Most of us know slathering on sunscreen is a must before hitting the beach. But for one-third of Americans, sunscreen is just an unused bottle of gloppy goo that lives at the bottom of a long-forgotten beach bag.

Skipping this skin-saving step can lead to a red, itchy sunburn or even skin cancer. And for some people, spending too much time in the sun sans sunscreen can have another immediate consequence… sun poisoning, or polymorphous light eruption, as it’s called in science-speak.

The question is… what is it?

Sun poisoning goes beyond your typical, run-of-the-mill sunburn and often includes a large rash, blisters and other symptoms such as fever and nausea.

People seem to be more susceptible to these severe sunburns when their bodies are less accustomed to sunlight. If you’re spending your winter bundled in parkas and take a January sojourn to a more tropical locale, your sun-deprived skin but not be able to handle the heat. People with fair skin are at a greater risk, as are folks taking medications that make them sensitive to light, such as antidepressants.

After several days, the symptoms of sun poisoning usually go away. But see your doctor or seek emergency care if you’re in serious pain or if your symptoms don’t get better.

Of course, the best medicine for sun poisoning is prevention. The sun’s rays pack the biggest punch between 10 a.m. and 4 a.m., so limit the time you spend outside then.

And don’t forget the sunscreen.