Here's something you probably didn't know about sunscreen: Those SPF numbers on the bottles, the ones that stand for "sunburn protection factor," the ones you think you're taking full advantage of by diligently slathering, are based on studies that measure a different amount of lotion than most people actually use. In reality, the majority of boaters apply one-half to one-quarter of the "tested concentration" of any sunscreen.

Here's something else you probably didn't know: Some studies show that skin damage can occur with just one-tenth of the solar exposure required to produce a sunburn.

So you're using sunscreen, but not enough, and you're injuring your skin, but you don't know it. Wait – did I mention the rashes?

UVA VERSUS UVB

Sunburn is just one form of solar injury. Others include photosensitivity, which causes a reaction similar to a drug allergy rash on parts of the skin exposed to the sun, or an exaggerated sunburn; eye damage, which can combine the pain of a burned cornea with the cloudy existence of a world seen through cataracts; photo aging (80 percent of visible aging can be attributed to sun exposure); and skin cancers, including malignant melanoma.

The twin villains in this horror production are UVA and UVB, two forms of ultraviolet rays, explained Dr. Michael Jacobs, a specialist in internal medicine, co-author of "A Comprehensive Guide to Marine Medicine" and a lifelong boater.

UVA remains fairly constant throughout the day and plays only a minor role in sunburns – which is why SPF numbers currently refer only to UVB rays – but UVA is the type of ray responsible for those nasty photosensitivity rashes, which can be triggered by cosmetics or antibiotics that make your skin more sensitive to the sun. Yes, you read that correctly. Taking antibiotic – or using certain cosmetics, creams and even sunscreens – can greatly enhance your odds of suffering a solar injury. Both UVA and UVB contribute to skin aging, eye injury, and skin cancer.

"Every landscaper where I practice medicine on Martha's Vineyard gets Lyme disease," said Jacobs, who spoke at the week-long "Medicine for Mariners and Safety at Sea" conference at The Bitter End Yacht Club on Virgin Gorda in January. "I have to avoid certain antibiotics because these guys are out in the sun, and they'll get more intense sunburns."

UVB, while innocent in the photosensitivity department, is the culprit when it comes to regular sunburns. And unlike UVA rays, UVB rays do vary in intensity. They are strongest between 10 a.m. and 2 p.m. You're rarely protected if you live the boating lifestyle, especially during peak hours. You can wear a baseball cap, but UVB rays will reflect off the waves and your boat's deck. You can jump into the water, but 80 percent of ultraviolet radiation penetrates at least 12 inches beneath the surface. A wet, white T-shirt is a sieve that lets 50 percent of ultraviolet rays through. And if you consider salt spray to be nature's own refreshing cool-down, consider that when your skin is wet, it allows even better ultraviolet penetration than when it's dry.

PROTECTING YOURSELF

Prevention, in the case of solar injury, is generally the cure. To fully protect yourself against UVA and UVB rays, you need to know that UVA wavelengths are 315 to 400 nanometers (nm), and UVB wavelengths are 280 to 315 nanometers. Many common sunscreen ingredients cover parts of each spectrum, but not the full 280 to 400 nanometers for both. If the label reads "benzophenone," you're getting 250 to 350 nm protection; with "oxybenzone," it's 270 to 350 nm; and Octyl Methoxycinnamate offers 290 to 320 nm. Remember, those super-high SPF numbers only refer to UVB. Look at the sunscreen's ingredients to determine what kind of protection you're really receiving against UVA and UVB combined.

Barring a day spent on the boat wearing dark, tightly-knit fabrics, or special sun protective clothing, your best bet is to look for sunscreen containing ecamsule, which covers you from 290 to 400 nm. It's relatively new in the United States, having earned Food and Drug Administration approval in 2006. Look for it under the name Anthelios Sx, from the La Roche-Posay company.

Another good ingredient to seek out is helioplex, which offers equal UVA protection to Anthelios. Neutrogena Ultra Sheer Dry Touch SPF 45, recommended by Consumer Reports, comes in a "with helioplex" formula at most drugstores. That means you have the high SPF for UVB rays plus the helioplex to block UVA.

You also can go with opaque tried-and-true sun blockers such as zinc oxide, the stuff most frequently seen in thick white gobs on beach bums' noses. "Surfing stores tend to have the best products for your lips and face," Jacobs says. "A brand like Sex Wax may not be familiar to boaters, but it is an opaque barrier, and it works."