Skincare Ingredients

Hydrogenated Starch Hydrolysate

Hydrogenated Starch Hydrolysate is a sugar-alcohol humectant/emollient blend used to improve moisturization and texture, typically at a few percent up to higher levels in leave-on products. It is generally well-tolerated and not a common irritant in patch testing, but like other polyols it can occasionally sting or feel irritating on severely compromised barriers (e.g., active eczema or post-procedure skin), especially in higher-load formulas. Given that small but real reactivity can occur in highly sensitive patients, it fits best as very gentle rather than fully inert. Safety Notes: Hydrogenated Starch Hydrolysate is used primarily as a humectant/sugar-alcohol blend and sensory modifier, and in many commercial leave-on products (lotions, serums, sunscreens, makeup) it appears at low supporting levels around 0.1–2% to boost moisturization and slip. In richer moisturizers, barrier creams, hand creams, and some “waterless” balm/cream formats it is used much higher as a major polyol/humectant phase component, with consumer-available products reaching roughly 10–25% while maintaining stability and acceptable tack. Rinse-off cleansers and shampoos typically sit on the lower end (about 0.1–5%) due to cost and diminishing benefit with short contact time.

Hydrogenated Styrene/Isoprene Copolymer

Hydrogenated Styrene/Isoprene Copolymer is an inert, high–molecular weight synthetic film-former/texture agent used at low-to-moderate levels to improve slip, viscosity, and wear, with minimal skin penetration. Clinical experience and patch testing trends for this class of polymers show very low irritation and sensitization potential, though rare mechanical/occlusive discomfort can occur in highly reactive or compromised skin. Given its overall biologic inertness but acknowledging nonzero risk in severely sensitized patients, it aligns best with an exceptionally gentle score rather than absolute inert. Safety Notes: In commercial skincare, Hydrogenated Styrene/Isoprene Copolymer is most often used as an oil-phase rheology modifier/film former in leave-on products (e.g., lotions, creams, sunscreens, primers) where it can appear at very low levels (~0.05–0.3%) as a minor viscosity/texture aid. Typical mainstream use is commonly around ~0.5–5%, while the upper end is observed in high-structure anhydrous or high-oil systems (e.g., balms, stick products, waterless occlusive/primer-like textures) where it can reach ~10–20% to build gel networks, cushion, and water resistance; such high levels are uncommon in rinse-off formats due to sensorial and deposition constraints.

Hydrogenated Vegetable Glycerides Citrate

Hydrogenated Vegetable Glycerides Citrate is primarily an emollient/structuring agent (often used in leave-on products at low-to-moderate percentages) and is generally well tolerated, with low rates of irritant reactions in patch testing compared with surfactants, acids, or fragrance components. While it is not a potent active, its waxy lipid nature can still rarely provoke irritation or dermatitis in highly reactive/eczema-prone individuals, so it is best categorized as very gentle rather than completely inert. Safety Notes: Hydrogenated Vegetable Glycerides Citrate is used as an emollient/co-emulsifier and structuring lipid, so it appears at very low levels (~0.05–0.3%) in some leave-on lotions/serums primarily for texture and dispersion support. In mainstream creams, balms, and stick products it is more commonly in the ~1–5% band, while consumer-available high-lipid anhydrous balms, cleansing balms, and solid formats (sticks/ointments) can push it into the ~8–12% range as part of the primary structuring/emollient phase; higher levels are uncommon due to waxy feel and potential crystallization/texture constraints.

Hydrogenated Vegetable Oil

Hydrogenated vegetable oil is primarily an emollient/occlusive used at moderate to high levels in creams and balms, and it is generally well-tolerated with low irritation potential because it is chemically stable and non-volatile. Clinically, true irritant reactions are uncommon, but in highly reactive or eczema-prone skin it can occasionally contribute to discomfort or follicular occlusion in leave-on routines, so it is not scored as completely inert. Overall, it fits best as a very gentle base ingredient appropriate for most sensitive skin. Safety Notes: Hydrogenated vegetable oil is used at very low levels (~0.1–1%) as a secondary emollient/structurant in lotions, creams, cleansers, and makeup, where it mainly boosts slip and viscosity alongside other oils/waxes. In anhydrous sticks (lip balms, deodorant/skin-protectant sticks), balm salves, and solid cleansing bars, it can function as a primary base wax/fat phase and is observed in the tens of percent up to ~95% when the product is essentially a hydrogenated-oil base with minor additives (flavor/fragrance/actives). It is not subject to a specific EU/FDA concentration cap as a cosmetic ingredient; practical limits are driven by melting point/texture, pay-off, and sensory/stability needs, with the highest levels primarily in leave-on anhydrous formats rather than rinse-off.

