Published: April 2011

Best options for straight hair

Methods to flatten waves and frizz include everything from flat-ironing blown-dry hair a few times a week to enduring a multi-hour formaldehyde process that lasts three months or longer.  EWG researchers reviewed the options and assessed the risks.

None of the longer-lasting chemical treatments passed muster with our scientists.

Formaldehyde, found in some hyped products like Brazilian Blowout, is found in embalming fluid and many adhesives.   Some hair relaxers are made of lye, also the active ingredient in drain cleaners like Drano.

Hair straighteners work by rearranging the chemical bonding within hair.  As a general rule, longer-lasting hair straighteners break and/or form very strong bonds within hair, but they use harsh and usually hazardous chemicals to do so.

Hair chemistry – the basics.
Hair, nails and skin are comprised primarily of the protein keratin. Keratin forms a long helical chain like a very thin strand of rope. Groups of these thin strands form filaments. Groups of filaments, in turn, form the bulk of the hair strand’s inner layer, called the cortex. Hair straighteners change how strands of keratin stick together.

Different hair-straightening methods target various chemical connections that hold keratin strands together.

Although advertisements for chemical hair straighteners like Brazilian Blowout claim the method has been discovered recently, a strikingly similar process was patented in 1945 (Calva 1945).  Permanents for curling straight hair were introduced in the early 20th century.  “Cold” permanent waves based on chemicals, not heated rollers, were developed during World War II (Robbins 2002).  These methods rely on some of the same chemicals used to straighten hair.


Hair straightening options and safety concerns

Hot flat iron, no chemicals


Running a flat iron through blown-dry hair breaks the hair’s hydrogen bonds. It flattens hair until water or water vapor (from humidity, drizzle or the shower) penetrates the hair fiber and allows the hydrogen bonds to revert to their natural positions. The flat-iron process is the top choice among EWG researchers because it is chemical free. You can pay a stylist or do it yourself at home. You can easily touch up your hair with a flat iron when a night’s sleep musses it up. The biggest danger is the hot iron.

Brazilian-style keratin treatments with formaldehyde


Brazilian Blowout and other popular chemical hair straighteners rely on a combination of formaldehyde and keratin. They claim results last 12 weeks or longer. A stylist coats hair with a keratin/formaldehyde solution. Formaldehyde molecules diffuse into the hair and cross-link strands of keratin and cross-link new keratin with the keratin in the customer’s hair. By blowing saturated hair dry and compressing it with a flat iron, the stylist accelerates the cross-linking reaction, transforming curly hair to flat. (Drahl 2010).

Testing results and product instructions show that products with formaldehyde concentrations of more than 6 percent can be washed out of the hair immediately. But products with less formaldehyde, typically 2 percent, must be left in the hair for up to three days to increase the number of keratin cross-linking bonds that determine how long the hair will stay straight. Hair continuously releases formaldehyde as it slowly reverts to its natural curly or frizzy state. Scientists have not tested this phase of the straightening process to measure how much formaldehyde vapor emanates from straightened hair over time.

Straighteners using cross-linking alternatives to formaldehyde


Cross-linking processes have been extensively studied because they are most often used to repair damaged wool textiles (Crawshaw 2002, Whewell 1960). Many molecules other than formaldehyde can cross-link hair or fiber (Whewell 1960), but most would be too hazardous for use in hair straighteners.

Some companies use “formaldehyde releasers” instead of formaldehyde to cross-link hair keratin. These chemicals decompose to formaldehyde under the heat of a blow drier or flat iron, leading to the same health concerns as formaldehyde-based products. Other companies use alternate “aldehydes,” compounds in the same chemical family as formaldehyde. Many alternative aldehydes present health concerns similar to those of formaldehyde: allergic response, irritation and cellular damage, technically known as cytotoxicity (Kieć-Swierczyńska M et al.1998).

Liquid Keratin, advertised  as “the first safe, at home alternative” to formaldehyde-based salon products lists biformyl, also known as glyoxal, on its ingredient list (Liquid Keratin. 2011B). The Cosmetic Ingredient Review, an industry panel, has concluded that glyoxal is safe for use on nails in concentration less that 1.25 percent (CIR 2007).

