Sodium Hydroxide

Sodium hydroxide lye relaxers operate at pH 12–14, swelling the cuticle and allowing hydroxide ion to penetrate the cortex where it breaks disulfide bonds via lanthionization—a permanent, irreversible chemical change. Extreme alkalinity causes chemical burns and scalp lesions that significantly enhance percutaneous absorption of co-formulants. NIEHS Sister Study (2022) found HR 2.55 for uterine cancer in frequent users. JNCI 2026 follow-up found HR 2.66 for pancreatic cancer, HR 1.71 for thyroid cancer, and HR 1.62 for non-Hodgkin's lymphoma. Epidemiological signal is strong; causal pathway likely includes co-formulant absorption (formaldehyde-releasers, parabens, phthalates, BPA, metals) through burn-damaged scalp. Disproportionate risk burden on Black and African American women, many starting use in childhood/adolescence. EU Annex III restricts concentration and mandates professional use for higher-concentration products. The EU classifies Sodium Hydroxide as "restricted". Safety rating: POOR. 7 toxicological study endpoint(s) are available in our database.

Sodium Hydroxide EU regulatory status: restricted. Maximum allowed concentration: Annex III entry 25: max 3.0% as hair straightener in professional products; max 2.0% in general consumer products; pH limits apply. Not for children.. This is based on EU Regulation 1223/2009 and its amendments.

Sodium Hydroxide functions as: alkali; cuticle-swelling agent; disulfide bond-breaking agent (lanthionization); pH modifier. It is classified as a Active Ingredient in our database. CAS number: 1310-73-2.

not established The Margin of Safety (MoS) is calculated using SCCS methodology. A MoS above 100 is generally considered safe. Use the MoS Calculator tool to calculate MoS for your specific formulation and product category.

The NOAEL (No Observed Adverse Effect Level) for Sodium Hydroxide is 147 mg/kg bw/day based on a Special Toxicology Study study via oral route in rabbit. A total of 7 study endpoints are available. Source: COSMOS_DB.

Sodium Hydroxide carries a GHS signal word of "DANGER". Hazard statements: Harmful if swallowed Causes severe skin burns and eye damage May cause respiratory irritation. This classification is based on the ILO/WHO International Chemical Safety Card and ECHA C&L inventory data. Note: GHS classification applies to the pure substance — at cosmetic use concentrations, hazard thresholds may not be met.