The massive Gulf of Mexico oil spill has not fouled the vast majority of the area's beaches but is still scaring tourists away, according to a report to be released Wednesday.

Almost all beach closures from the spill have been clustered in three areas of Louisiana, according to the 20th annual report on water quality at U.S. beaches by the Natural Resources Defense Council. The council is a non-profit, non-partisan environmental advocacy group that used government data for its "Testing the Waters" report.

But the spill, which has spewed as much as 184 million gallons of oil since it began April 20, is worrying tourists and prompting dozens of health advisories throughout the Gulf.

Closures or advisories have been issued for 49 of 253 monitored beach sites in Alabama, Florida, Louisiana, and Mississippi, the report says. Taken together for all these beaches, they cover a total of 1,972 days so far this year, whereas similar ones last year covered 237 days.

"We have not had any oil, any tarballs, anything within hundreds of miles of our beaches," said D.T. Minich, executive director of Visit St. Petersburg/Clearwater tourism authority. "But perception is the problem."

Already several conventions and an international volleyball tournament scheduled for the fall have canceled because of fears the area's beaches will be polluted by then, Minich said. "They're concerned about buying non-refundable airline tickets into a place where they are concerned there might be a problem in the fall."

"Tourists are being driven away by the specter of oil "mouse," tar balls, tar mats and even liquid oil on the sand and in the water," the report says.

"The oil spill has been a tragedy" that has hurt both the Gulf economy and its environment, says David Beckman, water program director of the Natural Resources Defense Council. "The impact varies widely from place to place."

Mississippi has been hit hard. It posted advisories for 16 of 20 beaches, covering 479 days, the report says. A beach was closed for a week earlier this month in Hancock County but others have remained open, says Robbie Wilbur of the state's Department of Environmental Quality.

Far fewer warnings have been issued in Alabama and Florida, and none in Texas, but officials take the spill seriously.

"We've never had oil on our beaches before," said Tim Hatch of Alabama's Center for Emergency Preparedness, which issued advisories for one-fourth of its beach areas.

"We strongly encourage people to avoid contact with the oil," said Susan Smith of Florida's Department of Health. Oil contains heavy metals that can cause skin and breathing irritation, as well as other problems, the report says.

In Panama City Beach, Fla., tourism tax collections rose 7.7% in May, before tumbling 4.6% in July, according to Dan Rowe, president of the convention and visitors bureau there. About half of all tourism dollars are spent during the three summer months, he said.

"The spill didn't really begin to impact us until late April," Rowe said. "Then in May, anecdotally, we were hearing that a lot of people were rushing to get a visit in here before the bad stuff hit the beaches — but it's never hit the beaches."

At Alabama beaches where warnings are posted, Hatch says there are fewer swimmers. But he adds: "People are still in the water."

Contributing: Dan Reed in Fort Worth