A burst of new products is now appearing on store shelves, colorful pages of mail-order catalogues and Internet sites with bold claims that they can relieve one of new parents' worst fears: sudden infant death syndrome.

The trend is alarming medical and safety experts, who say most of the products that claim to reduce SIDS have never been adequately tested and may actually heighten the risk.

The cause of SIDS is far from certain. But that's not stopping eager entrepreneurs and major baby-product manufacturers from developing and promoting products as SIDS preventatives.

There are special mattresses with mesh coverings or built-in fans, mattress pads and crib sheets with specially formulated air holes -- all designed to increase the flow of fresh air to a baby's face. There's also a polyethylene "cot wrap" from New Zealand, which covers a mattress to keep any possible toxins away from a baby. And there are dozens of sleep positioners devised to keep infants from sleeping on their stomachs, a position medical experts now believe increases the risk of SIDS.

Experts say none of these devices is needed, noting that medical research has shown that the surest and simplest way to reduce risk of SIDS is to put a baby to sleep on the back, on a firm mattress, with no comforters, pillows, stuffed toys or other soft bedding items.

They also point to a recent study by the St. Louis University School of Medicine that revealed that most of these new mattresses and mattress pads do not decrease the amount of carbon dioxide around a baby's face, as they claim, but rather increase it. Only one mattress -- a $200 model with a fan -- did result in lower carbon dioxide, addressing some researchers' belief that SIDS may occur when babies, sleeping face down, rebreathe their own exhaled carbon dioxide and asphyxiate.

"Any time there's a new area of research, products come in to fill the gap," said Judith Jacobson, executive vice president of the SIDS Alliance, a nonprofit voluntary organization. "Many of these products are playing on parents' fears, but all these products are really unnecessary if a baby is sleeping on her back on a firm, flat crib mattress."

"No products have ever been shown to decrease the incidence of SIDS," added John Kattwinkel, chairman of the American Academy of Pediatrics' task force on infant positioning and SIDS. "My main concern is that parents may be misled into thinking that if they use some of these devices, they won't need to pay attention to our recommendation to put babies on their back."

No sales figures are available on SIDS-related products, but experts say they detect a surge in offerings. There's the Safe & Sound Sleep System for babies, for example, a sleep positioner to make sure babies stay on their backs. The box it comes in declares it's "a must in every nursery." In even larger print, it adds: "Safety Product. Helps Reduce SIDS."

The Sleep-Safe crib sheet has been on the market for only six months. Its packaging message is a little more subtle: "A Breath of Fresh Air for Your Baby . . . Safety Engineered . . . Pediatrician Recommended." But the pamphlet that's tucked inside is less ambiguous: "Reduce the Chance of SIDS With the All New Sleep-Safe."

And just new to the market is a $99 baby monitor that sounds an alarm if a baby's movement stops for 20 seconds. While this "Movement Reassurance System" is not specifically promoted as a product to prevent or reduce the risk of SIDS, the package says it "monitors baby's slightest sounds and movements . . . day or night for parent's peace of mind."

SIDS deaths have declined sharply since 1992, when the American Academy of Pediatrics began telling parents to put their babies to sleep on their backs or sides -- not their stomachs. In 1992, there were 4,891 SIDS deaths; in 1997, the latest year for which figures are available, there were 2,705 -- a 45 percent drop. The death rate attributed to SIDS for babies born in the United States today is 0.69 out of l,000 live births -- or put another way, 99.931 percent of babies will not die of SIDS.

In December 1996 the academy took an even stronger stand on back sleeping, issuing a new position paper saying that placing infants "wholly on the back confers the lowest risk and is preferred." Side sleeping, it noted, carries the danger that babies may roll over onto their stomachs.

But that hasn't stopped manufacturers of baby products from making special positioners to keep babies on their sides while they sleep -- and noting prominently in their packaging that these sleepers help keep infants in the sleeping position recommended by the American Academy of Pediatrics in an effort to reduce SIDS.

In essence, these products consist of a set of cushioned wedges that are attached by Velcro to a thin flannel pad. The adjustable wedges are placed next to the infant to keep the baby in place during sleep.

The products, which range from $7 to about $20, are "extremely popular and very much in demand," said Dan Bilger, product manager for Graco Children's Product Inc.'s baby-care line, which has four different types of positioners. "Not all parents are 100 percent comfortable in putting a baby on the back, which is why we offer the side sleeper."

Bilger said that while the current academy statement says back sleeping is preferred, it adds that sleeping on the side "is a reasonable alternative and is safer than sleeping on the stomach."

