"When in doubt, don't go out." This classic lifeguard advice applies to big surf and strong currents, but also, these days, to waterborne pathogens that can cause gastroenteritis, rashes, ear, nose and throat infections, and (in rare cases), more. The primary source: storm water runoff after heavy rainfalls that bypasses treatment plants and sweeps human and animal waste into the sea. Last year, runoff caused more than 10,000 beach closures and advisory days. Next highest source: sewage spills and overflows. According to a report released this week by the Natural Resources Defense Council (NRDC), "our nation's beaches continue to suffer from serious water pollution that puts swimmers at risk."

Nationwide, more than 20,000 beach closure and advisory days were logged in 2007, the second-highest number in the 18 years since NRDC began their annual reports. What's even scarier is that, because monitoring frequency, standards and reporting vary widely from state to state, you might not find out that the water was contaminated until days after you swam in it! 

For the sake of mental as well as physical well-being, here are some tips for ensuring that you and yours don't take the polluted plunge.

* Swim at beaches with good water quality records. For the first time, NRDC includes a five-star rating guide for several popular beaches nationwide. There are also national programs that certify clean beaches; for more info, click here.

* Watch the weather leading up to your day at the beach. If it's rained a day or two before, ask the local health department if the water is safe for swimming.

* Use your eyes, nose and good judgment. If there's rafts of trash on the beach beneath the high tide line, it's likely the water's not too clean. Advisories or not, if the water looks cloudy, dirty or murky, and/or smells bad, pack up and try another beach.

* Swim at beaches with lifeguards. While they're not there to rescue you from pollution, they'll be the first to know, and let beachgoers know, if conditions are unhealthy.

* Take care not to swim with open cuts, sores or bad scratches and scrapes.

* When on vacation in a new place, think twice about diving into the ocean or into natural, freshwater streams, pools or ponds, no matter how clean and inviting the water may appear. Waterfall pools in Hawaii, for example, often harbor dangerous pathogens.

For more information, see the new U.S. Environmental Protection Agency (EPA) beaches report, also released in July, and click your state on the map.

The surf zone also gets trashed; see the Surfrider Foundation's State of the Beach report and Clean Water Initiative.  

A nice roundup of swimmability and other water safety tips can be found at Earth911

Remember, when in doubt, don't go out!  And wear a nontoxic green sunblock.

This article originally appeared in Plenty in August 2008. The story was added to MNN.com.

Copyright Environ Press 2008