Skincare Ingredients

Hydroxyethylcellulose

MVP Approved - Hydroxyethylcellulose is a widely used thickening and stabilizing agent in skincare formulations that enhances product texture without directly conferring active skincare benefits.

Hydroxyethylpiperazine Ethane Sulfonic Acid

Hydroxyethylpiperazine ethane sulfonic acid (HEPES) is a buffering/pH-adjusting agent typically used at low concentrations (about 0.1–1%) to stabilize formulas, and it is generally well-tolerated in patch testing because it is not a potent bioactive. That said, as a pH-modifying buffer it can contribute to stinging in compromised barriers or when paired with other actives (acids/retinoids), so I rate it as very gentle rather than inert for high-sensitivity patients. Safety Notes: Hydroxyethylpiperazine Ethane Sulfonic Acid (HEPES) is used in cosmetics primarily as a buffering/pH-adjusting agent and (secondarily) to support exfoliating systems; in commercial leave-on serums/lotions it is often present around ~0.1–1% with low-end usage (~0.05%) seen where it only fine-tunes pH. Higher-strength OTC “HEPES exfoliating” leave-on products and peel-like treatments available to consumers can reach about ~2–3%, above which irritation risk and pH-control diminishing returns tend to limit market use. Rinse-off cleansers may use similar or slightly lower levels since contact time is short, and there is no specific EU/FDA maximum set for HEPES beyond general cosmetic safety requirements.

Hydroxylated Lecithin

Hydroxylated lecithin is a phospholipid-derived emulsifier/liposome former used at low concentrations (typically ~0.1–2%) to support barrier-friendly delivery and skin feel. Human patch/usage data and clinical experience generally show low irritation potential, but as a modified lipid mixture it can still sting or trigger reactivity in a minority of highly compromised or eczema-prone patients, so it is best classified as very gentle rather than inert. Safety Notes: In commercial skincare, hydroxylated lecithin is most often used as a barrier-supporting phospholipid/emulsifier at low levels (~0.05–0.3%) in serums, lotions, and sheet-mask essences where it primarily functions as a skin-conditioning/lamellar-structure aid. Mid-range use (~0.5–2%) is common in leave-on moisturizers and creams to improve skin feel and support delivery of actives. High-strength consumer-available formulas (typically richer barrier creams, lipid concentrates, and some encapsulation/delivery bases) can reach ~3–5%, above which stability, viscosity, and tackiness usually become limiting; rinse-off products generally sit toward the low end since deposition benefits are reduced.

Hydroxymethoxyphenyl Decanone

Hydroxymethoxyphenyl Decanone is a synthetic aromatic ketone used as an antioxidant/skin-conditioning agent in low concentrations (typically well under 1%). Available safety/patch-test signals for similar phenolic aromatic compounds suggest generally low irritancy at cosmetic use levels, but the aromatic structure and phenolic functionality mean a small subset of highly reactive or barrier-impaired patients can still experience stinging or dermatitis. Given the limited broad clinical irritation data specifically for this INCI and my duty to err toward patient safety in severe sensitivities, I rate it as gentle but not “very gentle.” Safety Notes: Hydroxymethoxyphenyl Decanone (commonly used as an anti-irritant/soothing agent in the same general class as ginger-derived sensory modulators) is typically found in leave-on moisturizers, serums, and after-sun products at very low levels (~0.01–0.1%) where it provides comfort/anti-redness support without impacting sensorial profile or stability. Higher-strength OTC “calming,” post-procedure-style, or redness-relief leave-on products observed on the market can reach ~0.5–1.0%, which is generally near practical formulation limits due to solubility, cost, and potential odor/sensory impacts; rinse-off products, when used, are usually toward the low end because of short contact time.

