Exxon has also agreed to pay fines of US$8.7 million and spend US$9.7 million to upgrade city buses with low emission technology, restore coastal habitats in Louisiana and fund a range of environmental projects near their refinery sites.

The refineries targeted by the settlements provide 11% of US refining capacity and the public has been given 30 days to comment before the agreements are approved by the courts.

Under the terms of the agreement, annual emissions of sulphur dioxide are to be reduced by 42,000 tonnes and nitrogen oxides are to be cut by 11,000t, with technology upgrades to be installed to detect leaks, minimise hazardous gas flaring and cut pollution from sulphur recovery plants and benzene waste handling facilities.

ExxonMobil agreed to the settlement to avoid the government filing suit, allowing it to continue expanding fuel production in accord with the Clean Air Act.

According to Prem Nair, a spokeswoman for ExxonMobil, the company denies claims it violated any regulations and the settlement "is in the best interest of the company and supports the continued trend of emission reductions".

The settlement is the latest in a string of similar actions and means that almost 77% of US domestic refining capacity is now operating under a consent decree to enforce clean air laws.

"That will lead to reductions of more than 315,000 tons of pollutants annually from the 17 refining companies that have agreed to come into compliance," said Granta Nakayama, the US EPA assistant administrator at the Office of Enforcement and Compliance Assurance.

The ExxonMobil refineries covered by the settlement are in Baton Rouge, Chalmette, Baytown, Beaumont, Billings, Joliet, and Torrance.