Skincare Ingredients

10-Hydroxydecanoic Acid

10-Hydroxydecanoic acid is a medium-chain hydroxy fatty acid used as an antimicrobial/sebum-modulating component (notably in royal jelly derivatives) and is typically present at low levels, but as a free acid it can still sting or provoke erythema in compromised or highly reactive skin. Human irritation/sensitization data are limited compared with common fatty acids, so I do not treat it as inherently “gentle,” especially given its association with royal jelly–related allergenicity in some individuals. In real-world routines (often combined with other actives/preservatives), a mild irritation risk is plausible, warranting a cautious “mild” score. Safety Notes: 10-Hydroxydecanoic acid (a royal jelly–associated hydroxy fatty acid) is typically used in consumer skincare as a minor active within “royal jelly/10-HDA” complexes, where finished-product levels can be as low as ~0.001–0.01% in lotions/creams/serums that rely on extracts or blends. High-strength OTC products marketed around “10-HDA” (typically leave-on serums/spot products) are observed up to about 1% before sensory issues (waxy/soapy feel), solubility and pH-dependent salt formation, and irritation risk usually constrain further increases; rinse-off formats generally sit toward the lower end due to short contact time.

1,2-Hexanediol

1,2-Hexanediol is a humectant/solvent and preservative booster typically used around ~0.5–2%, and it is generally well tolerated but not fully inert. Patch testing and real‑world reports show occasional irritation/stinging—especially on compromised barriers (eczema, post‑procedure) or when paired with other preservatives/actives—so I score it as mild to reflect a small but meaningful risk in highly reactive patients. Safety Notes: In commercial skincare, 1,2-hexanediol is most often used as a multifunctional humectant/solvent and preservative booster, commonly appearing around ~0.3–1.0% in leave-on serums/creams and rinse-off cleansers, with low-end usage observed around ~0.1% when it is a minor co-solvent within broader preservative blends. High-strength consumer-available formulations (especially “preservative-free” systems relying on glycols/diols for antimicrobial support, some minimalist K-beauty/toner/essence bases, and certain anhydrous or water-light formulas) can push total 1,2-hexanediol to about 3–5% for solubilization and preservation support, above which irritation/tack and sensorial limits typically constrain use in mainstream OTC products. It is not specifically concentration-restricted as a preservative in EU/FDA cosmetics, so the upper bound is primarily set by tolerability and formulation performance rather than a hard regulatory cap.

2,3-Butanediol

2,3-Butanediol is a low–molecular weight diol used mainly as a humectant/solvent at modest concentrations, and diols in this class are generally well tolerated with low rates of irritation in standard patch testing. In very reactive or barrier-impaired skin it can still cause occasional stinging or dryness (especially alongside other actives), but the overall intrinsic irritancy is low, supporting a very gentle score. Safety Notes: 2,3-Butanediol (often listed as butanediol) is used primarily as a solvent/humectant and occasional fragrance/processing aid; in many mass-market leave-on skincare products it appears at very low levels (trace to ~0.1%) as part of an extract/ferment carrier system. In higher-solvent, water-based leave-on formats (serums/essences/ampoules, especially ferment-focused products), it can be deliberately dosed in the low-single-digits up to around 10% to support solvency, sensorial slip, and humectancy; rinse-off products typically sit toward the lower end due to wash-off and cost/need considerations. No specific EU/FDA maximum applies beyond general safety/GMP; practical limits are driven by odor/feel and compatibility with the rest of the solvent system.

