As another hot summer looms, consumers are once again stocking up on products that promise scientifically formulated sun protection. Only now, amid a steady drumbeat of bad-news stories about global warming, manufacturers are upping the ante with whole new categories of chemically treated products that purport to block ultraviolet light. The products range from clothing and shoes to makeup and umbrellas. There are even sunscreen bikinis that pledge to shield those patches of skin that they actually cover.

But consumers and dermatologists have their doubts. Among those doctors who view this new breed of products as just so much marketing is Dr. Naomi Lawrence, head of procedural dermatology at Cooper University Medical Center in Camden, N.J.

“When it comes to sun protection, you really can’t beat a dark shirt with a tight weave and a good hat,” she said. “There is a lot you can do and not spend a lot of money.”

Which is not to say that many UV-protective products do not do what they promise. UV-protective clothing — once the realm of specialty retailers catering to skin cancer patients, but now a hot seller for brands like the Gap, Izod, Uniqlo and Lands End — add protection by infusing fabric with chemicals that absorb UV rays, like titanium dioxide or Tinosorb. Sunscreen-infused laundry additives work the same way. With the infusion, summer-ready materials like cotton and linen can keep harmful rays from reaching the skin, even if the fabric is white, yellow or light blue, for example.

Because standard clothing must be densely woven or dark colored to offer advanced UV protection, these specially treated clothes are “good if you want something long-sleeved that is also lightweight,” Dr. Lawrence said.

But as sun-protective clothing has made its way into the mainstream, seemingly obvious features like sleeves have occasionally been sacrificed, defeating part of the purpose. Lesser offenses include shorts and sleeveless shirts, while items like bikinis — which claim to offer the maximum degree of sun protection — might be a bit of a stretch.

Consumers seem understandably puzzled by these UV-protective wares, perhaps buying them with fingers crossed. “I have used this product occasionally for several years with the hope that it works,” said one reviewer on Amazon.com for Sun Guard, a laundry treatment that says it puts “an invisible shield into clothing that helps block more than 96 percent of the sun’s harmful rays from reaching your skin.” The reviewer added, “Preferring white shirts for outdoor activities, I trust that this dye is blocking UV radiation, but I can’t say for sure.”

To some critics, introducing chemicals to a naturally sun-protective category like clothing seems wasteful. “Our recommendation is that you stick to clothes with tighter weave and that will provide adequate UPF protection,” said David Andrews, a senior researcher with the Environmental Working Group, which compiles an annual guide to sunscreens. “Not to mention there’s no need to do a full load of wash with all your undergarments and everything to make them UPF protected.”

The Food and Drug Administration briefly regulated sun-protective clothes in the early 1990s, classifying them as medical devices. While it no longer does that, the Federal Trade Commission does monitor marketing claims about garments and sun protection. A measurement called UPF, or ultraviolet protection factor, is the standard for UV-protective clothing. Developed in 2001 by ASTM International, a product-testing agency, a UPF ranges from 1 to 50, with 50 being the most UV light that is blocked by a garment.

The F.D.A. does continue to regulate claims made by traditional sunscreens, which will soon undergo the first significant changes in decades. Manufacturers will have to show that their products protect against both UVB rays, which cause burns, and UVA rays, which are linked to skin cancers. Gone will be labels of “waterproof” or “sweatproof,” and lotions will have to indicate how often they need to be reapplied. Originally slated to go into effect in June, the new labeling requirements have been delayed until December to give manufacturers time to comply.

Other products that are not traditionally associated with sun protection seem to be marketed to imply that they can do more than they can deliver. Sun-related claims are now common among hair products, like TRESemé’s “Climate Control” shampoo and conditioner, which both use ingredients like olive oil, keratin and a UV-blocking chemical called avobenzone to prevent frizz and drying. Outside of those benefits, the products offer no actual UV protection, according to the company.