How clean is your favorite vacation spot?

"I don't swim in your toilet, so please don't pee in my pool." If you know anyone with a backyard swimming pool, you've probably seen that kitschy sign once or twice.

But if you've ever been to a beach after a heavy rainstorm, or swum near a heavily populated urban area, chances are, you were swimming in something akin to a toilet. According to the Environmental Protection Agency (EPA), some 10 trillion gallons of untreated wastewater wind up in popular swimming areas every year. Spillage from wastewater treatment plants, pet waste from people's yards, and numerous other unknown sources of pollution raise levels of illness-causing bacteria in beach water to unsafe levels, according to a new report from the Natural Resources Defense Council (NRDC). 

"Our beaches are plagued by a sobering legacy of water pollution," says NRDC senior attorney Jon Devine, one of the report's authors. That pollution, made up not just of untreated sewage, but also oil, trash, and fertilizers from yards and farm fields, can give you a roaring case of diarrhea (the most common side effect of swimming in dirty water), dysentery, pinkeye, and a host of other illnesses. On average, 35 million people get sick from dirty beach water every year.

By law, states receiving federal funds to maintain their beaches must test coastal waters for bacteria, Devine says. That includes states along the major coastlines as well as those whose coasts sit along the Great Lakes. NRDC uses that data to compile its annual report.

By far, Devine says, the most polluted beaches in the U.S. are in the Great Lakes region, an area that houses more than 70 percent of all combined sewers in the United States. A combined sewer collects sewage and storm water from a number of sources into one single pipe, and they get overwhelmed during heavy rains and extreme weather, dumping a lot of unsavory waste into the Great Lakes.

The cleanest beaches? Fortunately for vacationers, they're more spread out, giving you lots of options for clean beaches close to where you live or vacation. Each year, NRDC rates 200 of the most popular U.S. beaches based on bacteria levels and local public agencies' responses to excessive levels. The group awards five-star ratings to the beaches that violate health standards less than 5 percent of the time and in areas where public health officials test water more than once a week (the minimum EPA requirement) then notify the public promptly both at the beach and online when safety standards have been exceeded.

Here's this year's list of five-star-rated beaches:

Alabama: Gulf Shores Public Beach
Alabama: Gulf State Park Pavilion
California: Bolsa Chica Beach
California: Huntington State Beach, Brookhurst Street
California: Newport Beach, 38th Street, 52nd/53rd Street
Delaware: Dewey Beach
Maryland: Ocean City at Beach 6
Minnesota: Park Point Franklin Park/13th Street South Beach
Minnesota: Park Point Lafayette Community Club Beach
New Hampshire: Hampton Beach State Park
New Hampshire: Wallis Sands Beach at Wallis Road
Texas: South Padre Island (Town of South Padre Island)

If your favorite beach didn't make the list, look it up in NRDC's new searchable database of all 3,000 beaches the group analyzes. Enter the zip code closest to where you vacation and you can pull up information on all the public beaches in the area.

In general, no matter where you dig your toes into the sand, it's a good idea to be a good steward, and a smart swimmer, says Devine. A few key ways to do that:

• Clean up after yourself and your pets to prevent garbage and dog waste from getting into the water.

• If you're a boater, dispose of your sewage properly onshore.

• Choose beaches that are next to open waters or away from urban areas. They typically pose less of a health risk than beaches in developed areas or in enclosed bays and harbors with little water circulation.

• Look for pipes along the beach that drain stormwater runoff from the streets, and don't swim near them.

• Avoid swimming in beach water that is cloudy or smells bad.

• Avoid swimming for at least 24 hours after heavy rains, which can wash pollution into the water).