Hydroxypalmitoyl Sphinganine

Hydroxypalmitoyl sphinganine is a synthetic ceramide-like lipid used at low concentrations (typically well under 1%) to support barrier repair and reduce transepidermal water loss. Human repeat-insult patch testing and post-market experience for ceramide/sphingolipid ingredients generally show very low irritation and low sensitization risk, including in compromised skin when formulated appropriately. Because rare intolerance can still occur in highly reactive eczema patients or in complex formulas, it is best classified as very gentle rather than completely inert. Safety Notes: Hydroxypalmitoyl sphinganine (a skin-identical ceramide-like sphingoid base) is typically used at very low levels in barrier-repair and anti-aging leave-on products, often as part of multi-ceramide/lipid complexes where effective use levels commonly start around ~0.0001–0.01%. High-strength consumer barrier serums/creams and post-procedure support products have been observed/formulated up to ~0.1–0.2% when positioned as a primary active within a lipid system; higher levels are uncommon due to cost, solubility/dispersion limits, and risk of waxy feel or instability. It is rarely a meaningful active in rinse-off products (usually trace levels only), so the upper end mainly reflects leave-on formulations.

Hydroxyphenyl Propamidobenzoic Acid

Hydroxyphenyl Propamidobenzoic Acid (often marketed as SymSitive 1609) is a soothing/anti-itch ingredient typically used around ~0.1–0.5% to help reduce stinging and neurogenic redness in sensitive skin. Available safety and patch-test experience generally show low irritation and low sensitization potential at these concentrations, including in reactive-skin formulations. While not completely inert and rare individual reactions can occur, its clinical role and typical use levels support a very gentle score. Safety Notes: Hydroxyphenyl Propamidobenzoic Acid (commonly marketed as SymSitive 1609) is used in consumer soothing/anti-redness products at low levels around 0.05–0.10% to support sensitive-skin claims while minimizing cost and sensory impact. Many mainstream leave-on creams/serums cluster around ~0.10–0.30%, while high-strength OTC calming concentrates and barrier-repair serums marketed for reactive skin are observed up to about 1.0% in leave-on formats. It is less commonly used in rinse-off products, where levels tend to sit toward the low end due to short contact time.

Hydroxypinacolone Retinoate

Hydroxypinacolone retinoate (HPR) is a retinoid ester used at low but active concentrations (commonly ~0.1–1%) to drive retinoid-like epidermal turnover and collagen signaling. While often marketed as “gentler” than retinoic acid, clinical experience and retinoid class effects still include meaningful rates of dryness, stinging, and dermatitis—especially in eczema-prone or barrier-impaired skin and when combined with acids or other actives. Given its genuine biologic activity and the real-world cumulative irritation risk, it warrants careful introduction rather than being treated as a benign additive. Safety Notes: In real-world OTC skincare, Hydroxypinacolone Retinoate (often marketed as “HPR” or “Granactive Retinoid”) is found at very low levels (~0.01–0.05%) in gentle, multi-active leave-on serums/creams and eye-area products where irritation risk and cost drive down use level. Most mainstream leave-on formulas cluster around ~0.1–0.5% HPR, while high-strength consumer-available specialty retinoid serums reach ~1–2% HPR (commonly via concentrated retinoid blends/solutions) while remaining non-prescription; it is rarely used meaningfully in rinse-off due to limited contact time and retinoid stability/benefit considerations.

Hydroxyproline

Hydroxyproline is an amino-acid derivative used mainly as a skin-conditioning/humectant or collagen-support ingredient, typically at low concentrations (generally well under a few percent). Available safety and patch-test experience suggests a low rate of irritant reactions, with sensitization being uncommon, but compromised barriers (eczema/post-procedure) can still sting with otherwise mild amino-acid actives. Given limited large-scale irritation datasets compared with inert excipients, I score it as very gentle rather than inert for patient safety. Safety Notes: In commercial skincare, hydroxyproline is most often used as an anti-aging/firming amino-acid active in leave-on serums and creams, commonly appearing at low “label-presence” and supportive levels around 0.01–0.1% and more typically ~0.2–1%. Higher-strength consumer-available products (usually niche/dermocosmetic firming concentrates) can reach ~2–5% when positioned as a primary collagen-supporting active; above this, solubility, tackiness, and formulation stability/feel constraints tend to limit broader market use. Rinse-off products generally sit at the low end because contact time is short and cost/benefit is weaker versus leave-on formats.

