The failure to measure its sulfur content in some gas streams generated by burning natural gas took its toll on the Exxon Mobil oil company, as officials from the Environmental Protection Agency (EPA) and the US Justice Department announced on Wednesday that the firm would have to pay in excess of $6.1 million in civil penalties to the state, for breaking the rules specified in the Clean Air Act, a court-approved decision.

Refinery furnaces went on without supervision on behalf of Exxon, which thus broke a 2005 settlement that it had with federal officials, which required a constant monitoring of the sulfur levels emitted at all the plants the company operated.

Exxon defended itself by saying that the deposits it had found in Texas were not included in the 2005 agreement, and that their properties were only being assessed, during a corporate internal audit. Its press release also says that it made all of the findings known to the Agency, and that all streams of natural gas it now operates are securely within federal operating standards.

"The 2005 settlement has already resulted in major reductions in air emissions from the company's refineries, but we need full compliance to realize all the benefits of the settlement," EPA's Office of Enforcement and Compliance Assurance assistant administrator, Granta Nakayama, said in a recent communicate.

The agreement, which was reached three years ago, threatens Exxon with having to pay $7.7 million in civil penalty, a further $6.7 million in environmental investments at sites around its refineries, as well as with having to install costly carbon-reducing technology at six of its US refineries, in order to bring them into full compliance with federal norms.

Exxon Mobil is the largest oil and gas corporation in the world, having totaled a $404.5 billion in revenues for the fiscal year of 2007 alone. Last year, it drew a net income of almost $40 billion, making it number one in the world. Over the years, the company has been responsible for a number of catastrophic oil leaks, including the Exxon Valdez and Exxon Brooklyn, and is known and criticized for having paid as much as $16 billion to groups claiming that global warming was not associated with the burning of fossil fuel.