4-T-Butylcyclohexanol

4-T-Butylcyclohexanol is primarily used as a fragrance/perfuming agent and odor-masking component, typically at low concentrations, but fragrance-class materials are a common cause of stinging/irritant reactions in eczema and highly reactive skin. While it is not a strong active like an acid or retinoid and is generally tolerated by many users, its sensory function and contact exposure profile justify a mild irritation rating to reflect real-world risk in compromised barriers. Safety Notes: 4-T-Butylcyclohexanol (commonly used as a cooling/soothing anti-sting agent for sensitive skin, often referenced in industry as SymSitive® 1609) is found at very low levels (~0.005–0.05%) in mass-market moisturizers, cleansers, and after-sun products where it is used as a sensorial modulator rather than a primary active. Most leave-on soothing serums/creams cluster around ~0.1–0.5% per supplier/industry formulation practice and efficacy-use levels, while the highest consumer-available “intensive anti-redness/anti-stinging” leave-on products and concentrates can reach about 1.0% (higher levels are uncommon due to sensorial limits and diminishing returns).

Acacia Farnesiana Extract

Acacia farnesiana (sweet acacia/cassie) extract is a botanical extract often used for skin conditioning or fragrance-adjacent effects, and plant extracts can contain allergenic small molecules even at low use levels. Reports of contact allergy exist for A. farnesiana-derived materials (especially when used for scent), and in eczema-prone or barrier-impaired skin this makes irritation/sensitization plausible in real-world routines. Given the variability in extract composition and the higher-risk sensitive-skin population, I rate it as a moderate irritant where patch testing is prudent. Safety Notes: In mass-market leave-on skincare, Acacia farnesiana extract is often used at trace levels (≈0.0001–0.05%) as a supporting botanical/skin-conditioning claim ingredient or as part of a multi-extract blend where the effective individual extract level is very low. More “natural/actives-forward” consumer products (serums, masks, firming/anti-wrinkle leave-ons) can push total Acacia farnesiana extract to ~1–5% when supplied as a cosmetic-grade glycerin/propylene glycol/water extract and used for film-forming/skin feel benefits; rinse-off products typically sit toward the low end due to short contact time and cost/solids constraints.

Acacia Senegal Gum

MVP Approved - Acacia Senegal Gum is a natural film-forming, thickening, and stabilizing agent used in cosmetics, known for its soothing properties and low irritancy profile.

Acacia Seyal Gum Extract

Acacia seyal gum extract is a film-forming polysaccharide used mainly as a thickener/binder at low concentrations (typically well under a few percent), and it is generally well tolerated on compromised or sensitive skin. While true irritation is uncommon, plant gums can occasionally trigger stinging or contact allergy in highly reactive or eczema-prone patients, so it is not fully inert but remains very gentle overall. Safety Notes: In commercial leave-on skincare (serums, moisturizers, eye products), Acacia seyal gum extract is most often used as a film-former/texture modifier at very low levels (~0.01–0.3%), frequently as part of a complex (e.g., “acacia gum + maltodextrin” or other carriers) where the active gum solids are diluted. Higher-strength OTC “lifting/tensor” gel-serums and mask-like formulations can push total extract usage into the ~1–5% range to maximize immediate tightening/film effects, with higher levels becoming increasingly tacky and prone to pilling; rinse-off products generally sit toward the lower end due to wash-off and foam/viscosity constraints.

Acer Saccharum Extract

Acer Saccharum (sugar maple) extract is a botanical extract typically used at low percentages as a skin-conditioning/antioxidant component, but it is also marketed as a source of natural AHA-like compounds that can increase stinging in compromised barriers. While not a high-potency exfoliating acid on its own, botanical extracts carry variability and a measurable risk of irritant reactions in highly sensitive or eczematous skin, especially when layered with other actives. Given this and the need to err on patient safety, it fits best as a mild irritant potential ingredient. Safety Notes: In commercial skincare, Acer Saccharum (sugar maple) extract is most often used as a minor botanical adjunct in leave-on creams/serums/toners and in rinse-off cleansers, where it commonly appears at very low levels (~0.0005–0.1%) as part of multi-extract blends or to support “AHA/enzymatic” positioning. Higher consumer-available usage is seen in exfoliating/brightening leave-on formulas and some masks that feature the extract more prominently, where formulators may dose it at ~1–5% depending on the supplier’s standardization and solvent system (e.g., glycerin/water). There is no specific FDA/EU maximum for this INCI; practical upper limits are driven by stability, color/odor, tackiness, and irritation potential when combined with other exfoliating actives.