Hydroxypropyl Cyclodextrin

MVP Approved - Hydroxypropyl Cyclodextrin is commonly used as a solubilizer and stabilizer in skincare formulations, with minimal irritation and a possible mild moisturizing (hydrating) effect.

Hydroxypropyl Guar

Hydroxypropyl guar is a cellulose-like polysaccharide derivative used mainly as a thickener/film-former in low concentrations (typically <1%) and is generally well tolerated, including in sensitive-skin formulations. Human repeat-insult patch testing and post-market experience suggest a very low rate of irritation or sensitization, with occasional reactivity usually tied to overall formula factors rather than the polymer itself. Because rare irritation can still occur in highly reactive or eczematous skin—especially under occlusion—I score it as very gentle rather than completely inert. Safety Notes: In commercial skincare, hydroxypropyl guar is most often used as a rheology modifier/film former at low levels (~0.05–0.3%) in lotions, cleansers, and micellar/gel textures where it provides slip and mild thickening. Higher levels are seen in consumer-available high-viscosity gels, barrier/anti-chafe balms, and peel-off/film-forming mask-type products, where total polymer load can push hydroxypropyl guar to ~0.8–1.5% before texture becomes overly stringy/tacky and processing becomes difficult. Rinse-off products typically sit toward the low-to-mid end for ease of spread and rinsability, while leave-on gels and masks more commonly reach the upper end for structure and film formation.

Hydroxypropyl Methylcellulose

Hydroxypropyl methylcellulose is an inert, high–molecular weight cellulose-derived polymer used mainly as a thickener/film-former in topical products, typically at low concentrations (~0.1–2%). Clinical and patch-test experience shows it is very well tolerated with irritation being uncommon, though a small minority of highly reactive or barrier-compromised patients can experience transient stinging or rare contact reactions, so it is not scored as completely inert. Safety Notes: In commercial skincare, Hydroxypropyl Methylcellulose is commonly used at very low levels (~0.05–0.3%) as a secondary thickener/suspending and stabilizing aid in leave-on serums/lotions and rinse-off cleansers. Higher levels (about 1–3%) are seen in consumer gel-cream masks, peel-off/film-forming gels, and some high-viscosity styling/skin gels where it functions as a primary gellant/film former; the upper end (~4–5%) occurs in niche high-build gel or mask formats available OTC, with viscosity and film formation becoming the main practical constraint rather than a specific cosmetics regulatory limit.

Hydroxypropyl Starch Phosphate

Hydroxypropyl starch phosphate is a modified starch used primarily as a thickener/texture enhancer and oil-absorbing agent, typically at low single-digit percentages, and it is generally considered non-reactive with a low rate of irritation in patch testing for leave-on products. In very compromised skin it can rarely contribute to dryness/tightness (especially in mattifying formulas), so I do not score it as fully inert, but overall it remains very gentle for most sensitive-skin patients. Safety Notes: In commercial leave-on skincare and color cosmetics, hydroxypropyl starch phosphate is commonly used at very low levels (~0.05–0.5%) as a rheology modifier, stabilizer, and sensory/blur agent in emulsions and gels. Mid-range usage (1–5%) is widely seen in mattifying lotions/primers and powdery-feel creams where a stronger oil-absorbing and slip effect is desired. High-strength consumer products such as oil-control primers, blurring balms, and some sunscreen/BB-type bases can reach ~8–10%, with higher levels limited by texture (dryness/drag, pilling) and viscosity/dispersion constraints rather than specific regulatory caps.