Hydrogen Peroxide

Hydrogen peroxide is a strong oxidizing agent; at common consumer/OTC concentrations (around 3% for antiseptic use, higher in professional settings) it frequently causes stinging, erythema, and chemical irritation, especially on compromised or eczematous skin. Patch/clinical experience consistently shows it can damage the stratum corneum and delay barrier recovery, with risk increasing markedly with occlusion, repeated exposure, or higher concentrations. Given the high likelihood of irritation in sensitive populations and the potential for caustic injury when misused, it warrants a very high irritancy score. Safety Notes: In consumer skincare/personal-care products, hydrogen peroxide is most commonly encountered at the low end around 0.5–1% in mild antiseptic/cleansing or acne-spot style products (where it is typically short-contact or used sparingly due to irritation/oxidation potential). The highest widely observed OTC consumer concentration for skin-related use is 3% (the standard “drugstore” hydrogen peroxide solution sometimes marketed for first aid/skin cleansing), with higher concentrations (e.g., 6–12%+) generally restricted to hair bleaching or professional/prescription contexts rather than typical leave-on facial skincare.

Hydrolyzed Acacia Macrostachya Seed Extract

This is a hydrolyzed botanical seed extract typically used at low concentrations as a conditioning/film-forming or soothing support ingredient, but hydrolyzed plant proteins/extracts can still provoke irritation or allergic-type reactions in eczema-prone and highly reactive skin. Direct clinical irritancy data specific to Acacia macrostachya seed hydrolysate are limited, so given the potential for delayed sensitization with botanical-derived hydrolysates and the high-risk sensitive-skin population, a mild (0.4) irritancy score is the safest clinically appropriate assessment. Safety Notes: In commercial skincare, hydrolyzed botanical seed extracts like Hydrolyzed Acacia Macrostachya Seed Extract are often used at very low levels as part of a broader “botanical complex,” with effective labeling use commonly starting around 0.001–0.01% in leave-on serums/creams and occasionally in rinse-off cleansers at similar or lower levels due to short contact time. Higher-strength consumer products (typically leave-on anti-aging/firming serums and masks positioned around a single hero extract) can reach ~0.5–2.0% active, with the upper end constrained by solubility, odor/color impact, and formula stability/compatibility rather than specific regulatory limits.

Hydrolyzed Algae Extract

Hydrolyzed algae extract is primarily a conditioning/humectant film-former used at low concentrations (typically <1–5%), and it is generally well tolerated in the broader population. However, algae-derived hydrolysates are complex biological mixtures that can carry a higher rate of irritant or allergy-type reactions than simple humectants in very sensitive or eczema-prone skin, especially when barriers are compromised. Given the variability in source/processing and documented sensitization potential for some seaweed/algae derivatives, a mild (not negligible) irritancy score is the safest clinical call. Safety Notes: In mass-market leave-on skincare (serums, lotions, eye creams) hydrolyzed algae extracts are commonly used as low-level supportive actives around 0.01–0.1%, reflecting typical supplier recommended use levels and the fact that many commercial products dose marine extracts for marketing/claims rather than primary efficacy. Higher-strength consumer products positioned around firming, hydration, or “marine collagen/seaweed” benefits (often gels, ampoules, and mask essences) can reach ~1–5% of the extract as supplied, with practical upper limits driven by odor/color, viscosity, and stability rather than specific regulatory caps; rinse-off formats usually sit toward the lower half of the range due to short contact time.

Hydrolyzed Collagen

Hydrolyzed collagen is a film-forming humectant/skin-conditioning peptide mixture typically used at low-to-moderate concentrations in moisturizers and serums, and it is generally well tolerated in patch testing with low inherent irritancy. However, as a protein-derived ingredient it can occasionally trigger stinging or contact reactions in highly reactive or eczema-prone patients, especially on compromised skin or when combined with other irritants in a routine. For patient safety in severe sensitivity populations, I rate it as very gentle rather than inert. Safety Notes: In commercial leave-on facial products, hydrolyzed collagen is often used at very low levels (~0.05–0.3%) as a marketing/label claim and for light film-forming/humectant support, while many mainstream moisturizers and serums sit around ~0.5–2%. Higher-strength consumer-available formulas (especially ampoules, collagen “boost” serums, sheet-mask essences, and some hair/skin treatments) commonly reach ~3–5% and can be found up to ~10% where viscosity, odor, tack, and microbial/stability constraints still allow. Rinse-off products (cleansers/shampoos) are typically lower than leave-on because deposition is limited, but high-viscosity mask/pack formats can support the upper end.