Acetic Acid

Acetic acid is a low–molecular weight acid that is clearly irritating at effective (low pH) use levels; even dilute solutions can sting and cause erythema on compromised skin, with higher concentrations being caustic. In cosmetic/skin-contact products it is typically used as a pH adjuster at low percentages, but because its irritancy is strongly pH- and concentration-dependent and sensitive/eczema-prone users can react sharply to acidic exposure, it warrants a very high irritancy score for patient safety. Safety Notes: In consumer skincare, acetic acid is most often used at very low levels (~0.01–0.3%) as a pH adjuster/acidulant in both leave-on and rinse-off products, and these trace levels are widely seen on INCI lists. Higher, consumer-available concentrations appear mainly in rinse-off chemical exfoliants/peels and callus/foot preparations where “acetic acid” is the declared active acid (commonly ~2–8%), while true household vinegar (~5% acetic acid) is not typically sold as a regulated cosmetic but informs the upper bound of consumer exposure in cosmetic-adjacent use; leave-on products at these higher levels are uncommon due to irritation potential and pH constraints.

Acetone

Acetone is a volatile solvent used at high concentrations (often near 100% in nail polish removers) that rapidly extracts skin lipids, causing dryness, barrier disruption, stinging, and irritant contact dermatitis in susceptible users. Clinical and occupational data consistently show it to be a common irritant with repeated exposure, with heightened risk on compromised or eczematous skin. In real-world routines where hands/face may already be exposed to surfactants and actives, cumulative barrier damage makes irritation likely, warranting a high score. Safety Notes: In consumer skincare/cosmetic-adjacent products, acetone is most often encountered as a solvent/quick-drying carrier at very low levels (trace to ~1%) in some astringents, wipe/cleanser-type products, and specialty solutions where it functions mainly as a volatile solvent. At the high end, OTC consumer nail polish removers and "pure acetone" removers are commonly sold at ~60–100% acetone, with 100% (or near-neat) being widely available; these are rinse-off/evaporative-use products rather than leave-on skincare due to irritation and defatting potential.

Acetylarginyltryptophyl Diphenylglycine

MVP Approved - Acetylarginyltryptophyl Diphenylglycine is a synthetic peptide used primarily in anti-aging skincare formulations for improving skin firmness and reducing signs of irritation.

Acetylated Lanolin

Acetylated lanolin is an emollient/occlusive derived from wool wax, typically used at low-to-moderate percentages to improve barrier feel, but lanolin derivatives have a well-documented history of causing contact allergy and irritation in a meaningful subset of patients—especially those with eczema or impaired barriers. Patch-test data and clinical experience place lanolin and its derivatives among more common moisturizer allergens in dermatitis populations, with reactions that can be delayed and clinically significant despite “gentle” marketing. Given the higher risk in sensitized and atopic individuals and the potential for cumulative exposure in leave-on routines, a moderate irritancy score is warranted and patch testing is advisable. Safety Notes: In commercial formulations acetylated lanolin is often used as a minor emollient/plasticizer in creams, lotions, and color cosmetics at ~0.05–1% (sometimes simply to improve slip and barrier feel). In richer leave-on moisturizers, hand/foot creams, and lip products it commonly appears around 2–10%, and high-occlusivity balms/salves and specialty barrier products marketed to consumers can reach ~15–25% to maximize substantivity and water resistance. It is less common at high levels in rinse-off products, where usage is typically kept lower to avoid deposition/feel issues.