Salon Favorite Do-It-Yourself Keratin Treatment, another home-use product, was found to be formaldehyde-free in tests conducted by Good Housekeeping Research Institute (GHRI 2011). Gluteraldehyde is potent disinfectant, marketed to health care facilities under trade names like Metricide® and Omnicide®.  Many facilities are phasing it out because it is a strong irritant, allergen and asthmagen.

Peter Coppola filed a patent in April 2009 for a product that uses keratin pre-reacted with formaldehyde. Coppola’s hair straightener would also contain glutaraldehyde (a chemical related to formaldehyde) or extra formaldehyde. Glutaraldehyde or formaldehyde may be replaced by alternate aldehydes such as citral, octanal or cinnamic aldehyde (CoppolaP, Bucario VC 2009). Coppola products currently on the market claim to use Timonacic acid and not formaldehyde, but testing by eight different organizations indicate the presence of formaldehyde.

In November 2010, following test results showing high levels of formaldehyde, the manufacturer of Trichovedic preemptively recalled the product, anticipating a mandatory recall ordered by the Australian Competition & Consumer Commission (ACCC 2010). The company reformulated its products. The most recent test reports indicate an average formaldehyde concentration of 0.046 percent, with no detected aldehyde alternatives (acetaldehyde, benzaldehyde, glutaraldehyde or octanal) (Trichovedic 2011B).

But tests published by the company show that the application process releases significant amounts of formaldehyde (Trichovedic 2011). The peak concentration of 0.279 parts per million recorded during a 13-minute application is nearly three times higher than 0.1 parts per million, the level at which the U.S. Occupational Safety and Health Administration requires a warning label. EWG researchers did not find this product for sale in the U.S.

Brazilian Blowout Zero (no formaldehyde)


Brazilian Blowout says the active ingredient in its new straightener is glycolic acid, derived from sugar cane. Tests show it is does not contain formaldehyde or release it during use (Brazilian Blowout 2011).

On its website the company claims that the product straightens the hair as effectively as its formaldehyde-based formulations. But in response to questions from EWG, a company representative said that Brazilian Blowout Zero does not leave the hair as smooth and flat as the original formulation.

Its ingredients and process are similar to a 2006 patent filing by L’Oreal that uses glycolic acid (or any other alpha-hydroxyl acid or keto acid) with a flat iron to smooth hair (Malle 2010).

The Cosmetic Ingredient Review has determined that glycolic acid is safe at concentrations of less than 10 percent in home use products and less than 30 percent in salon products.

Is this product safer than the original? Brazilian Blowout long denied the use of formaldehyde in their original product, raising questions about the veracity of their claims about Brazilian Blowout Zero. The new product may be safer, but use at your own risk.

Very long-lasting hair treatments (breaking disulfide bonds)

Products that break the strong disulfide bonds that hold adjacent hair strands together can create nearly irreversible changes to hair shape.  These changes, which last until the hair grows out and is cut off, are accomplished with lye and similar “hydroxide-based” relaxers, or with chemicals commonly used in permanent wave solutions —  thiolglycolate or sulfite-based reducing agents (Radisson 2010).  These processes can also cause irreversible damage to the hair.

Hair-perm treatments – typically ammonium thioglycolate


Permanent solutions can curl or straighten hair, depending on the hair is styled when they are applied.  The first solution breaks the hair’s disulfide bond. Then a fixative remakes the chemical bond in the new position. Water can serve as a fixative, but most brands rely on faster-reacting compounds like hydrogen peroxide (Robbins 2002).

Perm solutions are much less aggressive than lye-type relaxers. Drawbacks are unpleasant odors (a rotten egg smell) that must be masked by strong fragrances and degradation of the hair fibers. Over time, hair returns to its natural position (Radisson 2010). As the character Elle Wood in the film Legally Blond said “…isn’t it the first cardinal rule of perm maintenance that you’re forbidden to wet your hair for at least 24 hours after getting a perm at the risk of deactivating the ammonium thioglycolate?”  In other words, the chemical reaction continues after the client leaves the salon, exposing her to the chemical for more hours.