Hydrolyzed Corn Starch

Hydrolyzed corn starch is primarily a film-forming/texture and moisture-binding carbohydrate used at low to moderate concentrations in leave-on and rinse-off products, and it is generally well tolerated in patch testing with low irritancy. However, as a plant-derived polymer it can occasionally sting or provoke mild irritation on severely compromised eczema skin (especially in occlusive, multi-ingredient routines), so it is not truly inert. For patient safety in highly reactive populations, I rate it as very gentle rather than zero-irritation. Safety Notes: In commercial skincare and haircare, hydrolyzed corn starch is most often used as a low-level film-former/skin-feel modifier or viscosity aid in leave-on creams/lotions and rinse-off cleansers, with observed use starting around 0.05–0.2% when it is a minor texture additive. Higher-strength consumer products such as mattifying primers, oil-control lotions, and some “powder-gel” or soft-focus leave-on formulations can use starch derivatives at much higher loads, with hydrolyzed corn starch observed up to ~10–15% where it provides significant sensory, blurring, and oil-absorbing effects. Use above this range is uncommon in finished OTC products due to tack/film feel, pilling risk, and stability/processing limits, and it is not subject to a specific EU/FDA maximum beyond general cosmetic safety requirements.

Hydrolyzed Elastin

Hydrolyzed elastin is a low-molecular-weight protein/peptide conditioner used in moisturizers and serums (typically ~0.1–5%) and is not inherently acidic or keratolytic, so it is generally well tolerated. However, as an animal-derived hydrolyzed protein, it carries a small but real risk of irritation or sensitization in highly reactive or eczema-prone patients (protein allergen potential and impurity variability), so I score it as gentle rather than very gentle. Safety Notes: In commercial skincare, hydrolyzed elastin is most often used as a low-level conditioning/film-forming protein in leave-on creams, serums, and eye products (commonly ~0.1–1%), with very low-end usage around 0.01% when part of multi-protein blends or as a label-support additive. Higher-strength consumer products marketed as “elastin concentrates/ampoules” and some masks/body creams can reach ~3–5% active, with the upper end typically constrained by tack/film feel, odor/color, and stability/viscosity impacts rather than specific regulatory limits (it is generally not concentration-restricted in major cosmetic regulations). Rinse-off formats (cleansers/shampoos) tend to sit toward the lower half of the range due to limited deposition, while leave-on products more commonly occupy mid-to-upper levels.

Hydrolyzed Eruca Sativa Leaf

Hydrolyzed Eruca sativa (arugula) leaf is a plant-derived protein/peptide extract used mainly as a conditioning/“hair-skin strengthening” agent, typically at low percentages, but hydrolysates and botanical extracts are not inert and can provoke stinging or dermatitis in reactive or eczematous skin. Clinical patch-test data for this specific hydrolysate is limited, so patient-safety weighting relies on class risk: botanical-derived proteins/extracts have a meaningful (though not high) potential for irritant reactions and occasional sensitization, especially on compromised barriers. Given uncertainty and the need to avoid underestimating risk in severe sensitivities, a mild irritancy score is appropriate. Safety Notes: Hydrolyzed Eruca sativa (arugula) leaf is typically supplied as a dilute active (often in water/glycerin or butylene glycol) used primarily in hair/scalp and lash/brow-style leave-on serums as a conditioning/fortifying botanical, with many mass-market formulas using it at ~0.01–0.3% as a label-claim minor active. High-strength consumer “booster”/serum products and some natural hair masks can push the supplier-recommended use levels to ~1–5% for the commercial ingredient blend, while higher levels are uncommon due to cost, sensory/stability constraints, and diminishing formulation benefit; no specific EU/FDA cosmetic maximum applies beyond general safety substantiation.

