updated 12/14/2010 2:26:46 PM ET 2010-12-14T19:26:46

The largest oil refinery in the United States released more than 8 million pounds of illegal pollution in the past five years, violating the federal Clean Air Act thousands of times, according to a lawsuit filed Tuesday by environmental groups in Texas.

The lawsuit against Exxon Mobil is the latest by Sierra Club and Environment Texas as part of their campaign to rein in what they call "illegal emissions" by dozens of refineries and chemical plants that operate in the Texas Gulf Coast. In recent months, the groups have reached multimillion-dollar, out-of-court settlements with Shell and Chevron Phillips after filing similar suits.

Exxon Mobil denied the allegations and said it would fight the lawsuit. In the past five years, the Exxon refinery in Baytown has produced annual emissions that are 40 percent lower than the limits set by the U.S. Environmental Protection Agency, spokesman Kevin Allexon said in a statement. Since 2003, Exxon has invested nearly $1.3 billion in the Baytown facility to improve environmental performance, he added.

"Exxon Mobil works hard to operate within regulatory standards while continuing to make significant improvements in its environmental performance through emissions controls, technology enhancements and process changes," Allexon said. "We are proud of our investment in — and improvement of — environmental performance at the Baytown complex."

Texas has more oil refineries, chemical plants and coal-fired power plants that any other state and is the nation's leader in greenhouse gases. The state produces more than 20 percent of the nation's oil and one-third of the country's gas is refined along the Texas Gulf Coast.

The Associated Press reported in July that the environmental groups had issued Exxon Mobil with a required 60-day notice on their intent to sue.

The federal lawsuit filed in Houston accuses Exxon of violating emission limits on sulfur dioxide, a component of acid rain; hydrogen sulfide, a toxic, flammable gas characterized by a rotten egg smell; such cancer-causing agents as benzene and butadiene; carbon monoxide; and the smog-causing agent nitrogen oxide.

It says the "emissions events" are usually caused by equipment breakdowns and malfunctions. For example, in October, there were two such events in the same day, according to the lawsuit. The first occurred at 6:10 a.m. when a broken hose caused more than 15,000 pounds of toxic and cancer-causing chemical fumes to be released into the air in just 30 minutes. Later in the afternoon, an underground pipe began leaking, releasing more than 80 pounds of cancer-causing benzene before it was shutdown 28 hours later.

"Air quality in Harris County and especially along the Houston Ship Channel continues to be among the worst in the nation," Sierra Club chemist Neil Carman said. "Whether you're talking about high levels of ozone that make it difficult to breathe or about toxic and carcinogenic chemicals, industrial facilities like Exxon's Baytown plant are major contributors to the problem."

The legal maneuvers are part of the broader accusations by the environmental groups and the U.S. Environmental Protection Agency that state regulators are not properly monitoring and enforcing federal emissions standards.

State regulators say their rules decrease pollution, but are not so stringent that it becomes too expensive to operate in the state.

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