(Reuters) - The U.S. Environmental Protection Agency (EPA) on Thursday issued an emergency waiver to help alleviate fuel shortages in four states where the supply of gasoline has been impacted by the shutdown of the Exxon Mobil Refinery in Joliet, Illinois.

Exxon Mobil ( XOM ) shut down its 251,800 barrel-per-day refinery at Joliet, Illinois, last month due to a power outage after a storm. Although power was restored to the refinery by July 24, it is likely to take the company at least a few weeks to restart the facility.

The EPA's waiver, which applies to Michigan, Wisconsin, Indiana and Illinois, suspends federal rules requiring the sale of low Reid Vapor Pressure gasoline during the summer months.

Higher RVP fuels evaporate more easily and are linked to ozone pollution and smog.

The waiver applies through Aug. 20, the EPA said. It comes in response to requests from governors of the four states.

Wholesale and retail gasoline prices have surged in the Chicago market as a result of the refinery outage. The Chicago market is the main hub for trading of refined products in the Midwest. [PRO/U]

Gasoline stockpiles in the U.S. Midwest declined by 2.3 million barrels over the past two weeks to 45.81 million barrels by July 26, the lowest since November.