The 2005 settlement already required ExxonMobil to pay a $7.7 million civil penalty, perform an additional $6.7 million in supplemental environmental projects in communities around the company's refineries, and install pollution controls at six of its U.S. refineries.

"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," said Granta Y. Nakayama, assistant administrator for EPA's Office of Enforcement and Compliance Assurance.

The agreement penalizes ExxonMobil for failing to comply with the 2005 settlement at four refineries in Beaumont and Baytown, Texas; Torrance, Calif.; and Baton Rouge, La. Most of the penalties are for failure to monitor and control the sulfur content in certain fuel gas streams burned in refinery furnaces, as required by the 2005 settlement and EPA regulations. The other two refineries covered under the 2005 settlement are located in Joliet, Ill. and Billings, Mont.

Between approximately 2005 and 2007, ExxonMobil did not monitor the sulfur content in some fuel gas streams and subsequent testing revealed sulfur content in excess of EPA limits. The burning of sulfur-containing gases emits sulfur dioxide, which can cause serious respiratory problems.

The 2005 settlement and the penalty issued on Dec. 17 were reached as part of a broader EPA initiative to reduce air pollution from refineries nationwide,according to the EPA. To date, 95 refineries located in 28 states, representing  more than 86% of the nation's refining capacity, have been required to install new controls to significantly reduce emissions.