The application technique is similar to the Japanese Yuko System, in that the hair is heated with flat irons to bind the product into the hair cuticle.

Depending on the treatment used, the customer may have to avoid washing, wetting, pinning or constraining the hair in any way (e.g. with hair clips, pony tails, or headbands) for some period after application, up to three days.

It is also confused or mistaken for keratina, another hair treatment.

The name "straightener" is arguably incorrect because it does not chemically alter the structure of the hair.

Between 2010 and 2012, there was a dispute between the manufacturers and importers of Brazilian hair straightening products on one hand, and various US federal and state health agencies on the other, about the proper labeling and health warning of the products. The dispute centered about the chemical compounds formaldehyde (H 2CO) and methylene glycol (H 2C(OH) 2), which are the active ingredients of the process, responsible for binding the keratin to the hair. Formaldehyde is a gas that, when dissolved in water (including fluids in the human body), converts partially to methylene glycol. In the liquid, the two compounds coexist in a chemical equilibrium.
Formaldehyde in the air has long been recognized as a major occupational hazard in many industries; such as those using urea-formaldehyde (UF) adhesives, or UF-based products like fiberboard, and UFFI spray foam insulation. Levels below 0.05 parts per million (ppm) were found to be positively correlated with eye and nasal irritation. It has also been implicated in the development of childhood asthma, and, in 2006, the WHO International Agency for Research on Cancer (IARC) classified it as a 'known human carcinogen'.

Because of these concerns, since 1976 or earlier, European regulations on cosmetics banned products containing or releasing formaldehyde. Products containing any aldehydes more than 0.001% in leave-on products or 0.01% in rinse-off products were required to list the ingredients explicitly in their product labels.

The US Federal Occupational Safety and Health Administration (OSHA) sets a long-term permissible exposure limit (PEL) of 0.75 ppm of formaldehyde in the air, and a short term exposure limit of 2.0 ppm.
OSHA issued a hazard alert and created an informational site in response to an investigation into complaints from stylists and hair salon owners about exposure to formaldehyde while using hair smoothing products such as Brazilian Blowout (Acai Professional Smoothing Solution, Professional Brazilian Blowout Solution), Brasil Cacau Cadiveu, Keratin Complex Smoothing Therapy (Natural Keratin Smoothing Treatment, Express Blow Out, Natural Keratin Smoothing Treatment Blonde), and Marcia Teixeira (Advanced Brazilian Keratin Treatment, Extreme De-Frizzing Treatment). OSHA conducted air sampling at multiple salons and found formaldehyde in the air when stylists were using hair smoothing products. Some of these products were labeled "formaldehyde free" or did not list formaldehyde on the product label or in the Material Safety Data Sheet (MSDS). In most cases, where the label did not state that the product had formaldehyde in it, OSHA found that hair salon owners using those products did not know that hair smoothing products contain or could expose workers to formaldehyde because manufacturers, importers, and distributors did not include the correct hazard warnings on the product's label or MSDS.

During Federal OSHA investigations, air tests showed formaldehyde at levels above OSHA's limits in salons using Brazilian Blowout Acai Professional Smoothing Solution, labeled "formaldehyde free", and Brasil Cacau Cadiveu. Both Federal and State OSHA have found violations at several manufacturers, importers, and distributors (GIB LLC dba Brazilian Blowout, Keratronics Inc., Pro Skin Solutions, M&M International Inc., Copomon, INOVA Professional). The violations include failing to list formaldehyde as a hazardous ingredient o…

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The U.S. Food and Drug Administration (FDA) warns that Brazilian Blowouts are hazardous to the health of those who use them and the hairdressers who apply them.

Concerns over the presence of formaldehyde in various hair smoothing products at significant concentrations centered whether methylene glycol could legally be synonymous with formaldehyde. Anhydrous formaldehyde gas readily dissolves in and reacts with water to form an equilibrium solution of methylene glycol. When heated, the equilibrium shifts and favors the production of formaldehyde and water. Thus, the manufacturer of Brazilian Blowout argued that methylene glycol is in their products, not formaldehyde, and therefore they can claim that their product was formaldehyde-free. The first involves nomenclature. The second issue is the method by which formaldehyde concentration is measured. The third involves measurements of formaldehyde concentration in bottles of the product in which the reported concentration is dependent upon both the method of measurement and nomenclature. However, the company reached a settlement with the state of California and is no longer claiming their products are formaldehyde free.
The Chemical Abstracts Service (CAS) lists formaldehyde (50-00-0 ) and methylene glycol (463-57-0 ) as two different substances. The compounds have two different chemical structures, exist in two different chemical families and exhibit different physical properties. Formaldehyde is a colorless gas with chemical structure HCHO. Formaldehyde is a listed carcinogen. NTP notes methylene glycol as the primary chemical form of formaldehyde in water. When heat is applied in the Brazilian blowout process causes the methylene glycol to dehydrate, yielding formaldehyde gas and water vapours. According to Golden and Valentini even in worst case experiments less than half of the methylene glycol result in measurable formaldehyde gas, also an unknown portion of this measurement is due to the combining of the volatilised methylene glycol being reported as gaseous formaldehyde, furthermore the reverse reaction is much more rapid and much more favourable at STP equilibrium, which is not taken into account in the analysis. They contend that the chemical equivalence assumption is incorrect.
The Brazilian Blowout company (GIB LLC) has argued that Eastwood's lab and government labs in California and Oregon performed improper tests to determine formaldehyde concentration, arguing instead that an NMR spectroscopy test is superior.
Some manufacturers of products containing formaldehyde and methylene glycol have complained that the method of testing for formaldehyde—which does not distinguish between formaldehyde and methylene glycol—is not a reliable indicator of the toxicity of the product.

The American Chemistry Council issued an official statement, where they stated the following: "Formaldehyde content—in both gaseous and aqueous forms—should be accounted for when measuring the formaldehyde content of a product. ... Federal OSHA correctly defines formaldehyde as 'formaldehyde gas, its solutions, and materials that release formaldehyde.' This comprehensive standard is the cornerstone for the protection of people who work with and around formaldehyde." However the assumption of chemical and toxicological equivalence between Formaldehyde (a reactive gas) and methyl…

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