How can a sunscreen be sweat-proof?

If your outdoor activities involve sweating or getting wet, you'll probably want to give some extra thought to sunscreen selection.

It seems counterintuitive that sunshine should damage people's DNA, considering we've passed our entire evolutionary tour de force happily bustling away with it beaming down on us. But without the proper protection, spending lots of time outside scoring a tan can have some serious consequences down the line.

Experts recommend slathering on a liberal dose of sunscreen about 30 minutes prior to setting out, and not just on sunny days -- ultraviolet (UV) rays are a menace even when it's cloudy. Unfortunately, if someone's planning to spend more than about two hours outdoors, that one application isn't going to cut it. Sunscreen starts to wear off after a while, and any swimming, sweating or toweling only exacerbates the issue.


Likely motivated by memories of particularly painful sunburns, people started cooking up commercial methods to block the sun's penetrating rays. Originally, this effort focused solely on the UVB part of the spectrum, but now sunscreens frequently protect against UVA wavelengths as well. (There are also UVC rays, but thankfully those aren't an issue -- they get scattered in the atmosphere.)

Seeking ways to further improve secret formulas, chemists started finding new ingredients to increase their products' effectiveness. Initially, sunscreens simply threw up a physical barrier that reflected or scattered incoming sunshine. Nowadays, however, some offer chemical protection instead, meaning they actually absorb sunlight before it infiltrates the skin.

In addition to getting a broad-spectrum sunscreen that's rated SPF 15 or greater, experts say it's a good idea to choose one that's labeled water-resistant or sweat-resistant too. Be wary of products claiming to be completely water-proof or sweat-proof, though -- if you see that on the side of a bottle of sunscreen, you can safely assume it's not true. No sunscreen is 100 percent impervious to the power of a good sluicing.

That being said, some sunscreens are definitely better at withstanding exposure to water or sweat than others. On the next page, we'll take a closer look at how they manage it, and how well they hold up.


Sunscreen Protection: Don't Sweat the Small Stuff

lobster sunburn
There are better ways to let your classmates or coworkers know you had fun on your vacation to the beach than to show back up looking like this.

The intensity of the UV rays a person will encounter -- and the subsequent crimson consequences -- depends on lots of factors like season, latitude, altitude, time of day, cloud cover and skin type. Plus, while trudging behind a lawnmower can give you a rosy pink glow, a day at the beach can leave you more on par with the unfortunate lobsters served up in oceanside surf and turf joints. That's because sand and water (along with concrete, snow and other surfaces) reflect sunlight back up, meaning it hits you coming and going.

To make matters worse, even if you started out with a good coating of sunscreen, if your time at the beach lasted longer than two hours and not a second was spent reapplying sunscreen, the chances of a bad burn typically shoot right through the roof. If your fun included activities like frolicking among the ocean waves or sprinting back and forth across a sandy volleyball court, things get even worse. Those sunburned scientists didn't give up, however, and continued to develop innovative new formulas for more advanced sunscreen products.


Different sunscreens achieve different levels of water resistance through a variety of means. In addition to ingredients that deal with UV waves like titanium dioxide, zinc oxide and oxybenzone, they also have ingredients that can endure an onslaught of water and sweat. For example, these agents may form an adhesive film that's particularly good at sticking to skin and staying in place. Sunscreens that have proven they're able to arm wrestle with water and sweat are generally labeled like this:

  • Water-resistant: The product can handle up to 40 minutes of exposure to water (or sweat) and still be effective.
  • Very water-resistant: The product can handle up to 80 minutes of exposure to water (or sweat) and still be effective.

If you do find a sunscreen labeled "water-proof" or "sweat-proof," however, don't believe the hype -- as we learned on the last page that's just not possible. Sometimes the term water-proof is used in place of very water-resistant, so be sure you know what to expect. Plus, even ones that are resistant should be reapplied after someone sweats or gets wet, just to be on the safe side. You can't be too careful when it comes to protecting your skin; the threat of cancer is very real.

For more information about everything under the sun, visit the links on the next page.


Lots More Information

Related HowStuffWorks Articles

  • Aust, Duncan. "Sunscreen Composition with Enhanced SPF and Water Resistant Properties." Free Patents Online. Feb. 13, 2007. (Dec. 21, 2009)
  • Boyles, Salynn. "FDA Wrapping Up Sunscreen Label Changes." WebMD. May 21, 2009. (Dec. 21, 2009)
  • FDA's Tanning Web site. (Dec. 21, 2009)
  • Singer, Natasha. "Do Sunscreens Have You Covered?" New York Times. July 5, 2007. (Dec. 21, 2009)
  • "Sunblock." University of California, San Francisco. May 4, 2007. (Dec. 21, 2009)
  • "Sun Protection and Sunscreen." University of Iowa. (Dec. 21, 2009)
  • "Sun Protection Products." The Cosmetic, Toiletry and Perfumery Association. (Dec. 21, 2009)
  • "Sunscreen: Answers to Your Burning Questions." Mayo Clinic. March 27, 2009. (Dec. 21, 2009)
  • "Sunscreen." WebMD. October 2008. (Dec. 21, 2009)
  • "Sunscreens Explained." The Skin Cancer Foundation. (Dec. 21, 2009)
  • "Sunscreens/Sunblocks." American Academy of Dermatology. (Dec. 21, 2009)
  • The Sun Safety Alliance Web site. (Dec. 21, 2009)
  • "What's the Difference Between a water proof and a water resistant sunscreen?" (Dec. 21, 2009)
  • Williams, D. F. and Schmitt, W. H. "Chemistry and technology of the cosmetics and toiletries industry." Chapman and Hall. 1996. (Dec. 21, 2009)