Hydroxypropyl Tetrahydropyrantriol

Hydroxypropyl Tetrahydropyrantriol (commonly known as Pro-Xylane) is a humectant/skin-conditioning agent typically used around ~1–5% in anti-aging formulas and is not pH-dependent or exfoliative. Available tolerability and consumer-use data generally show a low rate of stinging or erythema, with irritation more often attributable to the overall formula rather than this molecule itself. For severe sensitivity/eczema populations I still assign a small but real risk (particularly on compromised barriers), so it scores as very gentle rather than inert. Safety Notes: Hydroxypropyl Tetrahydropyrantriol (Pro-Xylane) is most commonly used in leave-on anti-aging serums/creams, where it appears at low supportive levels around 0.05–0.5% in multi-active moisturizers and at higher headline levels around 2–5% in targeted firming products. High-strength OTC specialty formulations marketed around this technology have been observed up to ~10% (typically leave-on; rinse-off products, when present, tend to sit at the lower end due to short contact time and cost). No specific FDA/EU maximum concentration restriction is generally cited for this cosmetic ingredient, so the upper bound is driven by product positioning, sensory/stability, and raw material use levels seen in commerce.

Hydroxypropyltrimonium Hyaluronate

Hydroxypropyltrimonium Hyaluronate is a cationic (quaternized) hyaluronic acid derivative used as a film-forming humectant/conditioning polymer, typically at low concentrations (often well under 1%). Hyaluronic-acid based polymers have very low irritancy in patch testing and clinical use, and this derivative is generally well tolerated even in sensitive skin, with irritation most often attributable to the overall formula rather than the polymer itself. Because the cationic modification can very occasionally increase sting in highly compromised barriers compared with plain sodium hyaluronate, I score it as very gentle but not fully “exceptionally gentle.” Safety Notes: In commercial leave-on skincare and haircare/conditioner-type products, Hydroxypropyltrimonium Hyaluronate (a cationic hyaluronate derivative used for substantivity/conditioning) is commonly used at very low levels, with effective deposition and sensory benefits observed around ~0.01–0.05% in serums, lotions, toners, and rinse-off conditioners. Higher-strength consumer products (e.g., “intense” hydrating/conditioning serums, masks, and premium hair treatments) are seen up to ~0.3–0.5%, which is near the practical upper end due to viscosity/film-feel, formula salt sensitivity, and cost; no specific EU/FDA concentration cap applies, so the upper bound is driven by formulation performance and stability rather than regulation.

Hylocereus Undatus Fruit Extract

Hylocereus undatus (dragon fruit) extract is typically used at low concentrations as a soothing/antioxidant, humectant-supporting botanical, and it is generally well tolerated in standard patch testing of cosmetic fruit extracts. However, as a complex botanical mixture (sugars, organic acids, phenolics, trace proteins), it carries a small but real risk of irritant or allergy-like reactions in highly reactive or eczema-prone skin, especially in leave-on products and when layered with other actives. Given this baseline variability with botanicals but lack of strong evidence for frequent irritation at typical use levels, it fits best as “gentle” rather than very gentle. Safety Notes: In mass-market lotions, toners, and cleansers, Hylocereus Undatus (dragon fruit) extract is typically used as a label-claim botanical at very low levels (often ~0.001–0.1%) due to cost, odor/color impact, and the fact that many commercial extracts are supplied in dilute solvent/glycerin bases. Higher-end serums and masks marketed around “dragon fruit” commonly push botanical extract loading into the ~1–5% range, especially in leave-on products where marketing and sensorial benefits justify higher usage; rinse-off products generally sit toward the lower end because of short contact time. There are no specific FDA/EU cosmetic maximum limits for this extract, so the observed ceiling is primarily set by stability, color/fragrance, and supplier-recommended use levels seen in consumer products.

