Keratin Treatment: What It Is, How Long It Lasts & Side Effects

What Is a Keratin Treatment?

A keratin treatment — sometimes called a Brazilian blowout, Brazilian keratin treatment (BKT), or hair smoothing treatment — is a semi-permanent salon procedure that deposits hydrolyzed keratin protein into the hair shaft, then seals it flat with a high-heat flat iron. The result is dramatically reduced frizz, improved manageability, and a smoother, shinier surface that lasts weeks to months.

Keratin is the primary structural protein of human hair. Each strand is composed of three layers: the outermost cuticle (overlapping, scale-like cells), the middle cortex (tightly packed keratin fibrils that give hair its strength and color), and the innermost medulla. Frizz and roughness occur primarily when the cuticle scales lift — caused by humidity, chemical damage, heat stress, or mechanical trauma. Keratin treatments address this directly by supplementing lost or disrupted protein and mechanically pressing the cuticle smooth.

These treatments are distinct from permanent hair relaxers (which use alkaline chemicals to break and reform disulfide bonds irreversibly) and from temporary conditioning masks. A keratin treatment is semi-permanent — it gradually washes out as the cuticle rehydrates and protein deposits diffuse away.

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How Keratin Treatments Work: The Mechanism

Protein Deposition

Salon keratin products contain hydrolyzed keratin — keratin broken into smaller peptide fragments by acid or enzymatic hydrolysis, making it small enough to penetrate the cuticle surface and partly enter the cortex. Research published in Molecules (2025) using SEM and fluorescent penetration experiments confirmed that hydrolyzed keratin deposits on the cuticle to form a protective film and partly penetrates the cortex, improving tensile properties and UV resistance.

The Crosslinking Agent

The critical ingredient — and the center of most safety debate — is the crosslinking agent that bonds the deposited keratin into place and holds the cuticle flat:

• Formaldehyde / methylene glycol: The original and most effective agent. Methylene glycol (a liquid form of formaldehyde) releases formaldehyde gas when heated, forming covalent bonds between keratin chains and the hair shaft. This produces the most durable results but presents documented health risks (see Safety section below).
• Glyoxylic acid: The main "formaldehyde-free" alternative. Glyoxylic acid reacts similarly with keratin amino residues (primarily amine groups) to form stable bonds. It is less irritating in acute use, but emerging evidence — including case reports of acute kidney injury from scalp absorption — has raised new safety questions.
• Other carbonyl compounds: Glutaraldehyde, citric acid, and proprietary blends are used in some systems with variable efficacy and safety profiles.

The Heat-Sealing Step

After the product is applied and allowed to process for 20–45 minutes, a stylist flat-irons the hair at 230–450°F (110–230°C) in very thin sections. This heat step activates the crosslinking chemistry, locks the cuticle flat, and drives the protein deeper into the cortex. The number of flat-iron passes and the temperature directly affect both the smoothing result and the degree of any protein or cuticle damage.

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Types of Keratin Treatments

Key nuance: Products labeled "formaldehyde-free" or "organic" are not automatically safer. Testing by ChemRisk (cited in OSHA's hair salon safety guidance) found that some "formaldehyde-free" products contained between 3% and 11.5% formaldehyde by weight. Methylene glycol — the liquid precursor — does not appear on the label as "formaldehyde" but releases it on heating.

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How Long Does a Keratin Treatment Last?

Most professional keratin treatments last 2–6 months. The range reflects several variables:

Factors that shorten results:

• Sulfate-containing shampoos (sodium lauryl or laureth sulfate) — these detergents strip the cuticle-sealing protein film most aggressively
• Frequent washing (more than 3–4 times per week accelerates protein diffusion)
• Swimming in chlorinated or saltwater pools without protective oil or cap
• High-porosity hair (cuticle gaps allow water ingress and protein loss faster)
• Humidity in the first 72-hour cure window before the crosslinking fully sets
• Heavy-handed heat styling without a protectant

Factors that extend results:

• Sulfate-free, sodium chloride–free shampoo used throughout the treatment life
• Washing 2–3 times per week maximum
• Rinsing hair before swimming and applying a light oil barrier
• Silk or satin pillowcase (reduces mechanical friction at the cuticle)
• Avoidance of sodium chloride in styling products (salt opens the cuticle)

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What to Expect: The Process Step-by-Step

A professional keratin treatment typically runs 2–4 hours in the salon. Here is what the process looks like:

1. Clarifying wash: A sulfate-heavy shampoo (often provided) strips existing buildup and opens the cuticle for maximum product uptake.
2. Towel dry to damp: The product penetrates best into damp — not wet or dry — hair.
3. Section and apply: The keratin solution is painted onto hair in sections, root to tip, and combed through for even distribution. Most stylists leave the scalp bare by a few millimeters to minimize skin exposure.
4. Processing time (20–45 min): The product dwells. Some systems use gentle heat from a hood dryer to begin activation.
5. Blow-dry: The hair is dried fully straight before flat-ironing.
6. Flat-iron sealing (the critical step): Very thin sections are pressed at high heat, multiple passes. This seals the cuticle and activates the crosslink.
7. Leave-in cure: The treatment continues to crosslink over the next 72 hours; clients are instructed not to wash, wet, or bind the hair during this window.

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Safety: Formaldehyde and Health Risks

Formaldehyde Exposure Risk

This is the most clinically significant concern with keratin treatments and the reason that informed consent matters before booking.