Hydrolyzed Extensin

Hydrolyzed extensin is a plant-derived hydrolyzed glycoprotein used as a film-forming/conditioning ingredient, typically at low percentages (about 0.1–2%) in moisturizers and soothing serums. Hydrolyzed proteins of this type are generally well-tolerated and not inherently irritating, but they carry a small, documented risk of irritation or sensitization in highly reactive or eczema-prone individuals due to their botanical protein nature. Given the low typical use levels and overall good tolerability but non-zero allergy potential, it fits best as very gentle rather than inert. Safety Notes: Hydrolyzed extensin (a hydrolyzed plant glycoprotein/“extensin” film-former used for conditioning/skin-feel) is typically dosed at very low levels in mass-market leave-on lotions/serums and rinse-off cleansers where it appears near the end of the INCI list (commonly around 0.001–0.1%). Higher-strength consumer products marketed for firming/tightening, lifting, or biomimetic protein film-forming effects (leave-on masks/serums) have been observed using about 0.5–2% depending on supplier activity and solids, with >2% being uncommon due to viscosity, tack/film feel, and cost constraints; no specific FDA/EU concentration limit is established beyond general cosmetic safety requirements.

Hydrolyzed Ginseng Saponins

Hydrolyzed ginseng saponins are surfactant-like glycosides used in low concentrations as botanical actives, but saponins can disrupt stratum corneum lipids and increase stinging/erythema in compromised skin (e.g., eczema) despite being marketed as “soothing.” Human irritation data for this specific fraction is limited and botanical extracts show variable purity, so I score it as mild: generally tolerated, but with a real risk of irritation in highly reactive or barrier-impaired patients, especially when layered with other actives. Safety Notes: In commercial skincare, hydrolyzed ginseng saponins (often supplied as a dilute solution or standardized extract) commonly appear at very low levels in multi-ingredient serums/toners and K-beauty-style essences, with effective on-label positioning achieved around ~0.0005–0.05% active due to strong surface activity and cost. Higher-strength consumer products marketed as “ginseng saponin concentrates/ampoules” and some mask/leave-on treatments can reach ~1–5% active-equivalent, typically requiring careful solubilization and irritation control; rinse-off cleansers generally sit toward the low end because foaming and eye/skin tolerance limit practical use levels.

Hydrolyzed Glycosaminoglycans

Hydrolyzed glycosaminoglycans are humectant/skin-conditioning polysaccharide fragments (often hyaluronic-acid–like materials) typically used at low concentrations (~0.1–1%) to support hydration and barrier feel, and they are generally well tolerated in patch-testing and clinical use. True irritation is uncommon but can occur in highly reactive or eczema-prone skin due to formulation factors (preservatives/impurities, high tack/occlusion, or compromised barrier), so it is not fully inert. Given its broad safety profile but nonzero risk in sensitized populations, it fits best as very gentle rather than completely irritation-free. Safety Notes: In commercial skincare, hydrolyzed glycosaminoglycans are typically used as a specialty humectant/skin-conditioning active at very low levels, with many leave-on moisturizers/serums and eye products listing it in the <0.1% range (often ~0.001–0.05%) to support hydration/feel without impacting viscosity or tack. Higher-strength consumer-available leave-on “hydration/plumping” concentrates and ampoules can reach ~0.5–2% (generally as the supplier’s active content rather than a dilute blend), while rinse-off cleansers tend to sit at the low end due to short contact time and cost. There is no specific FDA/EU maximum for this INCI, so the practical upper bound is driven by solubility, sensorial limits, and formula stability rather than regulation.

Hydrolyzed Hyaluronic Acid

Hydrolyzed hyaluronic acid is a low–molecular weight humectant used at low concentrations (typically well under a few percent) to bind water and improve hydration, and hyaluronans are generally well-tolerated in clinical use and patch testing. Compared with higher–molecular weight sodium hyaluronate, the hydrolyzed/low-MW form has a slightly higher likelihood of stinging on very compromised barriers and can rarely provoke irritation in highly reactive eczema patients, especially in leave-on products layered with other actives. Overall, its inherent irritancy is low and it is usually safe for sensitive skin, but not “inert,” warranting a very gentle (0.2) score. Safety Notes: In commercial skincare, hydrolyzed (low–molecular weight) hyaluronic acid is commonly used at very low levels (about 0.001–0.05%) in lotions, toners, and rinse-off cleansers as a secondary humectant/label claim, often alongside standard sodium hyaluronate. High-strength OTC leave-on serums and ampoules marketed for intensive hydration are observed up to ~0.5–1.0% hydrolyzed HA, with higher levels becoming impractical due to tackiness/film feel, viscosity changes, and diminishing sensory stability; rinse-off products generally sit at the low end of this range.

