Ingredient review
Calcium Hydroxide
INCI: Calcium Hydroxide
Calcium hydroxide is a strong alkaline ingredient used in hair relaxers and depilatories, but it can be highly irritating and is not recommended for sensitive or damaged skin.
In plain English
Calcium hydroxide is a white powder that makes water very alkaline (high pH). In skincare, it's mainly used in hair relaxers to break down hair proteins so hair can be straightened, or in depilatory creams to dissolve hair. Because it's so alkaline, it can irritate or burn skin if left on too long or used too often. It's not a moisturizer or active treatment ingredient—it's a chemical tool for changing hair structure.
Review score
Safety, usefulness, and evidence
Higher-caution ingredient
The evidence base is useful, but some claims depend heavily on the formula.
Main practical flags: irritation is high; clogging risk is low.
- Source
- synthetic
- Evidence
- moderate
- Irritation
- high
- Clogging risk
- low
Quick decision guide
Read the cautions before using
Calcium Hydroxide can be useful, but watch for high irritation potential.
Plain-English read
Treat this as a practical screening step before you compare products that contain this ingredient.
- Step 1Start with the score, then check the irritation and clogging risk before judging Calcium Hydroxide.
- Step 2Use the "Best for" and "Use caution if" sections to match the ingredient to your skin, not just to a marketing claim.
- Step 3If a product stings, breaks you out, or worsens irritation, judge the finished formula and stop using it even if the ingredient scores well.
Score terms in plain English
Irritation risk
highMore likely to cause dryness, stinging, peeling, or reactivity if used too aggressively.
Clogging risk
lowLess likely to feel heavy or contribute to clogged pores for most skin types.
Evidence level
moderateThere is useful support, but formula details and claim strength still matter.
What it is
Calcium hydroxide is an inorganic compound (Ca(OH)₂) produced by mixing calcium oxide with water. It has a very high pH (around 12–13) and is used in cosmetics to raise pH or to break down keratin in hair for straightening or removal.
How it works
In hair relaxers, calcium hydroxide raises the pH of the product, which swells and softens the hair shaft, breaking disulfide bonds so the hair can be reshaped. In depilatories, it helps dissolve the keratin protein in hair, making it easy to wipe away. Its strong alkalinity also helps adjust the pH of other ingredients in a formula.
Pros
Effective hair straightening
Calcium hydroxide is one of the most effective ingredients for permanently straightening curly or coily hair when used in relaxer formulations.
Depilatory action
It helps dissolve hair quickly in depilatory creams, making hair removal easier and less painful than shaving.
Cons and cautions
High irritation risk
Its very high pH can cause chemical burns, redness, and irritation, especially if the product is left on too long or used on sensitive skin.
Hair damage potential
Overuse or improper application can lead to hair breakage, thinning, and scalp damage, particularly with repeated relaxer treatments.
Best for
- People with resilient, non-sensitive scalps seeking temporary hair straightening
- Those using professional-grade depilatory products under guidance
Use caution if
- Anyone with sensitive skin, eczema, psoriasis, or scalp conditions
- People with damaged or chemically treated hair
Usage tips
Safety summary
Calcium hydroxide is safe for use in rinse-off hair products when formulated correctly and used as directed, but it poses a high risk of irritation and chemical burns if misused. It is not recommended for daily skincare or for individuals with sensitive skin.
Research notes
Research confirms calcium hydroxide's effectiveness in hair straightening and depilation, but also documents its potential for skin irritation and hair damage. Studies emphasize the importance of proper formulation and usage guidelines to minimize adverse effects.
Common label clues
- Typical concentration
- Typically used at 1–5% in rinse-off products; much lower in leave-on formulations
- Regulatory status
- Approved for use in cosmetics in the US (FDA) and EU, but restricted to rinse-off products and must be used at safe concentrations to avoid skin burns.
- Common uses
- Hair relaxers, Depilatory creams, pH adjustment
- Environmental note
- Calcium hydroxide is a naturally occurring mineral, but its production and disposal can raise water pH, potentially affecting aquatic life if not properly managed.
Good to know
- Calcium hydroxide is often used in combination with guanidine carbonate in 'no-lye' relaxers, which are less harsh than traditional lye relaxers.
- It is not typically found in leave-on skincare products due to its high irritation potential.
Common questions
What is Calcium Hydroxide in beauty products?
Calcium hydroxide is a white powder that makes water very alkaline (high pH). In skincare, it's mainly used in hair relaxers to break down hair proteins so hair can be straightened, or in depilatory creams to dissolve hair. Because it's so alkaline, it can irritate or burn skin if left on too long or used too often. It's not a moisturizer or active treatment ingredient—it's a chemical tool for changing hair structure.
What does Calcium Hydroxide do in a beauty product?
In hair relaxers, calcium hydroxide raises the pH of the product, which swells and softens the hair shaft, breaking disulfide bonds so the hair can be reshaped. In depilatories, it helps dissolve the keratin protein in hair, making it easy to wipe away. Its strong alkalinity also helps adjust the pH of other ingredients in a formula.
Is Calcium Hydroxide safe for most people?
Calcium hydroxide is safe for use in rinse-off hair products when formulated correctly and used as directed, but it poses a high risk of irritation and chemical burns if misused. It is not recommended for daily skincare or for individuals with sensitive skin.
Who should be careful with Calcium Hydroxide?
Anyone with sensitive skin, eczema, psoriasis, or scalp conditions People with damaged or chemically treated hair
Research sources
Ingredient reviews are educational and are not medical advice. Patch test new products and ask a licensed clinician about persistent irritation, allergies, pregnancy-specific questions, or diagnosed skin conditions.