Acetylated Lanolin Alcohol

Acetylated lanolin alcohol is an emollient derived from wool wax components, typically used at low-to-moderate percentages to improve texture and occlusivity, but lanolin-derived alcohol fractions are well-documented causes of allergic contact dermatitis in susceptible individuals. While many users tolerate it, patch-test data and clinical experience show a meaningful risk in eczema-prone and barrier-impaired skin, where even low exposures can trigger flare or delayed sensitization. Given this non-trivial allergy/irritation potential in sensitive populations, a moderate score is warranted and patch testing is prudent. Safety Notes: In commercial skincare and cosmetic products, acetylated lanolin alcohol is most often used as an emollient/co-emulsifier at low levels (~0.05–1%) in lotions, facial moisturizers, and makeup where it improves slip and barrier feel without heaviness. Mid-range usage (~1–5%) is common in richer creams and some lip products. The upper end (~10–15%) is observed in very occlusive consumer-available ointment-style barrier creams, heavy-duty hand/foot creams, and some balm/stick formats where lanolin-derived emollients are used as primary structuring/occlusive components; rinse-off products typically sit at the low end due to deposition/feel considerations.

Acetyl Carnitine Hcl

Acetyl carnitine HCl is primarily a conditioning/antioxidant and cellular-energy support ingredient used at low concentrations (typically well under 1–2%) and is generally well tolerated in leave-on products. It is not a strong acid/alkali or keratolytic active, and clinical/consumer safety data suggest a low rate of irritation, though the HCl salt and increased osmolarity can occasionally sting compromised barriers (eczema, post-procedure). Given sensitive-skin risk and real-world cumulative exposure, it fits best as a gentle but not fully “very gentle” ingredient. Safety Notes: In commercial skincare, Acetyl Carnitine HCl is most often used at very low levels (around 0.01–0.1%) in multi-ingredient antioxidant/anti-aging serums and eye products where it functions as a supportive bioactive and is easy to solubilize in the water phase. Dedicated leave-on face serums and scalp/hair-tonic style products marketed for energizing/firming commonly run 0.5–2%, with a small number of high-strength consumer-available formulas reaching ~3% before limits are typically set by sensory issues (tackiness/saltiness), pH/ionic compatibility, and cost rather than explicit regulatory caps.

Acetyl Cysteine

Acetyl cysteine (N-acetyl-L-cysteine) is used topically mainly as an antioxidant/reducing agent, typically at low percentages, and is generally well-tolerated. However, its thiol chemistry and characteristic sulfurous nature can be irritating for reactive or barrier-impaired skin, and mild stinging/irritant reactions have been reported with topical use in sensitive populations. Given real-world use on compromised skin and the need to avoid underestimating risk, it fits best as a mild irritant with occasional sensitivity possible. Safety Notes: In consumer skincare, acetyl cysteine (commonly listed as N-acetyl-L-cysteine/NAC) is most often encountered at very low levels (~0.01–0.1%) in antioxidant/multi-ingredient serums and creams where it functions as a supportive antioxidant/thiol and is limited by odor, color change, and compatibility. Specialized OTC “high-potency” antioxidant or anti-pollution leave-on serums and ampoules have been marketed in the ~1–5% range, and a small number of niche, consumer-available water-based solutions/serums approach ~10%, which is near practical upper limits for aesthetics and stability (especially pH- and oxygen-sensitive systems); rinse-off products typically sit toward the low end due to short contact time.

Acetyl Glucosamine

Acetyl glucosamine is a skin-conditioning/hydration-support and tone-evening ingredient typically used around ~1–5% in leave-on products, and it is generally well-tolerated in clinical and consumer use. It is not an exfoliating acid and is not strongly pH-dependent, so it usually does not create the stinging/burning profile seen with more reactive actives. Rare irritation can still occur in highly compromised eczema skin or when layered with other actives, but on its own its irritation potential is low. Safety Notes: In mass-market and dermatologist-brand leave-on moisturizers/serums, acetyl glucosamine is frequently used as a secondary brightening/barrier-support ingredient around ~0.1–1%, with some cleansers/rinse-off products sitting at the low end due to short contact time. Clinical and commercial pigment/texture-focused leave-on formulas commonly use ~2–4%, and a small number of high-strength OTC brightening serums/ampoules reach ~5–6% (often paired with niacinamide), which is near the upper practical limit before tackiness/solids load and stability/sensory issues become limiting.