Hydrolyzed Jojoba Esters

Hydrolyzed jojoba esters are primarily emollient/skin-conditioning wax esters used at low to moderate percentages to improve barrier feel and reduce transepidermal water loss, and they are generally well-tolerated in patch testing compared with fragrances or surfactants. Because they are lipid-like and non-volatile, immediate stinging is uncommon, but in severely compromised or highly reactive eczema patients, any leave-on lipid material can rarely provoke discomfort or follicular irritation, so I score it as very gentle rather than inert. Safety Notes: In commercial leave-on skincare and haircare, hydrolyzed jojoba esters are often used at very low levels (around 0.05–0.5%) as a slip/feel modifier and conditioning aid in lotions, serums, and cleansers. Mid-range use (1–5%) is common in richer creams, body lotions, and conditioning products to boost emolliency and barrier feel without excessive greasiness. The highest consumer-available levels observed are in anhydrous balms, body butters, and specialty “ester/oil-gel” type formulations where it can function as a primary emollient/structurant, reaching about 10–15%; rinse-off products typically stay at the lower end due to cost and deposition limits.

Hydrolyzed Opuntia Ficus-Indica Flower Extract

Hydrolyzed Opuntia ficus-indica (prickly pear) flower extract is primarily used as a soothing/humectant botanical at low concentrations, and botanicals of this type are generally well tolerated in routine patch testing when properly preserved and formulated. However, as a plant-derived extract (and hydrolyzed material), it carries a non-zero risk of irritant or allergic contact dermatitis in highly reactive or eczema-prone patients, particularly with leave-on use alongside other actives. Given the limited standardized clinical irritancy data specific to the flower hydrolysate and the need to err on patient safety, a gentle-but-not-exceptional score is appropriate. Safety Notes: In commercial leave-on products (serums, essences, creams), hydrolyzed cactus-flower extracts are often used as low-level “label actives” around 0.001–0.05%, frequently because the marketed supplier blend is dosed at ~0.1–1% but contains only a small fraction of the actual extract solids. At the high end, a few consumer-available ‘booster’/ampoule-style or minimalist formulas using the neat extract (or high-solids concentrate) can reach ~1–3% active-in-formula, above which sensory, color/odor, and stability constraints typically limit use; rinse-off products generally sit toward the lower half of the range due to short contact time.

Hydrolyzed Pea Protein

Hydrolyzed pea protein is a film-forming conditioning agent used in low percentages (commonly ~0.1–2%) in hair and skin products; it is not an inherently caustic active, but hydrolyzed proteins can still sting on compromised barriers and occasionally trigger irritation. Patch-test and consumer data for plant-derived hydrolyzed proteins show generally good tolerability yet a measurable subset of reactive/eczema-prone patients report burning or itch, especially with leave-on use. Given the nonzero irritation and sensitization potential in highly sensitive populations, I rate it as mild rather than “gentle.” Safety Notes: In commercial skincare, hydrolyzed pea protein is often used at very low levels (~0.05–0.3%) in leave-on moisturizers/serums as a marketing-friendly “protein/firming” additive or to support skin feel, with many formulas dosing it below 1% due to cost, odor/color, and tack/film risk. Higher-strength consumer products (especially hair-adjacent skin/hair/scalp treatments, firming masks, and barrier-support creams) commonly run ~1–3% active, and I have observed a practical upper end around 5% in OTC leave-on products before sensory and stability constraints (stickiness/film build, viscosity drift, potential allergen labeling concerns) become limiting. Rinse-off cleansers/body washes typically sit toward the low end because contact time is short and foam/clarity can be impacted at higher protein loads.

Hydrolyzed Pumpkin Seedcake

Hydrolyzed pumpkin seedcake is a hydrolyzed plant-protein/peptide conditioning ingredient typically used at low percentages in moisturizers and hair/skin formulas, and it is not inherently caustic like acids or retinoids. However, hydrolyzed proteins and botanical extracts have a documented pattern of causing occasional irritant or allergic-type reactions in sensitive or eczematous patients (protein/botanical sensitization risk), especially with leave-on use and compromised barriers. Given the uncertainty in standardization between suppliers and the higher-risk population, I score it as mild with occasional sensitivity possible. Safety Notes: Hydrolyzed Pumpkin Seedcake is typically supplied as a hydrolyzed protein/peptide-type conditioning active and is most often used at low levels (~0.05–0.5%) in mass-market leave-on moisturizers/serums and in rinse-off cleansers where it functions as a skin-conditioning/marketing botanical. Higher-strength consumer products (e.g., specialty peptide/enzyme “pumpkin” masks, hair/skin conditioning treatments) can reach ~1–5% when the formula is built around the ingredient or its trade-name complex, with the practical upper end constrained by odor/color, viscosity/tack, and stability/compatibility rather than a specific global regulatory limit.

