A June 28 Health article about sunscreens incorrectly said that UVB, or ultraviolet B, waves "travel at short frequencies." It should have said that UVB radiation has a shorter wavelength and higher frequency than UVA radiation. (Published 07/06/05).

So you're ready for a day of relaxing at the beach, lounging by the pool or (poor you) mowing the lawn. You've got your SPF 30 slathered over every square inch of exposed skin, from the tips of your ears to the backs of your calves. You'll dutifully reapply in a few hours -- sooner if you get in the water or get doused in sweat. When it comes to protecting yourself against the sun's damaging ultraviolet rays, you've got yourself covered. Right?

Sorry. With growing certainty and trepidation, dermatologists, cancer docs and anti-aging gurus over the past decade have come to realize that available sunscreens may fight only half the battle, fending off the sun's short-wave UVB radiation. What's missing is an equally useful weapon against longer-wave UVA rays, whose deleterious effects have only recently come to light.

UVB, or ultraviolet B, radiation is the kind that causes sunburn -- and skin cancer. Its waves travel at short frequencies, and they don't penetrate beyond the skin's outer surface. It's easy to see when a body's been overexposed to UVB: Both sunburn and golden tans signal too much UVB.

UVA radiation, on the other hand, travels in long waves that skip right past the skin's surface, holding their fire for the collagen that gives skin its elasticity. Scientists have come to understand that while UVA damage isn't immediately visible, it takes a long-term toll on the dermis. UVA exposure is now thought to be a prime culprit in premature wrinkling. More unsettling (and please tell me you agree that this seems worse), UVA now is believed to alter skin cells' DNA, causing mutations that can, like changes wrought by UVB, lead to the big C.

Existing sunscreens are aimed almost exclusively at blocking UVB radiation. Because UVA's role has for so long been shortchanged, neither experts nor the sunbathing public has demanded otherwise.

But nowadays everyone -- the American Academy of Dermatology, the Food and Drug Administration (FDA), and, of course, sunscreen manufacturers -- agrees that the American consumer deserves access to a sunscreen that's as effective at filtering UVA as it is at taming UVB.

So why don't we have one?

FDA, UVA, UVB, SPF . . .

In the United States regulatory scheme, sunscreen ingredients are regarded as drugs and are thus subject to intense scrutiny. Drugs' active ingredients have to pass FDA muster both in terms of their long-term safety (which manufacturers have to demonstrate through research) and their efficacy: You have to prove that these things really do what you say they do. This degree of regulation is both a blessing and a curse. It's meant to ensure that when you buy a bottle of sunscreen, it'll be safe and effective. But the process of securing the FDA's blessing can be long and arduous, and it can keep a promising product off the shelves for years.

In Europe, sunscreens are treated as cosmetics and are thus subject to far less oversight. That's why Europeans have access to 28 active ingredients in their sunscreens, whereas Americans are allowed only 16. Of those 16, seven, including avobenzone, titanium dioxide and zinc oxide, are known to provide some protection against UVA rays.

In May 1999, the FDA drafted an updated "monograph" on sunscreens. (The monograph for a class of drugs outlines standards for ingredients, labeling and testing with which manufacturers must comply.) The sunscreen treatise, which was to become effective in 2001 (allowing time for public feedback and for manufacturers to bring their products into compliance), barely addresses UVA radiation but promises that the FDA will publish guidelines on the issue.

Matthew Holman, interdisciplinary sciences team leader with the FDA, notes that the agency and the manufacturers simply have lacked sufficient understanding of how UVA works, how its effects can be measured and how products can be accurately tested and labeled for their effectiveness against UVA. For now, manufacturers are allowed to claim "broad spectrum" protection if their product provides any amount of UVA protection. But Holman notes "the UVA range is quite broad compared to UVB," and "we don't know what the 'active spectrum' is. We can't correlate specific wavelengths to specific damage." Should consumers have access to information about how many minutes UVA protection a product confers, along the lines of an SPF rating? Or should they be told what range of UVA light the product protects against?

Sunblock Blueprint

After issuing the draft monograph, the FDA got lots of feedback from various companies, each of which had its own ideas about how to assess ingredients' utility in fighting UVA radiation. Once the FDA sorts through that feedback and decides which tack to take, the monograph -- including guidelines for both kinds of radiation -- will be finalized.

But we're not there yet. Holman said he hopes the document will be completed by the end of this year; feedback and finalizing could take another year. "Rather than do it piecemeal -- UVB, then UVA," he said, "there was a cry for a comprehensive monograph that covered both."

In the absence of such a document, manufacturers have little incentive to submit applications for approval of new ingredients, a lengthy and expensive proposition, with no guarantee that their product will ultimately comport with the FDA's blueprint.

Several manufacturers do have applications before the FDA; three ingredients submitted for approval in August 2002 have yet to be approved.

Holman notes that the FDA is "making policy that will impact a lot of products" and that that impact warrants the agency's high level of scrutiny. "I think some in the industry say, 'Oh, I just want to create this one sunscreen.' But we're writing regulations for all products," he said. "Our understanding of UVA light and the damage it causes humans -- our science isn't right where you'd want it to be."

Make Mine Mexoryl

The sunscreen ingredient that tops many most-wanted lists, including that of Clay J. Cockerell, president of the American Academy of Dermatology, is Mexoryl. The drug, developed by L'Oreal, has been widely used in Europe and elsewhere since 1993.

Mexoryl, which Cockerell said protects against even the longest-wavelength UVA light and can block 75 percent to 90 percent of that light, is widely deemed more effective than any anti-UVA drug available in the United States.

So where does Mexoryl stand in the FDA approval process? The agency doesn't comment on such matters. Nor does Jennie James, vice president for media relations for L'Oreal USA, shed much light: "All we really say about Mexoryl is that we have initiated a process of discussion with the FDA about Mexoryl and are continuing to work closely with the FDA." A June 20, 2003, Patent and Trademark Office document states that L'Oreal's patent on Mexoryl was to be extended to accommodate the FDA review process, noting that "a New Drug Application for the human drug product Mexoryl . . . has been filed and is currently undergoing regulatory review before the Food and Drug Administration for permission to market or use the product commercially."

In the meantime, people concerned about UVA exposure can take a few extra precautions: Wear that broad-brimmed hat, avoid sun during peak hours. . . . Or maybe this (but don't say we told you): Anyone with a computer and a credit card can have all the Mexoryl-containing sunscreen he wants. Oodles of the stuff can be had -- on eBay, for example.

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Jennifer Huget writes frequently for the Health section.

A sunscreen with L'Oreal's ingredient Mexoryl, which one expert calls "the best individual agent" in fighting UVA radiation isn't FDA-approved, but is available abroad -- and online.