Acetyl Glutamine

Acetyl glutamine is a skin-conditioning/humectant amino acid derivative typically used at low concentrations (about 0.1–2%) and is generally well tolerated in clinical and post-marketing experience. It is not a pH-dependent exfoliating active, not a known sensitizer, and lacks the irritancy profile seen with acids, retinoids, fragrances, or harsher preservatives. Rare reactions can occur in highly reactive or barrier-impaired eczema skin, but overall its intrinsic irritation potential is very low, supporting a 0.2 score. Safety Notes: In commercial skincare, acetyl glutamine is most often used as a low-level humectant/skin-conditioning amino-acid derivative in leave-on creams, serums, toners, and eye products, commonly appearing at trace-to-low levels around 0.01–0.1% when part of broader amino-acid/NMF complexes. Higher-strength consumer-available formulas (typically leave-on serums/ampoules and barrier-repair moisturizers marketed for hydration/soothing) have been observed using dedicated acetyl glutamine dosing in the ~1–5% range; above this becomes uncommon due to cost, solubility/formulation practicality, and diminishing returns. No specific EU/FDA cosmetic concentration limit is set for acetyl glutamine, so market use is mainly constrained by formulation/stability and sensorial considerations rather than regulation.

Acetyl Hexapeptide-1

Acetyl Hexapeptide-1 is a small signaling peptide used in low concentrations (typically well under 1%) for anti-aging/skin-conditioning, and it is generally non-stinging and non-exfoliating at functional use levels. Clinical and post-market data suggest a low incidence of irritation or sensitization compared with acids, retinoids, fragrances, and many preservatives, though rare reactivity can occur from the peptide itself or formula context. For highly reactive or eczema-prone skin, it remains a very gentle ingredient overall but not truly inert, supporting a 0.2 score. Safety Notes: In commercial leave-on skincare, Acetyl Hexapeptide-1 is often used at very low levels because it is typically supplied as a dilute solution and dosed to deliver trace-active peptide levels; products listing it near the end of the INCI commonly fall around ~0.0001–0.005% active. Higher-strength consumer anti-aging serums/ampoules and targeted eye/face treatments (still OTC) can reach ~0.01–0.05% active when formulators intentionally dose a concentrated peptide raw material. Rinse-off products rarely justify higher levels due to short contact time, so the upper end is predominantly observed in leave-on formulas.

Acetyl Hexapeptide-37

Acetyl Hexapeptide-37 is a small signaling peptide used in anti-aging/hydration products, typically at very low concentrations (often well under 1%), and it is not pH-dependent or inherently exfoliating. Available cosmetic safety/usage data and clinical experience suggest peptides like this are generally well-tolerated with low rates of stinging or dermatitis, though rare reactions can occur in highly reactive or eczema-prone skin due to individual sensitization or formulation context. Given the low intrinsic irritancy but nonzero real-world risk in compromised barriers, it fits best as "very gentle" rather than completely inert. Safety Notes: In consumer leave-on serums/creams, Acetyl Hexapeptide-37 is most often used at very low levels because it is typically supplied as a diluted solution and is effective in the ppm-to-low-bps range; commercial products commonly land around ~0.001–0.01% active. The lowest observed levels in multi-peptide or “supporting peptide” formulas can be ~0.0001% active, while high-strength OTC peptide serums and booster-style products occasionally reach ~0.05% active (generally limited by cost, solubility in the chosen base, and preservation/stability considerations). Rinse-off use is uncommon and, when present, tends to sit toward the low end due to short contact time.

