US beaches can be dirty places, making about 3.5 million people sick each year from sewage in the water, said an annual study Wednesday that rates American beaches by how dirty they are.

The Natural Resources Defense Council report included 3,000 beaches nationwide and listed 15 "repeat offenders" that have turned up again and again in the pollution rankings.

Those included four beaches in Louisiana, Avalon and Doheny State Beaches in southern California, and select waterfront beaches in Ohio, Illinois, New Jersey, New York and Wisconsin.

Overall, the total number of beach closures and water quality advisories nationwide last year was the third highest in 22 years of monitoring, said the study.

"America's beaches are plagued by a sobering legacy of water pollution, including bacteria-laden human and animal waste," said NRDC Water Program Director Steve Fleischli.

"So when people dive into the ocean, it can make them sick with a range of waterborne illnesses including stomach flu, skin rashes, pink eye, ear nose and throat problems, dysentery, hepatitis, respiratory ailments, neurological disorders and other serious health problems."

Louisiana tallied the highest number of dirty samples (29 percent) that exceeded national standards for designated beach areas in 2011.

The southern state famed for its seafood and music and whose shoreline was among those sullied by the BP oil spill in 2010 ranked last out of 30 states measured for beachwater quality.

However, since the ranking was based on bacteria sampling in the waters, the problem was not solely attributable to the devastating oil spill, the NRDC said.

Instead, the main problem across most of America's beaches is polluted storm water runoff -- with more than 10 trillion gallons of untreated storm water making its way into surface waters each year, according to US Environmental Protection Agency estimates.

"The biggest known cause of beach closings and swimming advisories is polluted storm water runoff," said Fleischli.

"When it rains, the water carries trash, chemicals, oil, animal waste -- you name it -- off the paved streets of our communities into sewers and ultimately to our beach."

The environmental group appealed for tougher EPA standards on beach water quality and improved measures to halt runoff pollution and sewer overflows.

"EPA believes it is acceptable for one in 28 swimmers to become ill with gastroenteritis from swimming in water that just meets its proposed water quality criteria," said NRDC.

"This risk is unacceptably high and is not protective of public health."

As many as 3.5 million people get sick from coming in contact with raw sewage from sanitary sewer overflows every year, the EPA says.

The NRDC also listed the best beaches for water quality. The top beaches were in the northeastern states of Delaware (Dewey Beach) and New Hampshire (Hampton Beach State Park and Wallis Sands Beach).

Other five-star ratings went to Newport Beach in California, Ocean City Beach 6 in Maryland, and Gulf Shores Public Beach in Alabama.

No Florida beaches made the top 12 that received the five-star rating.