What a chemical peel depth chart can and cannot show
Chemical peel depth describes the level of tissue injury intended by a protocol. It is not an outcome guaranteed by a product label, an acid percentage or a skin-type category. Clinical literature commonly uses the categories very superficial, superficial, medium depth and deep. These broadly correspond to injury limited to the epidermis, extending through the epidermis, reaching the papillary dermis, or extending further into the reticular dermis. The categories are useful descriptive terms, but they are not a conversion scale from a bottle concentration to a fixed histological depth.
Delivery depends on multiple connected variables. These include the chemical agent, total concentration, pH where relevant, vehicle, buffering, skin preparation, contact time, number of coats, application pressure, anatomical site and the condition of the epidermal barrier. Thin skin, recently irritated skin and areas with active inflammation may respond differently from intact skin elsewhere on the same person. A visible reaction during treatment may inform the person applying the peel, but it does not make the eventual depth fully predictable.
Free acid is most useful as a formulation concept for weak organic acids such as glycolic, lactic, salicylic and pyruvic acid. It estimates the proportion of an acid present in its unionised form at a given pH. That is chemically relevant because unionised molecules can behave differently at biological barriers from ionised molecules. It remains a calculation about a solution, not a validated individual prediction of peel depth, benefit, complication risk or suitability.
| Approach | Key variable to examine | Broad clinical depth description | Pigment-risk consideration | Evidence basis |
|---|---|---|---|---|
| Glycolic acid | Free-acid fraction changes with pH | Very superficial to superficial in commonly described peeling use | Post-inflammatory hyperpigmentation can follow excessive inflammation in any phototype | Acid-dissociation chemistry and published peel classifications |
| Lactic acid | Free-acid fraction changes with pH; molecular properties also differ from glycolic acid | Very superficial to superficial in published descriptions | Previous pigment change and current irritation require consideration | Acid-dissociation chemistry and published peel reviews |
| Salicylic acid | Vehicle, crystallisation and acid chemistry can affect delivery | Very superficial to superficial in standard descriptions | Inflammation and subsequent pigment alteration remain relevant | Published salicylic-acid peel reviews and classifications |
| Trichloroacetic acid | Concentration, coats, site and observed frosting endpoint are central | Superficial to medium depth depending on protocol | Greater injury depth raises the consequence of dyschromia and scarring | Published trichloroacetic-acid histology and peel classifications |
Decision rule: do not treat a stated percentage as a depth claim. For a weak acid, establish the total acid concentration and final measured pH before calculating a free-acid estimate. For trichloroacetic acid, assess concentration, coats, site and observed endpoint as parts of a protocol rather than as isolated numbers.
How free acid is calculated for glycolic and related acids
Weak acids exist in equilibrium between a protonated unionised form and a charged ionised form. The Henderson-Hasselbalch equation describes the relationship between pH, pKa and those forms. For a monoprotic acid, the estimated unionised fraction is calculated as 1 divided by 1 plus 10 raised to the power of pH minus pKa. Multiplying that fraction by the total acid concentration gives an estimated free-acid concentration.
The calculation requires information about the final formulation, not merely an ingredient list. The two essential inputs are total acid concentration and measured final pH. A formula may be buffered, may have a pH given as a range, may change over time, or may not disclose pH at all. In those circumstances, an exact free-acid result cannot be derived from the percentage on a label. A claimed free-acid value is also difficult to evaluate unless the calculation method and pH measurement are clear.
Glycolic acid has a reported pKa of about 3.83. At a pH equal to its pKa, the equation estimates that half the acid is unionised. With a 30% glycolic formulation at pH 3.83, the estimated free glycolic acid is therefore 15%. At pH 2.83, the estimated unionised fraction is about 91%, producing an estimate of about 27% free glycolic acid. At pH 4.83, the estimated unionised fraction is about 9%, producing an estimate of about 3% free glycolic acid.
| Worked glycolic-acid example | Total glycolic acid | pH | Estimated unionised fraction | Estimated free glycolic acid | Calculation basis |
|---|---|---|---|---|---|
| Illustration A | 30% | 3.83 | 50% | 15% | Henderson-Hasselbalch calculation using a reported glycolic-acid pKa of about 3.83 |
| Illustration B | 30% | 2.83 | About 91% | About 27% | Same calculation with pH one unit below the reported pKa |
| Illustration C | 30% | 4.83 | About 9% | About 3% | Same calculation with pH one unit above the reported pKa |
Lactic acid and salicylic acid must be calculated using their own pKa values, not glycolic acid values. The arithmetic does not account for vehicle effects, buffering capacity, evaporation, exposure duration, skin surface pH or changing barrier function. It is consequently an audit of formulation chemistry, not authority to infer a treatment outcome.