Acetyl Hexapeptide-8

MVP Approved - Acetyl Hexapeptide-8 is a synthetic peptide commonly used in anti-aging skincare formulations to reduce the appearance of wrinkles by inhibiting muscle movement, with a secondary hydrating benefit noted by several sources.

Acetyl Methionine

Acetyl Methionine is an amino-acid derivative typically used at low concentrations as a conditioning/repair-support ingredient, and amino-acid based materials are generally well tolerated in leave-on skincare. It is not an exfoliating acid, solvent, or preservative and lacks the common mechanisms that drive irritant dermatitis, though rare individual sensitivity is still possible in highly reactive or barrier-impaired users. Given its low intrinsic reactivity and typical low-use levels, it fits best as "very gentle" rather than completely inert. Safety Notes: In commercial skincare, acetyl methionine is most often used as a niche amino-acid/skin-conditioning active within blends, where it commonly appears at very low levels (around 0.001–0.05%) in leave-on serums/creams and eye products as part of multi-ingredient complexes. Higher-strength consumer-available products that feature amino-acid/peptide-type actives more prominently have been observed up to ~1–2% in leave-on formulations, with the practical upper end driven by solubility, odor, and overall sensory/stability constraints rather than explicit regulatory limits. Rinse-off products typically sit at the low end (trace to ~0.1%) due to short contact time and cost, while the highest levels are mainly seen in concentrated leave-on treatments.

Acetyl Octapeptide-3

Acetyl Octapeptide-3 is a synthetic signal peptide used in anti-aging products at very low concentrations (typically well under 0.1–0.5%) and is not pH-dependent or inherently exfoliating/keratolytic. Clinical experience and patch-test data for cosmetic peptides generally show low irritation and low sensitization rates, with occasional stinging or erythema in highly reactive or barrier-impaired skin. Given the vulnerable population risk but overall benign profile at typical use levels, it fits best as very gentle rather than truly inert. Safety Notes: In commercial leave-on anti-aging serums/creams, acetyl octapeptide-3 is commonly supplied as a dilute peptide solution and is frequently dosed at very low active levels (~0.0005–0.005%) in multi-peptide blends or “wrinkle relaxing” products. Higher-strength consumer-available targeted serums and ampoules have been marketed at up to ~0.05% active for a single-peptide focus, with practical limits driven by cost and stability/compatibility rather than regulatory caps (no specific FDA/EU maximum for this cosmetic peptide). Rinse-off use exists but is uncommon and typically sits at the low end due to short contact time.

Acetyl Tetrapeptide-11

Acetyl Tetrapeptide-11 is a synthetic signal peptide typically used at very low concentrations (often well under 1%) in anti-aging formulas, and peptides in this class are generally non-stinging and well-tolerated in clinical and consumer safety data. While rare irritation can occur (often more related to the overall formula/preservatives than the peptide itself), its intrinsic irritancy potential is very low, making it appropriate for most sensitive-skin users with minimal risk. Safety Notes: In commercial leave-on skincare (serums, eye creams, anti-aging moisturizers), Acetyl Tetrapeptide-11 is most often dosed very low because it is commonly supplied as a diluted trade-material (e.g., in glycerin/water) and used at ~0.001–0.01% active, with some multi-peptide formulas using trace levels around 0.0001%. High-strength OTC specialty serums can reach about 0.05% active when the peptide is purchased as a higher-assay raw and positioned as a hero peptide, but concentrations above this are uncommon due to cost and diminishing practical formulation benefits. Rinse-off products, when they include it at all, tend to sit at the low end of this range due to short contact time.

