Heading out to the beach this summer? Before you take a swim, check out the Natural Resources Defense Council’s (NRDC) new evaluation of environmental pollution—it reveals some pretty surprising (and dirty) secrets about our nation’s coastal waters, the Washington Post reports.

For the annual survey, the environmental group tested samples from 3,458 beaches across the country, including shorelines from the Atlantic and Pacific oceans, the Gulf of Mexico and the Great Lakes; turns out that nearly ten percent of beaches have so many pathogens in their waters that they are considered unsafe by the Environmental Protection Agency’s beach safety standards.

The NRDC says most of the contamination is from storm-water runoff that triggers sewage overflows that can contaminate beachside waves. According to the report, Lake Erie has the dirtiest beaches in the country, especially in Ohio, where more than 25 percent of samples exceeded pollution standards for the past five years. Malibu Beach in California also topped the list of offenders.

Among the cleanest beaches in the country were Dewey Beach and Swedes in Delaware, Point Lookout State Park and Assateague State Park in Maryland, and Virginia Beach and Black Bay Beach in Virginia. And while you’re at the beach, don’t forget to bring sun protection! Black skin does crack (and get skin cancer) if it’s not protected from sun damage.