The Cosmetic Ingredient Review found in 1991 that three thioglycolate ingredients caused allergic responses at levels of 0.25 to 2.5 percent. Perm solutions contain up to 15.4 percent of these ingredients. The panel concluded that the products were safe if used at “infrequent intervals” and that consumers and especially hairdressers should avoid skin contact (CIR 2007).

Japanese thermal hair straighteners


These straighteners usually contain ammonium thioglycolate and work on the same principle as permanents. Stylists flat iron hair to speed the chemical reactions. The same risks and warnings apply as for perm solutions, including allergic reactions and hair damage.

Alkaline Sulfites

Not necessarily
Low to moderate risks

Sulfites and bisulfites can break the hair’s sulfide bond.  These chemicals react much more slowly with hair than the ammonium thioglycolate used in Japanese straighteners and are less irritating to the skin. They are common in home-use products (Robbins 2002). Sulfite treatments do not last long (Radisson 2010).

Although the chemicals appear to be less hazardous than other hair-straightening compounds, worker safety information published by a chemical manufacturer says they could trigger asthma attacks in sensitive individuals: “Some individuals are said to be dangerously sensitive to minute amounts of sulfites in foods. Symptoms may include broncho constriction, shock, gastrointestinal disturbances, angio edema, flushing, and tingling sensations. Once allergy develops, future exposures can cause asthma attacks with shortness of breath, wheezing, and cough” (J.T. Baker 2009).

Lye-based alkaline hair relaxers


Alkaline hair straighteners, most commonly based on lye, also known as sodium hydroxide, a highly caustic chemical that is also used in drain cleaners like Drano. Alkaline straighteners are said to smell better, work more quickly and last longer than perm treatments using ammonium thioglycolate and similar chemicals (Radisson 2010). They cause irreversible changes in hair strands. They can be caustic and cause irritation and burning, make the hair feel coarse and render it fragile and prone to breaking.  Prolonged exposure will dissolve hair (Radisson 2010).  Lye-based hair relaxers were the focus of Chris Rock’s 2009 documentary Good Hair, which vividly depicted pain and scalp burning.   Sodium hydroxide, a corrosive, can cause severe skin burns and irritation of nose, throat and respiratory tract, and can damage the eyes.

No-lye alkaline hair relaxers

Moderate risk

No-lye relaxers, typically less caustic and irritating than lye-based products, are common in home-use products. Between November 2008 and August 2010, FDA received at least five reports of severe adverse effects after the use of no-lye relaxers, according to documents obtained by EWG in response to a Freedom of Information Act request. In each case, the consumer lost hair. Some reports documented baldness and severe scalp burning. The active ingredient in no-lye relaxers is often calcium or potassium hydroxide or guanidinium hydroxide (which is generated by mixing calcium hydroxide and guanidine carbonate) (Radisson 2010).  This mixture is less caustic than a lye-based relaxer but poses similar dangers  —scalp and skin burns and hair damage. Some no-lye relaxers use ammonium thioglycolate.

Future technologies


Permanent press and other no-iron clothing historically rely on urea-formaldehyde to cross-link fibers. In 2010 the Government Accountability Office reported that formaldehyde releases from textiles had diminished since the 1980s (GAO 2010), following the industry’s transition from formaldehyde to urea-based chemicals because of formaldehyde-related health concerns. Although there are differences between the keratin strands in hair and the cellulose fibers in cotton or wool, aspects of this technology have been applied to hair-smoothing treatments.

In July 2010, a L’Oreal scientist filed a patent for a hair relaxing/smoothing method similar to Brazilian Blowout (Radisson 2010), but some of the listed potential active ingredients are urea-based molecules, similar to many chemicals used in non-iron shirts (GAO 2010).


People with curls have elliptical, not circular hair follicles. The elongated opening waves the hair like curling ribbon (Goldstein 2009). Hormonal changes and some drugs can alter the shape of hair follicles, changing curly hair to straight or vice versa  (Highfield 2005). No such pharmaceuticals are approved by the FDA for use as hair straighteners, but such products could be marketed eventually.