Ingredient review
Quaternium-91
INCI: Quaternium-91
A gentle synthetic conditioner that helps detangle hair and reduce flyaways without heavy buildup.
In plain English
Quaternium-91 is a man-made ingredient that sticks to the surface of hair and skin to make them feel smoother and softer. On hair, it reduces static electricity and frizz, making it easier to comb and style. On skin, it can leave a silky, non-greasy feel. It is considered very mild and unlikely to cause irritation or clog pores.
Review score
Safety, usefulness, and evidence
Potentially useful with some tradeoffs
The evidence base is useful, but some claims depend heavily on the formula.
Risk flags are low for most users, though the finished product can still irritate.
- Source
- synthetic
- Evidence
- moderate
- Irritation
- low
- Clogging risk
- low
Quick decision guide
Useful, but context matters
Quaternium-91 is generally a lower-concern ingredient when the full formula suits your skin.
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 Quaternium-91.
- 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
lowLess likely to sting, burn, or bother most users, though sensitive skin can still react.
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
Quaternium-91 is a quaternary ammonium compound (a type of positively charged molecule) designed to condition hair and skin. It is often used as a gentler alternative to older quats like Behentrimonium Chloride.
How it works
The positive charge of Quaternium-91 is attracted to the negative charge of damaged hair or skin surfaces. It forms a thin, invisible film that smooths the cuticle, reduces friction, and locks in moisture. This film also neutralizes static charge, preventing flyaways.
Pros
Gentle on sensitive scalps
Unlike some older conditioning agents, Quaternium-91 has a low irritation profile, making it suitable for people with sensitive skin or scalp conditions.
Lightweight conditioning
It provides noticeable slip and softness without weighing hair down, which is ideal for fine to medium hair types.
Cons and cautions
Potential buildup over time
Because it adheres to hair, repeated use without clarifying shampoo can lead to a dull or heavy feel, especially on fine hair.
Not biodegradable in all conditions
As a synthetic quat, its environmental breakdown can be slower than some natural alternatives, though it is generally considered safer than older quats.
Best for
- People with dry, damaged, or frizzy hair
- Those seeking a mild conditioner for sensitive skin or scalp
Use caution if
- People with very fine or low-porosity hair who experience buildup easily
- Those strictly avoiding all synthetic ingredients
Usage tips
Safety summary
Quaternium-91 is considered safe for use in rinse-off and leave-on products at typical concentrations. It has low skin and eye irritation potential and is not known to be sensitizing. No significant safety concerns have been raised by regulatory bodies.
Research notes
Most safety data comes from industry-sponsored studies and regulatory reviews (e.g., CIR). There are limited independent clinical studies, but the ingredient is well-established in cosmetic formulations with a long history of safe use.
Common label clues
- Typical concentration
- 0.1% to 2%
- Regulatory status
- Approved for use in cosmetics in the EU, US, and many other regions. It is not restricted at typical use levels.
- Common uses
- Conditioners, Leave-in treatments, Shampoos, Styling products, Body washes
- Environmental note
- Quaternium-91 is not readily biodegradable in all environments, but it is considered less environmentally persistent than some older quaternary ammonium compounds.
Good to know
- Quaternium-91 is often used in 'low-poo' or sulfate-free shampoos because it rinses out easily with mild cleansers.
- It is compatible with many other conditioning agents and can be part of a complex conditioning system.
Common questions
What is Quaternium-91 in beauty products?
Quaternium-91 is a man-made ingredient that sticks to the surface of hair and skin to make them feel smoother and softer. On hair, it reduces static electricity and frizz, making it easier to comb and style. On skin, it can leave a silky, non-greasy feel. It is considered very mild and unlikely to cause irritation or clog pores.
What does Quaternium-91 do in a beauty product?
The positive charge of Quaternium-91 is attracted to the negative charge of damaged hair or skin surfaces. It forms a thin, invisible film that smooths the cuticle, reduces friction, and locks in moisture. This film also neutralizes static charge, preventing flyaways.
Is Quaternium-91 safe for most people?
Quaternium-91 is considered safe for use in rinse-off and leave-on products at typical concentrations. It has low skin and eye irritation potential and is not known to be sensitizing. No significant safety concerns have been raised by regulatory bodies.
Who should be careful with Quaternium-91?
People with very fine or low-porosity hair who experience buildup easily Those strictly avoiding all synthetic ingredients
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.