Hypericum Perforatum Flower/Leaf/Stem Extract

Hypericum perforatum (St. John’s wort) extract is a botanical anti-inflammatory/soothing agent typically used at low percentages, but it contains bioactive compounds (notably hypericin/hyperforin) that can trigger irritant or allergic contact dermatitis in susceptible individuals. It also carries a clinically relevant photoreactivity risk, which can amplify redness and stinging when combined with UV exposure or other actives in a routine. Given the real-world variability of botanical extracts and the higher vulnerability of eczema-prone skin, a moderate irritancy score is warranted and patch testing is advisable. Safety Notes: In mass-market creams, toners, and cleansers, Hypericum perforatum (St. John’s wort) extract is frequently used as a minor soothing/“botanical blend” component at trace-to-low levels (often 0.0001–0.1%), especially when part of multi-extract complexes. Higher levels are seen in consumer botanical balms, post-sun/cica-style creams, and herbal “ointment” leave-on products where the extract is a primary active (commonly ~0.5–2% and up to about 5% in high-botanical formulations). Very high use is constrained in leave-on products by odor/color, potential photoreactivity of hypericin-rich extracts, and variability by solvent/extract ratio, so strong products more often use standardized extracts or oily macerates while rinse-off products typically remain at the low end.

Hypochlorous Acid

Hypochlorous acid is primarily used as an antimicrobial/anti-inflammatory in leave-on sprays or wound/eczema-adjacent care, typically at very low concentrations (~0.005–0.02%) and near-skin pH, where clinical use is generally well tolerated. However, as an oxidizing biocide it can still sting on compromised skin and can irritate eyes or reactive faces (especially if the formulation has higher free available chlorine or is off-pH), so it is best classified as very gentle rather than inert. Safety Notes: In consumer leave-on hypochlorous acid facial/body sprays and mists, the lowest commercially observed levels are around ~0.005% (≈50 ppm) to provide mild antimicrobial/soothing benefits with good tolerability, while the most common market level clusters near ~0.01% (≈100 ppm). High-strength OTC specialty sprays marketed for more intensive antimicrobial/deodorizing use typically top out around ~0.02–0.03% (≈200–300 ppm) because higher HOCl levels are harder to stabilize (pH/ORP constraints) and are more likely to be irritating; products above this are generally positioned as disinfectants or are not typical cosmetic leave-ons.

Ilex Paraguariensis Leaf Extract

Ilex Paraguariensis (yerba mate) leaf extract is used mainly as an antioxidant/soothing botanical, typically at low concentrations, but it contains multiple plant polyphenols and can retain trace caffeine/tannins that may sting or provoke reactivity on compromised or eczema-prone skin. While not a classic high-risk irritant like acids or fragrance allergens, botanical extracts have a consistent track record of occasional positive reactions in sensitive populations, so a mild score is the safer clinically aligned assessment. Safety Notes: In mass-market leave-on products (serums, lotions, eye gels) and rinse-off cleansers, Ilex paraguariensis (yerba mate) leaf extract is frequently used as a low-level antioxidant/soothing “label claim” botanical at ~0.001–0.1%, especially when supplied as a dilute glycerin/propylene glycol extract. Higher-strength consumer-available products (e.g., antioxidant/firming serums, cellulite/contouring creams, and some K-beauty toners) may use the extract as a primary botanical active, commonly ~0.5–3% and occasionally up to ~5% depending on extract potency and solubility; above this, stability, color/odor, and irritation/caffeine-related sensory limits typically constrain use in OTC leave-on formats.

Illite

Illite is a mineral clay (often used in masks/cleansers at high percentages) that functions mainly as an absorbent and viscosity/bulking agent. While not a classic chemical irritant, clays can cause dryness, tightness, and barrier disruption—especially in eczema-prone or compromised skin—leading to stinging and reactive dermatitis in real-world use. Patch testing generally shows low reactivity, but given typical use concentrations and the risk of over-drying in sensitive populations, it fits best as a gentle (not exceptionally gentle) ingredient. Safety Notes: Illite (a mineral clay, often listed as Illite/Yellow Clay/French clay) appears at very low levels (~0.1–1%) in emulsions and cleansers as an opacity/skin-feel modifier or mild oil-absorbing aid, and at moderate levels (~2–20%) in wash-off masks and scrubs for absorbency and texture. The highest consumer-available concentrations are found in powder masks and clay pastes where illite is a primary solid phase (commonly ~40–70% and up to ~80% when blended with minimal other powders/binders); these are typically rinse-off products, while leave-on products rarely exceed a few percent due to grit, color, and wearability.

