Exxon but have notified the Irving-based company, the Environmental Protection Agency and the Texas Commission on Environmental Quality of plans do so — a requirement under the Clean Air Act.

The Associated Press obtained copies of the groups' two 60-day notices, which outline violations Exxon measured and reported itself. Among other complaints, the notices accuse Exxon of violating emissions limits on sulfur dioxide, one of the components of acid rain; hydrogen sulfide, a toxic, flammable gas characterized by a rotten egg smell; cancer-causing agents such as benzene and butadiene; carbon monoxide; and the smog-causing agent nitrogen oxide.

The environmental groups' legal maneuvers are part of broader accusations by the organizations and the U.S. Environmental Protection Agency that Texas regulators are failing to properly monitor, control and enforce federal emission standards.

Exxon reported all of the incidents and claimed in some cases they were "not considered deviations" because they did not violate the limitations in the air operating permit for the refinery, which is in Baytown, about 35 miles southeast of Houston.

Exxon confirmed in a statement Wednesday that it had received a notice of the groups' intent to sue. The oil giant said it works closely with the EPA and Texas regulators to control and report emissions from the Baytown refinery and petrochemical plants. It said it had reduced benzene emissions by 70 percent between 1990 and 2008, as well as significantly reducing other pollutants in the past decade.

"We continue to make significant improvements in the environmental performance of our Baytown complex through emissions controls, technology enhancements and process changes," the statement said.

Sierra Club and Environment Texas hope that by investigating suspected violations by the dozens of refineries and petrochemical plants that line the Houston Ship Channel and filing suit against polluters, it will force the companies to act responsibly and push the Texas Commission on Environmental Quality to more closely monitor and maintain pollution standards, Neil Carman, a Sierra Club chemist who is spearheading the investigations and lawsuits, said.

"The TCEQ is not enforcing the Clean Air Act in Texas and these cases are clear evidence of the agency's failure in carrying out its mission in protecting public health in Texas," Carman said.

The Texas commission declined to comment specifically on the pending litigating but said in a written statement that it has an effective enforcement program that uses corrective actions when necessary.

"The TCEQ believes that enforcement is not a goal, but one tool among many available to protect the environment and people," the statement said. "The agency pursues strong and vigorous enforcement."

The agency also said it has been issuing more fines in recent years, increasing from $11.3 million in 2007 to $23 million last year.

About 70,300 people live in Baytown, which is home to the Exxon refinery and two accompanying petrochemical plants. The refinery, founded in 1919 with 100 employees, now has 4,000 workers on a complex stretching across five square miles. It can process nearly 570,000 barrels of crude daily.

The environmental groups' letters to Exxon and regulators are official notice that a lawsuit will be filed if a deal is not reached with Exxon to pay millions of dollars in fines and update equipment causing the thousands of incidents at the plant.

The letters were dated Nov. 30 and July 2 and show nearly daily violations and excess emissions reported by the company itself from 2005 until this year.

In one case, Exxon noted a roof drain failed but said emissions had not been calculated, as required. In other instances, the plant reported a small fire or failure to monitor pressure readings during a procedure. In a 20-day period in 2009, the company reported spills of nearly 50 gallons of different types of oils. In 2008, it reported it exceeded 24 hour averages of ammonia five consecutive times. In 2006, it failed to monitor certain pressure readings during a performance test and under corrective action listed it was talking to the Texas Commission on Environmental Quality about excluding such readings from the requirements.

"Based on available information ... Exxon Mobil has repeatedly violated, and will continue to violate, its air operating permits, the Texas State Implementation Plan and the federal Clean Air Act by emitting air pollutants into the atmosphere from the Baytown complex," attorney Joshua Kratka wrote in the letter dated Nov. 30.

The sides have met since the November letter was issued but have not yet reached an out-of-court agreement. Kratka's letter stated that any lawsuit would cover the five years preceding the letter and any incidents after Nov. 30.

Sierra Club and Environment Texas reached a $5.8 million settlement with Shell in April 2009 after filing a similar lawsuit against that petroleum giant. Shell also is required to make several costly fixes to its Houston-area facility to ensure it meets standards.

A similar lawsuit filed against Chevron Phillips Cedar Bayou plant in Baytown, Texas, is ongoing.