Aug 1 (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.

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.