The Natural Resources Defense Council released a report card for the nation’s most popular beaches, and some of the results may surprise you.

Besides sunburn and sand in uncomfortable places, American beach-goers are also being exposed to bacteria and viruses that can cut a long holiday weekend short.

The Natural Resources Defense Council (NRDC) said stormwater pollution and sewage spills were the most common reasons for beach closings and advisory days, but not all beach authorities notified visitors of contamination and other problems.

The major offenders in coastal waters are bacteria and viruses—such as Giardia and Cryptosporidium—that can cause severe intestinal upset, diarrhea, dehydration, and even death in some people. These can enter a person’s body cavities if he or she swims in contaminated water.

“Children, the elderly, pregnant women, cancer patients, and those with weakened immune systems are most likely to get sick from swimming in contaminated water—they are also most likely to be seriously ill from exposure to waterborne illnesses,” the NRDC said on its website.

Last summer, beaches in the U.S. were closed for a combined total of 20,120 days.

The NRDC has created an interactive list that rates 200 of America’s most popular beaches along the coasts, the Great Lakes, and the Gulf of Mexico. You can check the status of the beaches nearest you by visiting the site here.

From Gulf Shores in Alabama to San Clemente State Beach in California, 13 beaches received five-star ratings based on their water sample scores, monitoring frequency, and whether or not they post advisories warning beach-goers about potential contamination.

Other beaches didn’t do so well, and the majority of them were near heavily populated urban areas.

Popular San Francisco destinations like Ocean and Baker beaches received one-star ratings because as many as 26 percent of water samples tested in 2010 exceeded national pollution standards set by the Environmental Protection Agency, and beach-goers weren’t notified of the potential dangers.

Other major offenders include beaches around the Great Lakes.

“In 2012, out of all the areas measured, the Great Lakes region had the highest percentage of monitoring samples that exceeded the Environmental Protection Agency’s health standards,” the NRDC said on its website. “Approximately one in every 10 samples taken in the region last year was more contaminated than EPA standards allowed.”

There are plenty of things besides a gut full of harmful bacteria that can leaving you dehydrated and feeling worse for wear this summer. Make sure to drink plenty of water while on the beach, especially if you’re consuming alcoholic or caffeinated beverages.

Of course, everyone should be mindful about sun exposure, but white men need to be especially careful. A recent study from the American Medical Association showed that while white women get melanoma skin cancers more often, young white men are 55 percent more likely to die from them.

Being proactive by regularly wearing sunscreen and reapplying it every two hours, avoiding the sun between 10 a.m. and 4 p.m., and wearing hats and other protective clothing are ways to prevent sunburn and long-term skin damage.

Playing it safe can help you get the most out of your summer and many more summers to come.