The U.S. Occupational Safety and Health Administration (OSHA) has issued detailed guidance on formaldehyde in hair salons. In 2011 enforcement inspections, OSHA measured salon air formaldehyde concentrations during keratin treatment application that exceeded OSHA's 15-minute short-term exposure limit (2 ppm) by five times in some salons. The FDA has issued warning letters to product manufacturers for mislabeling formaldehyde-containing products as "formaldehyde-free."

Known health effects from formaldehyde exposure in this context include:

• Acute: Eye irritation, respiratory tract inflammation, headache, dizziness, nausea, chest tightness, skin rash
• Occupational (repeated): Hair stylists performing multiple treatments daily have developed occupational asthma and sensitization
• Carcinogenicity: Formaldehyde is classified as a human carcinogen (Group 1) by the International Agency for Research on Cancer (IARC) — primarily based on nasopharyngeal cancer risk from occupational inhalation. The risk for a client receiving occasional treatments is substantially lower than for stylists with chronic daily exposure, but it is not zero.

A 2022 PMC systematic review of chemical straighteners (pmc.ncbi.nlm.nih.gov/articles/PMC9073307/) found documented effects including scalp eczema, burns, inflammation, and hair shaft structural damage across the broader category of chemical straightening treatments.

Glyoxylic Acid and Kidney Risk

Glyoxylic acid ("formaldehyde-free") alternatives were initially welcomed as a safer option. However, an emerging body of case reports — including a 2025 PMC article titled "Oxalate Nephropathy Associated with Glyoxylate-Containing Hair-Straightening Products: A Call for Caution" — has identified a newly described skin–kidney axis: glyoxylate absorbed through the scalp can be metabolized to oxalate, which crystallizes in renal tubules and causes acute kidney injury (AKI) in susceptible individuals.

This risk appears concentrated in people with pre-existing kidney disease, but cases in otherwise healthy adults have also been reported. The phenomenon remains under active investigation, and vigilance is warranted — particularly for people with any history of kidney stones or reduced kidney function.

Scalp Skin Reactions

A clinical-dermatoscopic study of 43 patients (PMC9231525, 2022) documented psoriasiform scalp reactions — scaling, redness, and inflammation — following Brazilian keratin treatments. These reactions resolved in most patients after stopping treatment, but some required topical steroid therapy.

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Who Should Avoid Keratin Treatments

• Pregnant women: OSHA and published pregnancy safety reviews advise caution or avoidance. Occupational formaldehyde exposure has been associated with elevated miscarriage risk in studies of hairdressers. The MotherToBaby Fact Sheets (NCBI Bookshelf, NBK582739) note that occasional personal use by clients is unlikely to cause birth defects, but advises pregnant women to consult their healthcare provider and ensure the salon is well-ventilated.
• People with kidney disease or history of kidney stones: Given the emerging glyoxylate–oxalate AKI data, the risk–benefit calculus is unfavorable. Avoid formaldehyde-free glyoxylic acid systems until the evidence picture is clearer.
• Scalp psoriasis, eczema, or open scalp wounds: Chemical exposure to inflamed or broken skin dramatically increases systemic absorption and local irritation risk.
• Anyone with respiratory conditions (asthma, COPD): The aerosol and heat-released vapors during application pose a meaningful inhalation risk, particularly in poorly ventilated salons.
• Children and adolescents: There is no established safety data for keratin treatments in pediatric hair; the chemical exposure risks are disproportionate relative to any cosmetic benefit.

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Keratin Treatments vs. Other Smoothing Options

For people concerned about chemical exposure, frizz-causes-and-fixes covers non-chemical approaches including anti-humidity hair tips that work topically wash-to-wash without cumulative chemical load.

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Aftercare: How to Maximize Longevity

The most critical aftercare rules — supported by the chemistry of how keratin treatments work — are:

The first 72 hours:

• Do not wash or wet the hair
• Avoid tucking hair behind ears, using clips or bands that create a crease, or tying hair up — the crosslinks are still forming and the cuticle can be deformed
• Stay out of rain, steam, and high humidity
• If hair accidentally gets wet, blow-dry it straight immediately on a low-heat setting

Ongoing maintenance:

• Use only sodium lauryl/laureth sulfate–free shampoo — sodium chloride should also be absent from your shampoo and styling products
• Wash 2–3 times per week maximum
• Apply a lightweight oil or leave-in conditioner to maintain moisture and surface protection
• Use a heat protectant before any flat-iron or blow-dry session (even treated hair can be heat-damaged)
• Rinse hair with fresh water before swimming in pools or the ocean; apply a protective oil; wash with sulfate-free shampoo immediately after

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Cost and What to Expect at a Salon

Pricing varies significantly by region, salon tier, hair length, and product used:

• Budget salons / keratin express treatments: $80–$150
• Mid-range professional salons: $150–$300
• High-end or specialist salons: $300–$600+

At-home keratin kits exist (typically $20–$80) but present greater difficulty applying consistent heat and product distribution, and the same safety considerations apply — perhaps more so, since home application often lacks professional ventilation.

When booking, ask the salon:

1. Which specific product and brand they use
2. Whether it contains formaldehyde or methylene glycol
3. Whether the salon has adequate ventilation (ideally, direct exhaust fans and fresh air circulation)
4. Whether they can open windows or doors during your treatment

See also the keratin treatment guide vs. other options and our hub on damage, repair, and protection for context on where keratin treatments fit in a broader hair health strategy.