Imidazolidinyl Urea

Imidazolidinyl urea is a formaldehyde-releasing preservative typically used around ~0.1–0.6% in leave-on and rinse-off products. It is a well-documented trigger for allergic contact dermatitis (and can sting/irritate compromised skin), with sensitization risk heightened in eczema-prone and barrier-impaired patients. Given its recognized allergen potential and the real-world likelihood of cumulative exposure across multiple products, it warrants a notable irritancy score for sensitive-skin safety. Safety Notes: In consumer cosmetics, imidazolidinyl urea is typically used as a formaldehyde-releasing preservative around 0.1–0.3%, with very low-end uses (~0.05%) occurring in multi-preservative systems where it is only a minor contributor (common in both leave-on lotions and rinse-off cleansers). Higher-strength OTC products (especially economical creams/lotions and some haircare where stronger preservation is needed) are observed up to about 0.5–0.6%, which aligns with common supplier/industry maximum-use guidance for cosmetic preservation rather than prescription/professional-only levels.

Inonotus Obliquus Extract

Inonotus obliquus (chaga) extract is typically used at low concentrations in cosmetic formulas as an antioxidant/soothing botanical; available human data suggest low irritancy for most users, but evidence is limited compared with well-characterized actives. As a complex natural extract containing multiple bioactive compounds, it carries a small but real risk of irritation or allergic contact dermatitis in highly reactive or eczema-prone patients, especially when combined with other potential irritants in a routine. Safety Notes: Inonotus obliquus (chaga) extract is typically used as a marketing/antioxidant-support botanical in leave-on facial products at very low levels (often ~0.001–0.1%) when supplied as a dilute liquid extract or as part of a blended “mushroom complex,” and it can appear similarly low in rinse-off cleansers where contact time is short. Higher-strength consumer serums, ampoules, masks, and some creams using concentrated glycerin/butanediol extracts or higher active loading commonly reach ~1–3%, with a small number of OTC “concentrate” formulas pushing to about ~5% before sensorial, color/odor, and stability constraints (solids/viscosity and potential tack) typically limit further increases.

Inositol

Inositol (myo-inositol) is a skin-conditioning/humectant ingredient typically used at low percentages and is highly water-soluble, with a long history of good tolerability in leave-on formulations. Clinical experience and patch-test data suggest a very low rate of irritant reactions, though any leave-on ingredient can rarely sting on severely compromised eczema skin. Given its minimal inherent reactivity and low sensitization potential, it fits the 'exceptionally gentle' category rather than fully inert. Safety Notes: Inositol is used in many commercial skin and scalp products primarily as a humectant/skin-conditioning and barrier-support ingredient, where it often appears at low “label” levels (~0.01–0.3%) in multi-ingredient leave-on serums/creams and in rinse-off cleansers. In more targeted OTC offerings (e.g., hydrating/barrier formulas, scalp tonics, and some anti-breakage hair/skin treatments), inositol is formulated at ~1–5% as a primary polyol-type active; above ~5% is uncommon in consumer skincare due to solubility/processing limits and increasing tackiness, and there is no specific EU/FDA concentration cap for inositol in cosmetics.

Inula Helenium Extract

Inula helenium (elecampane) extract is a botanical anti-inflammatory/soothing agent typically used at low concentrations, but it contains sesquiterpene lactones (e.g., alantolactone/isoalantolactone) that are well-documented contact allergens in patch testing and can trigger dermatitis in sensitized or eczematous patients. Because allergy-driven irritation can be delayed and clinically significant even at small doses—especially with cumulative leave-on use—I rate it as a moderate irritancy risk requiring patch testing in sensitive skin. Safety Notes: Inula helenium extract is typically used as a supporting botanical/soothing antioxidant at very low levels in mainstream leave-on products (often as part of a multi-extract blend), with commercially observed use starting around ~0.0005% when present mainly for label/marketing or trace-level contribution. Dedicated botanical serums, creams, and scalp treatments marketed as high-strength “plant extract” formulas can reach ~1–2% for the standalone extract in OTC leave-on products, with higher levels being uncommon due to odor/color impact and potential sensitization constraints; rinse-off products generally sit toward the lower end because of short contact time.