Hydrolyzed Rice Protein

Hydrolyzed rice protein is primarily a conditioning/film-forming protein used in low concentrations (typically <1–2%) in hair and skincare, and it is generally well-tolerated without intrinsic irritant activity. However, as a hydrolyzed protein it can rarely provoke irritation or allergy in highly atopic or protein-sensitized individuals (including some eczema patients), especially on compromised skin barriers, so it cannot be scored as exceptionally gentle or inert. A cautious “gentle” score reflects low baseline irritation with nonzero sensitization risk in reactive populations. Safety Notes: In commercial skincare and haircare, hydrolyzed rice protein is often used as a minor conditioning/film-forming active at ~0.01–0.2% in leave-on lotions/serums and some cleansers (especially when supplied as a dilute aqueous solution), with typical mainstream usage around 0.1–1%. Higher-strength consumer products marketed for “strengthening/plumping” (notably hair masks, conditioners, and some leave-on treatments) can reach about 2–5% active hydrolyzed protein; above this, sensory issues (tackiness/film), stability, and viscosity changes commonly limit use. Rinse-off formats can tolerate the upper end more readily than elegant leave-on facial products, which more commonly stay ≤1–2%.

Hydrolyzed Sodium Hyaluronate

Hydrolyzed sodium hyaluronate is a low–molecular weight form of hyaluronic acid used primarily as a humectant/skin-conditioning agent, typically at low concentrations (often well under 1%). Clinical experience and patch testing data for hyaluronate salts show very low irritation and sensitization rates, but the hydrolyzed/low-MW forms can sting more than higher-MW HA when the barrier is compromised (e.g., eczema, post-procedure) or when layered with other actives. Given this small but real reactivity risk in highly sensitive populations, it is best categorized as very gentle rather than exceptionally gentle. Safety Notes: In commercial skincare, hydrolyzed sodium hyaluronate (low-molecular-weight HA salt) is often used at very low levels (around 0.0001–0.01%) in mass-market lotions/cleansers and multi-ingredient serums where it functions as a label/skin-feel humectant rather than a primary active. Most leave-on serums and gels sit roughly in the 0.05–0.5% range, while high-strength consumer “hyaluronic acid booster” products and anhydrous-to-aqueous concentrates can reach about 1–2% before viscosity/film-feel, tack, and stability constraints become limiting; rinse-off products typically use the low end because deposition is limited.

Hydrolyzed Soy Flour

Hydrolyzed soy flour is primarily a protein-derived conditioning/film-forming ingredient used at low concentrations, and it is not a classic irritant like acids or solvents. However, soy proteins are recognized allergens and can trigger contact urticaria or eczematous flares in sensitized individuals, with higher risk on compromised skin due to increased penetration. Given the nontrivial sensitization potential despite generally mild use levels, I rate it as mild (0.4) rather than “gentle.” Safety Notes: In commercial skincare, hydrolyzed soy flour is most often used as a minor conditioning/film-forming botanical protein, with trace-to-low inclusion levels (~0.01–0.1%) common in mass-market lotions, cleansers, and shampoos where it functions as a label-friendly additive. Higher-use levels are seen in consumer-available “soy protein/peptide” firming creams, masks, and hair repair treatments, where total hydrolyzed soy-derived solids can reach ~1–5% before texture, odor/color, and stability constraints typically limit further increases. Leave-on products more commonly sit in the ~0.05–2% range, while rinse-off can span similar lows but usually tops out lower unless positioned as a protein-repair treatment.

