A hair relaxer permanently straightens curly or coily hair by breaking the protein bonds that give each strand its natural shape. The active chemicals penetrate the outer layer of the hair shaft, reach the inner cortex, and rearrange the structural connections between keratin proteins so the hair lies flat. The result is lasting straightening on the treated section of hair, though new growth will still come in with your natural curl pattern.
How a Relaxer Changes Hair Structure
Your hair gets its shape from connections called disulfide bonds, which act like tiny bridges linking the protein chains inside each strand. These bridges hold keratin (the main structural protein in hair) in its natural spiral or coiled formation. A relaxer uses a strongly alkaline chemical to break those bridges apart. While the bonds are broken and the keratin is soft, a comb is used to physically pull the hair straight, repositioning the protein chains. This rearrangement is permanent on the treated hair because the bonds reform in their new, straightened configuration.
The specific chemical reaction replaces about a third of the original bond-forming amino acids with a different, more stable type of cross-link. This is why relaxed hair can’t revert to its original curl pattern, even when wet. It also means the process can’t be undone. Once hair is relaxed, it stays relaxed until it’s cut off and replaced by new growth.
Lye vs. No-Lye Relaxers
All relaxers rely on a hydroxide compound as their active ingredient, but they’re divided into two categories based on which one they use.
- Lye relaxers contain sodium hydroxide. They tend to work faster and are considered the stronger option, with pH levels typically ranging from 12 to 14. They come in “super,” “regular,” and “low lye” strengths, with low lye formulas containing less than 2.5% sodium hydroxide.
- No-lye relaxers use calcium hydroxide, guanidine hydroxide, lithium hydroxide, or potassium hydroxide instead. Their pH levels generally fall between 9 and 11, making them slightly less alkaline. They’re often marketed as gentler, though they still carry similar risks for hair damage.
The strength of any relaxer comes down to how much hydroxide it contains and its pH level. For context, healthy hair sits at a mildly acidic pH of around 4.5 to 5.5. A study measuring the pH of commercially available relaxers, including those marketed for children, found a median pH of 12.36 across all products tested. That level is classified as corrosive to skin under occupational safety standards, which define anything at pH 11.5 or above as corrosive.
What Happens to the Hair Shaft
The outermost layer of each hair strand, the cuticle, is made of overlapping scales that protect the inner cortex (similar to shingles on a roof). Relaxers force these scales open to reach the cortex where the disulfide bonds live. This causes visible, measurable damage to the cuticle layer. Research examining relaxed hair under microscopy found severe cuticle lifting, fissures, and gaps that left the inner cortex exposed, particularly on curlier hair types.
This cuticle damage has practical consequences. Relaxed hair loses some of its ability to absorb and hold onto moisture, and its resistance to breakage drops. Studies comparing treated and untreated hair found reductions in breakage resistance, water retention, and overall hair mass after chemical straightening. The more often you relax, and the longer you leave the product on, the more pronounced these effects become.
Processing time plays a role in how much structural damage occurs. Relaxers are typically left on for 20 to 30 minutes, depending on hair texture and the product’s strength. Leaving a relaxer on too long, or overlapping it onto previously relaxed hair, significantly increases the risk of breakage and scalp irritation.
The Neutralizing Step
After the relaxer is rinsed out, a neutralizing shampoo is applied. This step is critical. The neutralizer’s job is to remove leftover alkaline residue from the hair and bring the pH back down into the acidic range, typically below 7 and ideally closer to 5 or lower. At that acidic pH, the rearranged bonds in the cortex lock into their new positions permanently.
Skipping or rushing the neutralizing step leaves alkaline residue on the hair and scalp, which can continue breaking down protein bonds after the treatment is supposed to be finished. This is one of the most common causes of post-relaxer breakage and scalp irritation.
Scalp and Hair Loss Risks
Because relaxers operate at pH levels that are corrosive to skin, scalp burns and irritation are common side effects, especially if the product contacts the scalp directly or is left on too long. Over time, repeated chemical treatments combined with tight styling can weaken hair follicles and contribute to traction alopecia, a form of hair loss caused by sustained tension on the hair root.
Frequent relaxer use is also linked to central centrifugal cicatricial alopecia (CCCA), a type of scarring hair loss that starts at the crown and spreads outward. It’s particularly prevalent among Black women. In CCCA, the hair follicle is permanently destroyed and replaced by scar tissue, meaning the hair loss is irreversible. Applying relaxers to already chemically treated hair increases this risk substantially.
Potential Health Concerns Beyond Hair
Recent research has raised questions about the systemic health effects of chemicals found in relaxer formulations. Several ingredients commonly detected in these products, including formaldehyde and formaldehyde-releasing agents, phthalates, and parabens, are classified as endocrine disruptors, meaning they can interfere with the body’s hormone signaling.
Formaldehyde, which can be released into the air during application, has been shown to bind directly to DNA and proteins, causing the type of cellular damage associated with cancer development. Phthalates are linked to inflammation and oxidative stress, while parabens can disrupt thyroid hormone regulation.
A large prospective study published in the Journal of the National Cancer Institute followed participants for a median of 13.1 years and found that use of hair straighteners and chemical relaxers was associated with a 71% higher incidence of thyroid cancer and a 166% higher incidence of pancreatic cancer compared to non-users. The researchers noted these findings need confirmation in additional studies, but the associations were statistically meaningful. Several of the implicated chemicals are now being considered for regulation or bans under California’s Proposition 65 and the European Union Cosmetics Directive.
Caring for Relaxed Hair
Because the relaxing process strips away some of the hair’s natural protective structure, relaxed hair needs consistent aftercare to stay manageable and minimize breakage. Protein treatments are one of the most effective tools. They work by temporarily patching weak spots along the damaged cuticle, laying those lifted scales back down so strands feel smoother, stronger, and less prone to frizz. Adding regular protein treatments to your routine helps compensate for the keratin lost during the chemical process.
Moisture is equally important. Since relaxed hair has a reduced ability to hold water, using deep conditioners and leave-in treatments between relaxer sessions helps prevent the dryness and brittleness that lead to breakage. If you also color your hair, wait at least one week and one shampoo after a relaxer service before applying any dye. Layering chemical processes too close together dramatically increases the chance of damage.
Avoiding overlap is one of the simplest ways to protect relaxed hair long-term. When you get touch-ups, the relaxer should only be applied to new growth near the roots, never re-applied to hair that’s already been processed. Overlapping onto previously relaxed sections is a primary cause of mid-shaft breakage, where hair snaps at the weakened point where old and new treatments meet.