Inulin

Inulin is a high–molecular weight plant-derived polysaccharide used mainly as a prebiotic/humectant or skin-conditioning agent, typically at low-to-moderate percentages, and it is not pH-dependent or intrinsically reactive. In clinical and consumer use it is generally well tolerated with low irritancy, but as a botanical-derived polymer it can rarely trigger stinging or contact reactions in highly reactive/eczema-prone individuals (especially on compromised skin), so it is best classified as very gentle rather than inert. Safety Notes: Inulin is used commercially as a prebiotic/humectant/skin-feel modifier, commonly appearing at low levels (~0.05–0.5%) in leave-on moisturizers, serums, and toners as part of a microbiome-support or moisturizing complex. In rinse-off cleansers and many leave-on products it more typically sits around 0.5–3% for noticeable conditioning and sensory benefits. High-strength consumer-available “prebiotic” masks, barrier creams, and anhydrous-to-emulsion balms can reach ~5–10%, with practical upper limits driven by solubility/viscosity, potential tackiness, and formula stability rather than specific regulatory caps.

Iodopropynyl Butylcarbamate

Iodopropynyl Butylcarbamate (IPBC) is a preservative typically used at very low concentrations (~0.01–0.1%), but it has well-documented potential to cause irritation and, more importantly, allergic contact dermatitis in leave-on products and compromised skin. Patch-test data and clinical reports place it among preservatives with a meaningful sensitization risk, so in sensitive or eczematous patients it warrants caution and patch testing. Given routine multi-product exposure and the stakes in reactive skin populations, it is best scored as a notable irritant/sensitizer despite its low use level. Safety Notes: In consumer cosmetics, iodopropynyl butylcarbamate (IPBC) is typically used at very low levels (around 0.001–0.005%) as part of preservative blends in leave-on products (creams, lotions, serums) and lower-water systems, reflecting both efficacy data and sensitization/iodine-related constraints. The highest over-the-counter levels observed are in rinse-off products (shampoos, body washes, cleansers) where formulas can legally/typically support up to ~0.02% in major markets (e.g., EU), and a subset of “high-preservation” or micro-challenging rinse-off products reach this upper end. Leave-on products generally top out much lower than rinse-off due to regulatory limits and irritation/sensitization risk.

Ipomoea Batatas Root Extract

Ipomoea batatas (sweet potato) root extract is primarily used as a botanical antioxidant/soothing agent, typically at low concentrations, and it is not a classic high-risk irritant like acids, surfactants, fragrances, or strong preservatives. However, as a plant-derived extract it can contain variable phenolics/proteins and may trigger irritation or allergy in a small subset of highly reactive or eczematous patients, so it cannot be scored as “very gentle” or “inert.” In routine leave-on formulations it is generally well tolerated, but I score it as gentle to reflect real-world sensitization/irritancy potential in compromised skin. Safety Notes: In mass-market leave-on skincare (serums, lotions, sheet masks) Ipomoea batatas (sweet potato) root extract is often used as a minor botanical/antioxidant claim ingredient at very low levels (typically via pre-diluted extracts), with observed commercial use down to ~0.0005%. Higher-strength consumer products marketed as “superfood,” “botanical,” or “antioxidant” concentrates can use the extract as a featured component, with observed levels up to ~5% in leave-on emulsions or gel serums; rinse-off products usually sit toward the lower-to-mid end due to brief contact time. There are no specific EU/FDA cosmetic maximum limits for this extract, so practical upper levels are mainly constrained by odor/color, stability, tack, and overall formula aesthetics.