Acetyl Tetrapeptide-2

Acetyl Tetrapeptide-2 is a synthetic signaling peptide used in low concentrations (typically well under 1%) for firming/anti-aging, and peptides as a class are generally non-stinging and well-tolerated in clinical use. Reported irritation and sensitization rates are low compared with acids, retinoids, or fragrance allergens, but any leave-on ingredient can trigger reactivity in highly compromised eczema skin, so it is best categorized as very gentle rather than inert. Safety Notes: In consumer leave-on anti-aging products (serums, eye creams, and firming moisturizers), Acetyl Tetrapeptide-2 is most often present at very low active levels because it is typically supplied as a dilute solution (often ~0.5–5% active in glycerin/water) and used at low inclusion rates; this yields finished-product actives around 0.0001–0.001% at the gentle/low end. High-strength OTC “peptide concentrate” serums and multi-peptide boosters occasionally push the finished active level up to about 0.01% when using higher supplier-recommended maximums and/or more concentrated peptide raw materials, but higher levels are uncommon due to cost, diminishing returns, and stability/formulation constraints; it is rarely used in rinse-off formats and is primarily a leave-on active.

Acetyl Tetrapeptide-40

Acetyl Tetrapeptide-40 is a synthetic signaling peptide used primarily for anti-inflammatory/soothing and anti-redness benefits, typically at very low concentrations (often well under 1%). Peptides of this type are generally non-sensitizing and show low rates of irritation in leave-on use, with reactions more often attributable to the formula’s preservatives/solvents than the peptide itself. Given rare but possible idiosyncratic sensitivity in highly reactive or barrier-impaired eczema patients, it rates as very gentle rather than fully inert. Safety Notes: Acetyl Tetrapeptide-40 is an anti-inflammatory peptide primarily used in sensitive skin and redness-reduction products. In commercial formulations, it typically appears at 0.001-0.5% in standard consumer serums and creams, with supplier recommendations generally around 0.5-2%. High-strength specialized anti-redness treatments and professional-grade leave-on products available to consumers can reach up to 3% concentration for maximum efficacy in calming applications.

Acetyl Tetrapeptide-5

Acetyl Tetrapeptide-5 is a small signaling peptide commonly used in eye products at low concentrations (typically well under 1%) to help reduce puffiness; it is not pH-dependent and is not an exfoliating or keratolytic active. Available safety and cosmetic patch-test data for peptides in this class show low rates of irritation and sensitization, but reactions can still occur in highly reactive or barrier-impaired eczema patients, especially in multi-ingredient formulas. Given the generally excellent tolerability yet non-zero risk in compromised skin, it fits best as very gentle rather than inert. Safety Notes: In commercial leave-on eye gels/serums, Acetyl Tetrapeptide-5 is often supplied as a diluted trade solution and ends up in finished formulas at very low active levels (around 0.0005–0.005%) in mass-market moisturizers and multi-ingredient eye products. Higher-end or "peptide-focused" OTC eye serums and targeted de-puffing treatments commonly land around 0.01–0.02% active, with a smaller set of high-strength consumer products pushing to about 0.05% active when the marketing claim is built around the peptide. It is predominantly used in leave-on products (especially eye-area skincare); rinse-off use is uncommon and typically at similarly low levels due to limited contact time.

Acetyl Tetrapeptide-9

Acetyl Tetrapeptide-9 is a synthetic signal peptide used mainly for anti-aging/firming benefits, typically at very low concentrations (often well under 1%) in leave-on products. Available clinical and cosmetic safety experience suggests peptides of this class have low inherent irritancy and are generally well tolerated, with occasional stinging or redness more likely driven by the overall formula or compromised barriers rather than the peptide itself. For highly reactive or eczema-prone patients, I still classify it as very gentle (not inert) because any leave-on bioactive can rarely trigger individual intolerance. Safety Notes: In consumer leave-on products (serums, eye creams, neck/firming creams), Acetyl Tetrapeptide-9 is often supplied as a dilute peptide solution and is used at very low active levels; the lowest market-relevant levels are typically around 0.0001% active where it appears near the end of the INCI list in multi-peptide blends. The highest OTC formulations observed generally top out around ~0.01% active (commonly corresponding to ~0.2–2% of a ~0.5% active commercial raw material), reflecting cost, stability, and sensory constraints for peptides in leave-on emulsions; rinse-off usage exists but is usually at the low end due to limited contact time.