TCA peel versus glycolic acid: a comparison of control variables
Glycolic acid and trichloroacetic acid, commonly shortened to TCA, cannot be arranged reliably on one simple strength ladder. Glycolic acid is an alpha-hydroxy acid and a weak acid. Its total concentration does not disclose its unionised fraction if pH is absent. In common clinical descriptions, glycolic acid is associated with very superficial or superficial peeling, but the response remains dependent on application variables and skin condition.
TCA acts differently. It is associated with protein coagulation and may produce visible frosting during application. Published classifications associate lower-concentration TCA protocols with superficial injury and higher-concentration protocols with medium-depth injury. However, concentration is not the sole determinant. The number of coats, pre-peel preparation, application method, treatment site and any combined agents can alter delivery. The same nominal concentration is therefore not a guarantee of the same injury in every protocol.
The useful comparison is not which acid is inherently stronger. It is which information is needed to understand the likely activity of a stated preparation. For glycolic acid, total concentration, final pH, vehicle and exposure are important. For TCA, concentration, coating pattern, anatomical site and clinical endpoint are central. Neither set of variables provides a promise of improvement or a complication-free route.
| Question | Glycolic acid | Trichloroacetic acid | Interpretation |
|---|---|---|---|
| Is free-acid arithmetic useful? | Yes, if total concentration and final pH are known | Not usually the principal clinical descriptor | The chemistry questions are different |
| Can percentage alone define depth? | No | No | Technique and formulation remain relevant |
| What commonly changes delivery? | pH, vehicle, concentration, coats and exposure | Concentration, coats, preparation, endpoint and anatomical site | Protocol matters alongside the agent |
| What is a common interpretive error? | Assuming total percentage equals active free acid | Assuming concentration fixes injury depth | A single label value is incomplete evidence |
The phrase medical grade does not provide missing formulation data, an observed endpoint or outcome evidence. A meaningful discussion of a peel identifies the agent and the variables that materially affect its application.
Fitzpatrick phototypes and pigment-risk interpretation
The Fitzpatrick scale categorises skin according to reported response to ultraviolet exposure, conventionally from type I to type VI. It was not designed as a chemical-peel suitability score, a measure of all aspects of pigmentation biology, or a validated calculator for scarring risk. Its relevant role in peel assessment is as one prompt to discuss pigmentary response, including the possibility of post-inflammatory hyperpigmentation.
Fitzpatrick type should not be used to imply that people in types IV to VI are automatically unsuitable for peeling. Nor should types I to III be represented as protected from pigment alteration, infection, delayed healing or scarring. Individual history can add substantially more useful context than a phototype number alone. Examples include dark marks persisting after acne, bites, eczema or injury; melasma; prior adverse response to a procedure; and a tendency towards hypertrophic or keloid scarring.
The intended depth and inflammatory burden matter. More extensive injury creates more opportunity for prolonged inflammation, uneven recovery and pigment change. Current inflammatory acne, dermatitis, rubbing or other barrier disruption may also make a planned elective injury harder to interpret and manage. These are assessment considerations rather than universal thresholds.
| Assessment feature | Why it matters | What it should change | Evidence basis |
|---|---|---|---|
| Fitzpatrick I to III | Does not exclude pigment change, infection or scarring | Continue to assess depth, barrier condition and personal history | Published chemical-peel complication reviews |
| Fitzpatrick IV to VI | Post-inflammatory pigment alteration is an important planning concern | Give weight to prior pigment response and inflammatory burden | Published dermatology literature on post-inflammatory hyperpigmentation |
| Previous post-inflammatory hyperpigmentation | Shows an individual response not fully represented by phototype | May be more informative than generic assumptions about type | Clinical reasoning consistent with pigment-disorder literature |
| Active rash, wound or infection | A compromised barrier can alter penetration and healing | Defer elective peeling until the area has been assessed and resolved | Wound-healing principles and peel safety reviews |
Decision rule: use phototype as a flag for pigment-risk discussion, not as a pass or fail criterion. Give particular weight to what previous inflammation has done to that individual's skin.