But Kattwinkel said he fears that infants are less stable sleeping on their side. If they do roll over, they may get "into compromising situations" with their faces pressed against the wedges, he added.

Federal safety officials say there have been no reported problems with the positioners, but Jacobson at the SIDS Alliance says the product is "something unnecessary" if the baby is put to sleep on its back. An infant can't roll over until the baby is about 5 months old and the peak incidence of SIDS involves babies between 2 and 4 months, said Phipps Cohe, national public affairs director for the alliance.

In fact, the alliance, in partnership with the academy and U.S. Public Health Service, has issued a policy paper as part of its "Back to Sleep" campaign specifically not recommending the use of positioners, noting that there have been no published studies examining their safety.

Now drawing the alliance's particular scrutiny -- and ire -- are the increasing number of mattresses and mattress pads that claim to prevent SIDS. In April, the group took the unusual step of issuing a product advisory against one particular mattress, the Kid Safe Baby Air Mattress, which has been marketed as a way to reduce SIDS, even for babies sleeping face down. "There is no proven advantage of its airflow system to specifically reducing or preventing SIDS," the alliance said in its advisory.

Habib Mahdavi, who created the air mattress after his baby daughter slept "face down no matter what I did," said he remains confident about his product, basically a piece of mesh fabric that is tightly drawn over a wood frame that is placed on top of a crib mattress. Although he has not done extensive testing, he said tests by a parachute company show that there is a good flow of air through the fabric -- proof, he says, that the product is "breathable" and there is no obstruction.

Mahdavi said he has sold about 3,000 Baby Air mattresses, which cost about $80 each. He said he has never claimed that his product reduces SIDS. Although his Web site displays a large headline, "Reduce the Risk of SIDS," that statement "is similar to the admonition `Drive Carefully' " and not a particular claim, he said. "We don't claim to prevent SIDS, we claim [the product] is a pediatrician-recommended mattress for SIDS. Those are two different things," said Mahdavi, who has letters of support from two pediatricians.

The SIDS Alliance, increasingly concerned about products like the Kid Safe mattress, recently asked the Consumer Product Safety Commission to investigate these new anti-SIDS products and their marketing claims. "They are being marketed in stores, catalogues and baby fairs to impressionable, young parents as a means of preventing or reducing SIDS risk," the alliance wrote the commission.

But, the group noted, in a recent test by the St. Louis School of Medicine, all but one of these products "did little to reduce, and in most cases worsened, carbon dioxide trapping," thereby increasing the potential risk of infant death. (The "Sleep-Safe" crib sheet came on the market after the test was conducted.)

The exception was the "Halo Crib Mattress" developed by William Schmid, who lost a daughter to SIDS in 1991. The mattress has a fan in the bottom to provide a gentle flow of air to the sleeping surface to eliminate any carbon dioxide that could accumulate near the baby's face.

"We believe babies should be put to sleep on their backs, but babies do roll over or, in some cases, they are placed inadvertently on their stomachs by care givers who don't know better," Schmid said.

The mattress is new to the market but the company has already sold several hundred -- through the Internet and some specialty catalogues -- even at its steep price of $200, with an additional $30 for the mattress pad/sheet.

CPSC Chairman Ann Brown said the agency is now investigating the products that claim to reduce SIDS. If the agency finds that the products don't do what they say, "we could require the manufacturer to change the way it promotes the product."

The agency could also refer any unsubstantiated claims to the Federal Trade Commission, which regulates advertising.

For the moment, manufacturers are convinced that their claims are valid -- even if they haven't done their own extensive research to back them up. John Hebric, chief financial officer of Basic Comfort Inc., which makes the Safe & Sound sleep system -- a back-sleep positioner -- said the product's claim that it "helps reduce SIDS" is "based on the recommendation by the SIDS Alliance and the American Academy of Pediatrics that babies sleep on their backs. Our product places the baby in the proper position to reduce SIDS. Then the product accomplishes that purpose."

SOME PRECAUTIONS

Here are ways to reduce the risk of sudden infant death syndrome:

* Always place your baby on the back to sleep. Stomach sleeping more than doubles the likelihood of SIDS. The risks also increase for babies used to sleeping on their backs who are inadvertently put to sleep on their stomachs.

* Use a firm mattress in a safety-approved crib. Avoid futons, water beds, beanbags, soft mattresses, sofa cushions and pillows as sleep surfaces.

* Remove blankets, stuffed toys and other soft bedding items from sleeping area.

* Keep your baby's face and head clear of blankets and other coverings during sleep. Consider a sleeper as an alternative to a blanket.

* Don't overheat the room; keep it at the same temperature that you find comfortable.

* Provide a smoke-free environment for your baby.