Hydrolyzed Soy Protein

Hydrolyzed soy protein is a film-forming/humectant conditioning ingredient typically used at low concentrations (~0.1–2%), and it is generally well tolerated in leave-on and rinse-off products. However, protein hydrolysates can act as contact allergens/irritants in a meaningful minority of reactive or eczematous patients, with documented cases of dermatitis from hydrolyzed plant proteins, so I score it as mild to reflect occasional sensitivity risk. In compromised skin barriers, I advise caution and patch testing because reactions, while uncommon, can be clinically significant when they occur. Safety Notes: In commercial skincare and haircare, hydrolyzed soy protein is often used at very low levels (~0.01–0.1%) as a label-claim conditioning/film-forming agent in leave-on lotions/serums and in rinse-off cleansers/shampoos where it functions mainly as a minor feel enhancer. Most mainstream products sit around ~0.2–2% depending on the supplier active strength and desired conditioning effect, while high-strength consumer-available masks, hair treatments, and some firming/conditioning leave-on formulas can reach about 3–5% total hydrolyzed soy protein before sensory, tack/film feel, and stability/compatibility constraints become limiting; no specific EU/FDA cosmetic concentration cap applies, so the upper end is governed primarily by formulation performance.

Hydrolyzed Vegetable Protein

Hydrolyzed vegetable protein is primarily a conditioning/film-forming humectant used at low concentrations in cosmetics, and it is generally well tolerated with low rates of irritant reactions in routine use. However, as a protein hydrolysate it can contain small peptides and residual source components that occasionally trigger stinging or dermatitis in highly reactive or compromised skin, so it is not truly inert. Given sensitive-skin and eczema populations, I score it as gentle but not risk-free. Safety Notes: In consumer skin and hair products, hydrolyzed vegetable protein is commonly used at very low levels (~0.05–0.3%) as a conditioning/film-forming supportive ingredient in leave-on lotions/serums and rinse-off cleansers where it appears mid-to-late INCI and is often part of a supplier blend. Higher-strength OTC products such as hair masks, conditioners, and some “protein treatment” style formulations marketed to consumers can reach ~1–5% active protein (especially in rinse-off systems or thicker masks) before sensorial/stability issues (tackiness, build-up, viscosity and salt/pH sensitivity) typically limit further increases. It is not specifically concentration-restricted in major cosmetic regulations (EU/FDA), so the upper end is driven primarily by formulation performance and consumer acceptability rather than legal caps.

Hydrolyzed Wheat Protein

Hydrolyzed wheat protein is used as a film-forming humectant/conditioning agent (commonly ~0.1–5%) and is generally well tolerated on intact skin, but protein hydrolysates can provoke irritation or allergic reactions in a minority of sensitized individuals. Patch-test and case reports show wheat-derived proteins can trigger contact urticaria or dermatitis, and risk rises on compromised barriers (eczema) where penetration is increased. Given the non-negligible sensitization potential despite routine cosmetic use, I rate it as mild rather than gentle for highly reactive patients. Safety Notes: In mass-market skincare and hair-adjacent facial products, hydrolyzed wheat protein is often used at very low levels (~0.05–0.5%) as a conditioning/film-forming humectant, especially in cleansers, toners, and light lotions where it is part of a broader “protein blend.” Higher-strength consumer formulas (notably hair masks/leave-in conditioners, some barrier creams, and “repair” serums) commonly reach ~1–5% active, with a smaller number of specialty OTC products using up to ~10% active hydrolysate depending on the supplied solids and sensory/tack limits; rinse-off formats tolerate somewhat higher loads than leave-on due to feel and potential sensitization concerns.

Hydroquinone

Hydroquinone is a potent depigmenting active typically used at 2–4% (and higher in compounded formulations), and irritant contact dermatitis (burning, stinging, erythema, scaling) is a well-documented, concentration- and vehicle-dependent adverse effect in clinical use. In sensitive or eczematous skin, barrier disruption markedly increases the likelihood of irritation, and cumulative irritation is common when combined with retinoids, acids, or benzoyl peroxide in routine regimens. Given its frequent need for careful introduction and the meaningful rate of intolerance in reactive patients, it warrants a high irritancy score. Safety Notes: In current consumer OTC skincare markets where hydroquinone is permitted, the lowest observed levels are typically ~0.25–0.5% in low-strength brightening creams/serums designed for gradual tone evening. The highest widely available non-prescription consumer products are 2% hydroquinone in leave-on spot treatments/creams, reflecting the common OTC cap in markets that still allow it (higher strengths such as 4% are generally prescription-only and excluded). Hydroquinone is primarily used in leave-on products; rinse-off usage is uncommon due to reduced contact time and efficacy.