Acetyl Tributyl Citrate

Acetyl Tributyl Citrate is primarily used as a plasticizer/solvent (often in nail products, fragrances, and some cosmetics) and is generally considered low-irritant in standard patch testing, with reactions being uncommon at typical use levels. However, because it is an organic solvent-like ester and is frequently used in products that can occlude or contact compromised skin (e.g., around nails/cuticles), mild stinging or irritant responses can occur in highly reactive or eczematous patients. For patient safety in sensitive-skin populations, it fits best as very gentle but not fully inert. Safety Notes: Acetyl Tributyl Citrate (ATBC) is used in cosmetics primarily as a plasticizer/film-forming aid and emollient for fragrances, hair styling polymers, nail products, and some long-wear color cosmetics; in these systems it is often present at low “functional” levels around 0.1–2% to improve flexibility and reduce brittleness. In high-solvent/film-forming consumer products (notably nail polishes/top coats, some long-wear liquid makeup, and certain hair styling films), ATBC is commercially observed at much higher levels, commonly in the 5–20% band and reaching ~30% when it is a major plasticizer component of the non-volatile phase. There is no specific EU/FDA maximum for ATBC as a cosmetic ingredient; practical upper limits are typically set by odor/feel, compatibility with resins/polymers, and VOC/solvent balance rather than regulatory caps.

Acetyl Tyrosine

Acetyl tyrosine is an amino acid derivative typically used at low concentrations (often ~0.1–2%) as a skin-conditioning or tone-support ingredient, and it is not pH-dependent or inherently reactive like acids/retinoids. Available cosmetic safety/patch-test experience suggests a low rate of irritation, but in severely compromised barriers (eczema, post-procedure) any small-molecule additive can still sting or trigger transient reactivity. Given that risk in highly sensitive populations while remaining broadly well tolerated, it fits a very gentle (0.2) profile. Safety Notes: In commercial skincare, Acetyl Tyrosine is most often used as a supporting amino-acid derivative in brightening/energizing or anti-aging leave-on serums and creams, where it appears at very low “label-claim” levels around 0.0005–0.05% (often within multi-ingredient complexes). Higher-strength consumer-available products (primarily leave-on serums/ampoules and targeted treatments) typically use ~0.5–2%, with a small number of high-claim formulations reaching about 3% while still remaining cosmetically stable and tolerable. It is uncommon in rinse-off products and, when present, is generally kept toward the low end due to limited contact time and cost-to-benefit considerations.

Acetyl Zingerone

Acetyl zingerone is an antioxidant/anti-glycation and photoprotective adjunct typically used at low leave-on concentrations (commonly around ~0.1–1%), and available patch/HRIPT-style safety data generally show low rates of irritation or sensitization. It is not a pH-dependent exfoliating acid and does not have the predictable stinging profile of stronger actives, so most sensitive users tolerate it well. Rare individual reactivity is still possible with any aromatic phenolic-derived molecule, but overall it fits a very gentle risk profile at typical use levels. Safety Notes: In consumer leave-on skincare (serums, moisturizers, sunscreens, after-sun/anti-pollution products), acetyl zingerone is most often used at very low antioxidant/photoprotective support levels around 0.005–0.05%, with some products/formulas and supplier-referenced use levels commonly in the 0.05–0.2% range. High-strength OTC “booster”/treatment-style products marketed for oxidative stress/photodamage support have been observed up to about 1.0%, which is near the practical upper end for aesthetics and cost while maintaining stability in typical emulsions. Rinse-off formats (cleansers) tend to sit at the low end of the range due to short contact time, while the upper end is primarily found in leave-on products.