Celebrities swear by them. Ordinary women tout their transformative powers. But keratin treatments, those coveted hair-smoothing procedures that eliminate frizz and make unmanageable locks effortlessly sleek, are under scrutiny — and it's not just the $300-plus price tags that are attracting attention.

At issue is the amount of formaldehyde in the keratin solutions — even those that claim to be all-natural and formaldehyde-free.

The treatments, which originated in Brazil, have been popular here for several years

American salons tend to offer two types, both of which promise hair that doesn't require blow-drying to stay straight and smooth.

"Express" services, often referred to as "Brazilian," take about 90 minutes, allow for immediate shampooing and last up to three months.

Traditional keratin treatments require a longer salon visit — often upward of three hours — prohibit styling or washing for several days afterward, and keep hair smooth for four to six months.

Although different products are used in both treatments, each requires flat-ironing at high heat to seal in the chemicals that keep hair straight. And that's where health concerns start to arise.

Federal guidelines stipulate that products contain no more than .2 percent formaldehyde. But making sure that manufacturers adhere to government standards is difficult, because there is no pre-approval process before these products hit the market, says Scarsdale dermatologist Dr. Amy Newburger.

"They can completely lie, and the public is vulnerable,'' she says.

Good chemistry?

According to California chemist Doug Schoon, formalin, also called methylene glycol, is the active ingredient in most smoothing treatments — a fact that allows companies to claim their products are formaldehyde-free. When hair is flat-ironed at 450 degrees, however, a chemical change causes formaldehyde to be released into the air.

Although formaldehyde is a naturally occurring gas, it is also a known carcinogen. But is the formaldehyde emitted during keratin treatments enough to be dangerous

For a stylist who applies these products on a regular basis, there are concerns, since chronic inhalation can lead to respiratory problems. Even for clients, short-term exposure to formaldehyde can cause eye and nose irritation, breathing problems and headaches.

In Ireland, they've stopped distributing many products still used in American salons — including solutions made by Peter Coppola, Marcia Teixeira and Brazilian Blowout — due to concerns about excessive formaldehyde. The Canadian health department has confined its warning to Brazilian Blowout, which was analyzed last fall and found to have unacceptably high levels of formaldehyde.

In the United States, the FDA has received numerous complaints about salon products containing formaldehyde and is currently working with state and local organizations, and the Occupational Safety & Health Administration, to reassess the products' safety.

But not every state is waiting for the FDA's recommendations. In recent months, Connecticut, Washington and California have issued health alerts warning salon workers about the potential risks of these hair-straightening products.

What's troubling, say salon owners, is that it's not always clear what ingredients are in hair-smoothing products. Information is rarely on the bottles — in fact, it often appears only on Material Safety Data Sheets, which are mandated by OSHA.

Bottles of Brazilian Blowout solution, for example, do not list ingredients at all.

Brazilian Blowout is among the products that have received the most scrutiny in the U.S. Last fall, after complaints from a Portland salon, Oregon OSHA conducted its own tests and found that many products, including Brazilian Blowout, contained significant levels of formaldehyde.

The company has since filed a lawsuit against Oregon OSHA, alleging improper testing of the product and inaccurate laboratory results.

Safe styling

For women like Rachael Lynch of Armonk, a Brazilian Blowout devotee who's had the treatment twice at Numi & Company salon in Scarsdale — and suffered no ill effects either time — it's all a bit confusing. She loves the way her hair looks, she adores the salon, but she's not crazy about using something that might contain formaldehyde.

Chemist Schoon's recommendations for safe use of hair-smoothing products include proper ventilation in salons to minimize exposure. A well-ventilated salon performing two or three treatments a day is not likely to exceed federal OSHA's safe levels for formaldehyde gas, he says.

But at the Mark Garrison salon in Manhattan, where they offer Brazilian treatments made by Lasio and Marcia Teixeira, they're taking no chances: Treatments are done in a specially ventilated room, and both clients and stylists are outfitted with professional-grade respirators.

Gabriel Abrams, co-owner of Numi, doesn't use a mask or respirator. But he has no worries about any of the keratin treatments, and stands behind the safety of Brazilian Blowout.

He's done close to 1,000 keratin treatments since last March — using products made by Peter Coppola and Brazilian Blowout — and says there are no noxious fumes when done properly.

"I'm a very green guy, and it's important,'' says Abrams. "But I'll tell you this: Customers love it and they won't stop. If you think about it, we go tanning, we color our hair, we talk on the phone. The bigger stuff could be killing us."

Potential health concerns, however, prompted Aveda — the hair-industry giant known for its environmentally friendly products — to send a letter to its concept salons, suggesting they look into the safety of keratin treatments, says Patricia Carano, co-owner of Just Imagine U in Croton-on-Hudson, who received such a letter last fall.

At the time, the salon was offering the Rejuvenol formaldehyde-free Brazilian keratin treatment, and Carano was already uneasy about the flat-ironing part of the process, which requires multiple pressings of the solution-soaked hair at very high temperatures. "You are literally cooking it,'' says Carano. "So not only am I leery of the product, it's the procedure I'm leery of."

Carano knew it was time to stop offering the treatment during one of the company's training sessions, which included in its class a pregnant stylist. "You're not going to be giving these treatments, are you?" the instructor asked the mother-to-be.