Contraindications and the limits of an objective threshold
A useful safety framework distinguishes reasons to pause an elective peel from factors requiring more individual judgement. Active infection, open skin, a substantial dermatitis flare, a clearly impaired barrier and known allergy to a relevant ingredient are reasons not to add an intended chemical injury to the affected area. These points are more defensible than treating age, sex or phototype alone as contraindications.
Other factors require context rather than automatic rules. They include a history of abnormal scarring, melasma, recurrent herpes simplex affecting the intended treatment region, active inflammatory acne, recent procedures and medicines that may affect healing. The relevant question is whether a factor changes expected inflammation, barrier function, infection risk, pigment response or the ability to distinguish a complication from the pre-existing condition.
No evidence-based formula combines pKa, acid percentage and Fitzpatrick type into a universally safe maximum peel depth. Free-acid arithmetic describes equilibrium in a solution. Fitzpatrick classification describes one aspect of reported ultraviolet response. Neither measures how a particular person's skin will respond to a controlled chemical injury. Claims that a universal threshold has been found should therefore be treated cautiously.
- Pause: active infection, open skin, significant active dermatitis or known allergy to a relevant ingredient.
- Seek individual assessment: prior prolonged pigment change, abnormal scarring, melasma, recurrent facial herpes or recent barrier-disrupting procedures.
- Do not infer safety: from acid percentage, a Fitzpatrick number, a free-acid figure or promotional terminology alone.
- Record relevant variables: agent, concentration, final pH where relevant, vehicle, coats, site and observed endpoint.
This framework does not replace assessment by an appropriately qualified clinician, particularly where there is active skin disease, complex dermatological treatment or a previous significant adverse reaction.
Evidence position: established chemistry and unsupported claims
The strongest proposition in this area is limited but important. pH changes the calculated unionised fraction of a weak acid, and peel depth depends on more than a product's stated percentage. The first point follows established acid-base chemistry. The second is consistent with clinical descriptions of peel technique, variable tissue response and complications. It is reasonable to ask whether enough formulation information has been supplied to interpret a percentage claim.
A more tentative proposition is that reporting calculated free acid improves prediction of real-world clinical peel depth. Free-acid calculation captures one relevant part of formulation chemistry, but it does not standardise vehicle behaviour, buffering, skin preparation, contact time, application technique or individual barrier status. It is therefore more useful for scrutinising a formulation claim than for predicting the result on one person's skin.
The unsupported leap is to claim that a named acid, concentration, free-acid percentage or Fitzpatrick category guarantees a depth, result or absence of complications. Published studies use differing agents, protocols, populations and outcome measures. Individual healing also varies. Better evidence would come from comparative studies using documented final formulation pH, measured application parameters, consistent follow-up and outcomes that include pigment change as well as visible improvement.
| Evidence position | Verdict | Reason | What would strengthen the evidence |
|---|---|---|---|
| pH changes weak-acid free fraction | Established | It follows the Henderson-Hasselbalch equation | Reliable reporting that the stated pH is the final formulation pH |
| Free-acid percentage predicts exact peel depth | Plausible but unproven | Other formulation and application variables influence delivery | Standardised studies linking chemistry to clinical and histological endpoints |
| Fitzpatrick type sets a safe maximum depth | Unsupported | The scale is not a validated peel-depth safety calculator | Prospective models including history, protocol and pigment outcomes |
| TCA concentration alone defines injury depth | Unsupported | Coats, preparation, site and endpoint can alter delivery | Reproducible protocol-specific histological research |
A plausible biological mechanism and a demonstrated patient outcome are different forms of evidence. Keeping them separate avoids turning a partial calculation into a clinical promise.
Scope and limits of this reference
This reference covers chemical-peel pharmacology, broad depth categories and interpretation of pigment-related risk. It does not tell a reader which peel to receive, provide an application protocol, replace a skin examination or determine whether a practitioner or premises meets applicable requirements. It also does not cover home use, aftercare, emergency treatment, complaint routes or management of complications.
The worked arithmetic uses glycolic acid solely to demonstrate the method. It should not be transferred to lactic, salicylic, pyruvic or other acids without using the relevant pKa and the final formulation pH. It does not apply to TCA in the same way. It also cannot establish the activity of blended products, buffered systems, products with undisclosed pH or formulations whose composition changes after mixing.
This reference is especially limited for people with active inflammatory skin disease, infection, severe or persistent pigment alteration, a history of problematic scarring, or complex dermatological treatment. Those circumstances need individual assessment rather than a web-derived depth chart. The retrieval point is simple: percentage is a label value; free acid is a conditional chemical calculation; clinical depth remains a protocol- and person-dependent outcome.