The dark side of sunscreen?

Think SPF 50 sunscreen will keep you completely safe from the sun's rays? Don't bet on it. According to a new study by the Environmental Working Group — a nonprofit that seeks to "expose threats to your health and the environment, and to find solutions" — three out of five sunscreens offer "inadequate protection from the sun or actually contain toxic ingredients." Only 5 percent of the roughly 1,400 lotions tested by EWG met the group's stringent safety standards, while 42 percent may actually contribute to the growth of skin cancer. How is that possible? Here, a guide:

Wait ... using sunscreen can promote skin cancer?

It seems almost cruel, but that's what the animal tests conducted by the Environmental Working Group suggest. The main culprit is vitamin A, which many sunscreen makers include in their products for its purported anti-aging properties. Unfortunately, says EWG, the common vitamin A derivative retinyl palmitate also speeds up the growth of cancerous cells in lab mice by as much as 21 percent when combined with sunlight. Skeptics caution, however, that just because this effect occurs in mice, doesn't mean it occurs in humans.

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Are there any other health concerns with sunscreen?

The EWG also warns against products — an estimated 60 percent of those on the market — that include the potentially "hormone-disrupting compound" oxybenzone, added to boost protection against UVA rays. There is no consensus on the issue, however, and many experts maintain that oxybenzone is safe.

Has the government taken a position on which sunblocks are safe?

Not yet. The Food and Drug Administration is issuing new safety and labeling guidelines for sunscreens — in the works since 1978 — as early as October. But the rules won't be enforced for a year or more, says the EWG, meaning "the first federally regulated sunscreens won't go on store shelves before the summer of 2012." The group also claims that the FDA has been aware of the problems with vitamin A for many years, but has neglected to alert the public or take any regulatory action.

Is EWG being overly alarmist?

The sunscreen lobbying group the Personal Care Products Council calls EWG's report "reckless," saying unfiltered exposure to the sun is a far greater threat to your health than the ingredients in any sunscreen. At least some skin doctors agree. "EWG is kind of the Chicken Little of the sunscreen arena," says Florida dermatologist James Spencer. EWG acknowledges that the evidence underpinning some of its warnings is "not conclusive," but argues, why risk using "suspect" products?

So which sunscreens should I use?

Avoid most of them, according to EWG. Of the 500 most popular sunscreens tested, the group recommends only 39. The top scorers include products from Jason Natural Cosmetics and All Terrain brands. (See the EWG's best-rated sunscreens.) EWG recommends lotions over sprays or powders. Consumer Reports, however, lists five spray-on sunscreens as its top protectors (Target tops the list) in terms of blocking UVA and UVB rays.

How can I best protect myself against the sun?

Cover yourself with hats and clothing, as much as possible. When you use sunscreen — and you should definitely use sunscreen — make sure you're applying the proper amount (about a shot glass full, or twice as much as people typically use, says the EWG) about 20 minutes before you go outside. Reapply the full amount every two hours — or sooner, if you've been sweating, swimming, or dried off with a towel.

Sources: L.A. Times, AOL News, Vanity Fair, Discovery, EWG.org, KCTV, Consumer Reports, WebMD (2), US News