Updated

Federal regulators will require sunscreen manufacturers to test their products' effectiveness against sun rays that pose the greatest risk of skin cancer. Under new rules published Tuesday, they also will have to follow stricter guidelines when describing how well their products block ultraviolet B rays.

The Food and Drug Administration announced new regulations Tuesday designed to enhance effectiveness of sunscreens and make them easier to use.

Sunscreens that don't protect against both ultraviolet A and B rays and have a sun protection factor, or SPF, below 15 will have to carry warning label: "This product has been shown only to help prevent sunburn, not skin cancer or early skin aging."

Currently, the FDA only requires testing for ultraviolet B rays that cause sunburn. That's what the familiar SPF measure is based on.

But the new regulations require testing for the more dangerous ultraviolet A rays, which can penetrate glass and are most commonly linked to wrinkles and skin cancer.

More On This...

"The FDA has evaluated the data and developed testing and labeling requirements for sunscreen products, so that manufacturers can modernize their product information and consumers can be well-informed on which products offer the greatest benefit," said Janet Woodcock, director of the FDA's Center for Drug Evaluation and Research in a press release. "These changes to sunscreen labels are an important part of helping consumers have the information they need so they can choose the right sun protection for themselves and their families."

FDA will also prohibit sunscreen marketing claims like "waterproof" and "sweatproof," which the agency said "are exaggerations of performance."

Products that protect against UVA and UVB will be labeled "broad spectrum." In an effort to clear up the confusing mix of numbers, acronyms and symbols on sunscreen labels, the FDA says manufacturers must phase out a four-star system currently used by some companies to rate UVA protection.

Any product that is not broad spectrum will be required to have a warning, which states that the product has not been shown to help prevent skin cancer or early skin aging.

"Most skin cancers are caused by sun exposure. The FDA encourages consumers to protect themselves," Woodcock said. "Not only should consumers regularly apply and reapply sunscreens with broad spectrum and SPF of 15 or higher, they should also limit sun exposure."

The FDA rules will also standardize the older SPF protection rankings for UVB rays. Only sunscreens with an SPF of 15 or higher can claim to lower the risk of cancer. The FDA also proposed capping the highest SPF value at 50, unless companies can provide results of further testing that support a higher number. Some products on the market claim to offer SPF protection of a 100 or higher.

The SPF figure indicates the amount of sun exposure needed to cause sunburn on sunscreen-protected skin compared with unprotected skin. For example, an SPF rating of 30 means it would take the person 30 times longer to burn wearing sunscreen than with exposed skin.

To ensure that existing sunscreen products meet modern safety standards, the FDA is also currently reexamining the active ingredients included in sunscreens on shelves today. The FDA stated in a press release that it does not have any reason to believe these products are not safe for consumer use.

The new regulations will become effective for most manufacturers in one year. Manufacturers with annual sales less than $25,000 have two years to comply.

FDA announced its intent to draft sunscreen rules in 1978 and published them in 1999. The agency then put the plan on indefinite hold until it could address issues concerning both UVA and UVB protection.

The delay in FDA regulations means many companies have already adopted the some of the language. For example, all Coppertone products from Merck & Co.'s Schering-Plough unit and Neutrogena Sunblock from Johnson & Johnson already boast "broad spectrum UVA and UVB protection."

Most dermatologists recommend a broad spectrum, water-resistant sunscreen of SPF 30 or higher every two hours while outside.

Last year an estimated 68,130 people in the U.S. were diagnosed with melanoma—the most dangerous form of skin cancer—and an estimated 8,700 died, according to the National Cancer Institute. Nearly $2 billion is spent treating the disease each year.

The Associated Press contributed to this report.