Click the green tab on the Layers panel to show or hide it.

The numbers and pins and data give you some information, but still don't convey the whole picture. For that, we turn to the NRDC's description of what it means when a beach exceeds national beachwater quality standards.

More than 80 percent of closings and advisories were issued because bacteria levels in beachwater exceeded public health standards, indicating the potential presence of human or animal waste in the water.

If a beach exceeded the national standard five percent of the time, that means that one out of every twenty samples had unhealthy levels of pollutants. Mostly fecal pollutants, that is.

And then we come back to that sickeningly low "one in 28" standard. We mean that literally. That standard is set at a level that allows 36 gastrointestinal illnesses for every 1,000 swimmers. So there's just enough pollution allowed to ensure that only 35 people out of every 1,000 get upset stomachs from ingesting human or animal waste. Says the NRDC: "Just imagine a restaurant where 1 in 28 people were allowed to get sick—these are simply unacceptable standards."

Remember, these data are from testing conducted in 2012. That's where the percent change map might be helpful. If you see a beach that got worse between 2011 and 2012, the change from 2012 and 2013 could be even worse still.

There's only one way to know how clean the water is: by testing it. That why we've included a map that shows the number of tests conducted per month. If a beach is tested daily, it's a decent bet that the water is safer to swim in; after all, dangerous levels of pollution (from, say, sewer run-off) would be detected sooner.

One final note. If you don't live near the coast, and perhaps think that the chlorinated water of a swimming pool is a better bet, you might want to think again. Nothing wrong with taking a cold shower and sitting in front of an air conditioner until autumn rolls around.

Photos: Competitors in the intentionally dirty New York Merrell Down & Dirty National Mud and Obstacle Series. (AP)

This article is from the archive of our partner The Wire.