Iris Florentina Root

Iris florentina root is primarily used as a botanical extract/fixative (orris root) in fragranced products, where it contributes aromatic compounds and can behave similarly to fragrance components. While often present at low concentrations, botanical fragrance materials have a documented risk of irritant and allergic contact dermatitis in sensitized or eczema-prone users, so it is not reliably “gentle” for compromised skin. Given the potential for delayed sensitization and cumulative irritation in routine use, a mild (but non-trivial) irritancy score is the safest evidence-aligned assessment. Safety Notes: In commercial skincare, Iris florentina root (typically used as an extract/powder for scent/skin-conditioning) is often present at trace levels around 0.001–0.05% in leave-on creams/serums and rinse-off cleansers where it functions more as a supporting botanical. Higher-strength consumer products marketed around “orris/iris root” (e.g., botanical masks, powder/enzyme-style cleansers, and some natural deodorant/body powders) can reach ~1–5% when used as a featured plant powder/extract for absorbency/sensory claims; above this becomes uncommon due to cost, odor impact, and formula aesthetics rather than a specific regulatory cap.

Iron Oxides

Iron oxides are inert mineral pigments used primarily as colorants in makeup and tinted sunscreens, typically at low-to-moderate concentrations, and they are not chemically active on skin. Clinical experience and patch testing data generally show very low irritation and sensitization rates; reactions are uncommon and more often related to contamination, particle friction, or the overall formula rather than the pigment itself. Given their strong tolerability even in sensitive populations, they fit an exceptionally gentle irritancy profile. Safety Notes: In commercial cosmetics, iron oxides are used as pigments and can appear at trace levels (~0.01–0.1%) in lightly tinted skincare (tone-up lotions, subtle color-correcting serums) or as part of blended color systems. At the high end, consumer-available color cosmetics with strong coverage—particularly cream foundations, concealers, camouflage makeup, and some pressed/powder foundations—can reach ~15–25% total iron oxides (often combined with TiO2) depending on shade depth and opacity. Use is primarily leave-on; rinse-off products typically contain none or only trace carryover from colorant systems.

Iron Powder

Iron powder is an insoluble particulate used mainly as a colorant/pigment in cosmetics; while it is not a biochemical “active,” fine particles can cause mechanical irritation (grittiness, microabrasion) and provoke stinging when applied to compromised barriers (eczema, post-procedure). Clinically, insoluble metal/oxide pigments are generally low in irritancy at typical use levels, but in sensitized or inflamed skin the physical friction and potential for trace metal contaminants justify a mild score from a patient-safety standpoint. Safety Notes: In consumer skincare, iron powder is most often encountered as a minor colorant/optical modifier or as a trace component in mineral blends, with commercial INCI lists supporting use down to ~0.001–0.01% in leave-on products where only a small amount is needed for tone adjustment. The highest observed OTC levels are in specialty exfoliating/polishing products (typically rinse-off) or novelty “magnetic”/metallic effect masks where a perceptible particulate load is required, reaching ~1–5% while still maintaining spreadability, skin feel, and acceptable irritation risk. Above this, products tend to become cosmetically unacceptable (gritty/heavy) and are more likely to fall into non-skincare or professional-use categories, which are excluded here.

Isoamyl Cocoate

Isoamyl cocoate is a fatty acid ester emollient used as a lightweight skin-feel and slip agent, typically at a few percent up to ~10% in leave-on products. As a non-volatile, non-acidic ester, it is generally well-tolerated with low rates of irritant reactions in patch testing compared with fragrances, solvents, or surfactants. Rare individual intolerance can occur (especially in highly compromised eczema skin), but at typical use levels its intrinsic irritancy is very low, supporting a 'very gentle' score. Safety Notes: Isoamyl cocoate is used at very low levels (~0.1–1%) as an emollient/slip agent or solubilizing co-emollient in lotions, serums, sunscreens, and cleansers where it is one of many oils/esters. Mid-range use (2–20%) is common in leave-on creams, makeup/primer textures, and hair conditioners as a primary sensorial emollient. At the high end, consumer-available anhydrous products (body oils, cleansing oils/balms, and silicone-free facial oils) can use it as a major oil-phase component, with market-observed formulas reaching ~50–70% when it functions as a main carrier ester; it is generally not specifically concentration-limited by major cosmetic regulations and is more constrained by product type, sensory targets, and solubility/stability.