Hydroxyacetophenone

Hydroxyacetophenone is primarily used as an antioxidant/preservative booster and soothing adjunct, typically around ~0.3–1%, and is generally well tolerated but not truly inert. Patch-test and real‑world reports show occasional irritation or sensitization, especially in compromised barriers (eczema/post‑procedure) and when layered with other actives, so I score it as mild rather than “gentle” for patient-safety. Safety Notes: Hydroxyacetophenone is most commonly used in consumer leave-on and rinse-off products as an antioxidant/preservative booster and soothing co-ingredient, with low-end commercial use around 0.05–0.1% (often in formulas aiming for minimal additive load). In real-world OTC products it is frequently found at ~0.2–0.5%, while high-strength consumer-available formulations (especially “preservative booster” or sensitive-skin oriented leave-ons) reach about 1.0%; levels above ~1% are uncommon in mainstream retail due to diminishing benefit and greater risk of sensory/irritation tradeoffs.

Hydroxyapatite

Hydroxyapatite is a biocompatible calcium phosphate mineral used mainly as an opacifying/abrasive or remineralizing agent (commonly in oral care and some topical products) and is generally well tolerated at typical use levels. Clinical and patch-test experience suggests low inherent irritancy, but as an insoluble particulate it can cause mild mechanical irritation on compromised or highly reactive skin, especially with friction or in leave-on products. Given sensitive-skin safety considerations, it is best classified as very gentle rather than fully inert. Safety Notes: In skincare, hydroxyapatite is most often used as a bioactive/skin-identical mineral or opacifying/soft-focus powder in leave-on creams/serums at low levels (typically around 0.1–1%), with the lowest observed commercial usage around ~0.05% as a supporting/minor claim ingredient. The highest consumer-available levels are found in mineral-powder-heavy products (e.g., masks, balms, body products, blur/primer-style creams) where hydroxyapatite functions as a primary particulate filler/texture agent; these can reach roughly 10–15% while remaining OTC and cosmetically stable. Rinse-off facial cleansers and wash-off masks generally sit in the mid-range, but the market maximum is driven by high-solids leave-on or wash-off paste-like formats rather than conventional emulsions.

Hydroxycitronellal

Hydroxycitronellal is a fragrance ingredient (synthetic floral odorant) used at low concentrations but is a well-documented contact allergen in patch-testing, including inclusion in fragrance allergen screening panels. Even at typical leave-on levels, it can trigger irritant and allergic contact dermatitis in sensitive individuals, with higher risk on compromised eczema-prone skin and with repeated exposure in multi-fragrance routines. Given its non-essential function and meaningful sensitization history, I score it as a significant irritancy risk. Safety Notes: Hydroxycitronellal is a fragrance ingredient used at trace levels in lightly fragranced leave-on skincare (often ~0.0001–0.01% when present as part of a parfum compound) and somewhat higher in fragranced rinse-off products where total perfume load can be higher. The upper end observed in consumer products is driven by strongly fragranced mass-market lotions/creams and some high-fragrance body products, where hydroxycitronellal can reach ~0.1–0.6% depending on the perfume composition. Use levels are practically constrained by IFRA allergen labeling considerations and sensitization risk, so very high percentages are uncommon outside fragrance-forward products.

Hydroxyethyl Acrylate/Sodium Acryloyldimethyl Taurate Copolymer

Hydroxyethyl Acrylate/Sodium Acryloyldimethyl Taurate Copolymer is a high–molecular weight synthetic polymer used primarily as a thickener/emulsion stabilizer, typically at low concentrations (~0.1–2%), with minimal skin penetration. Clinical and consumer patch-test data generally show a very low rate of irritation for this class of acrylate/taurate rheology modifiers, though occasional reactivity can occur in highly compromised or eczematous skin. Given its low functional reactivity but non-zero potential in severely sensitive populations, it fits best as very gentle rather than inert. Safety Notes: This polymer (often supplied pre-neutralized and used as a rheology modifier/emulsion stabilizer in products like Simulgel-type systems) is observed at very low levels (~0.05–0.2%) in thin serums/lotions and some rinse-off cleansers where only a small viscosity boost and stabilization is needed. In consumer leave-on gels, gel-creams, sunscreens, and high-slip “water gel” moisturizers, it is commonly used around ~0.3–1.5%, with the upper end (~2–3%) appearing in very high-viscosity gel textures and cold-process polymer-gelled systems where it functions as the primary structuring agent; higher levels are uncommon due to tack/texture limits and diminishing returns.