Urea, salicylic acid and lactic acid: where a balm stops being just occlusive

Urea hydrates below 10 percent and breaks down keratin above 20. Salicylic acid monograph levels, the anhydrous dispersion problem and the ammonia risk.

A balm made of wax, butter and oil can do one useful thing to a heel: slow the water leaving it. Urea, salicylic acid and the alpha hydroxy acids do something else, they act on the keratin itself, and all three were developed in vehicles that contain water. This page sets out which of them survive the move into an anhydrous base, what the concentration figures on a tube actually mean, and the point at which a foot product stops being a cosmetic.

• INCI: Urea
• Water about 108 g/100 mL at 20 C
• Oil solubility negligible
• Keratolysis above 20%
• EU salicylic acid cap 2%

Urea is a different ingredient at every concentration

Urea appears on ingredient lists anywhere between about 2 and 40 percent, and the number is not a strength setting on one effect. It is the difference between two mechanisms. Below roughly 10 percent urea works as a humectant, topping up a molecule the skin already makes: urea is one of the components of natural moisturising factor, the pool of small water-binding solutes inside corneocytes, where it accounts for around 7 percent by weight alongside free amino acids, lactate and pyrrolidone carboxylic acid. Above roughly 20 percent it starts competing for the hydrogen bonds that hold keratin filaments together, which swells and loosens the corneocyte stack rather than hydrating it.

The practical consequence is that a "urea foot cream" tells you nothing until you read the percentage. A 5 percent product and a 25 percent product are not the same product at different doses, they are a moisturiser and a keratolytic, and only one of them will do anything to a callus.

Why urea cannot act without water

Urea is small, with a molecular weight of 60.06, and it is a hydrogen bond donor and acceptor at both ends. Both of its jobs depend on that. As a humectant it binds water molecules and holds them in the corneocyte. As a keratolytic it inserts itself between keratin chains and takes over the bonds that were holding them in register. Neither is possible unless the urea is dissolved and mobile. A crystal sitting in a wax matrix has no water to bind and cannot get between anything.

So in a fully anhydrous balm the amount of urea in solution is zero. The general rule and the arithmetic behind it are set out in solubility in anhydrous formulas. What complicates the picture, and what suppliers lean on, is that water does eventually arrive: an occlusive film raises stratum corneum hydration underneath it, and some of the urea sitting at the interface will dissolve into that water and act. That is real, but the dose is unknown, uncontrolled and small, and it is nothing like the 25 percent solution the clinical studies used. Selling a 10 percent urea balm on the strength of it is selling a number that never gets into solution.

Three routes into an oil base, and what each one costs

Work the arithmetic before you buy the powder. Urea dissolves in glycerin at roughly half its own weight, so a glycerin pre-solution is the obvious idea. The problem is the carrier, not the urea: glycerin is itself insoluble in oils and starts beading out of an anhydrous base at around 3 to 5 percent, as covered in glycerin and humectants. Five percent glycerin will carry about 2.5 percent urea. To deliver 10 percent urea you would need 20 percent glycerin, which is not a balm, it is a slick that separates in the tin.

That last row is the one most makers resist and it is usually the right answer. Urea does the keratolysis in a vehicle built for it, the balm does the occlusion, and splitting the two removes the awkward conversation about what a 25 percent urea product legally is.

Ammonia, pH drift, and the one place urea keeps well

Urea hydrolyses in water. The reaction runs through ammonium cyanate to ammonia and carbon dioxide, and it is slow at room temperature, faster with heat, and much faster in the presence of urease from skin flora or contaminating bacteria. Two things follow. The first is the smell: a urea product that has been abused smells sharply of ammonia when you open it, and the customer reads that as spoilage, correctly. The second is pH. Ammonia is basic, so an aqueous urea formula drifts upward over time, which is why commercial urea creams are buffered to around pH 4 to 6 and why urea goes in at the cool down, below about 40 C.

Here is the awkward part. Dry urea powder in a wax and oil base is chemically stable, more stable than it would be in any cream, because the hydrolysis needs water it does not have. The anhydrous balm is the one format where urea keeps perfectly and the one format where it cannot work. What it is not is immune to contamination. Urea is hygroscopic, cakes above about 75 percent relative humidity, and is a nitrogen source. A wet spatula, condensation on a tin brought in from a cold van, or a customer with damp fingers in a foot balm gives you a local wet pocket with a nutrient in it and a pH heading upward, which is exactly the situation described in water contamination in balm. Read moisture control before deciding an anhydrous urea product needs no preservation at all.

Salicylic acid, the keratolytic an oil base can actually carry

Salicylic acid is the exception, and the reason is worth understanding because it is the opposite of the urea problem. Permeation of a weak acid across the stratum corneum happens as the un-ionised free acid, and salicylic acid has a pKa of about 2.97, so in an aqueous vehicle you have to hold the pH low to keep a useful fraction of it un-ionised. An anhydrous vehicle has no pH at all, so the whole dose is present as free acid, in its most permeable form. The chemistry is on your side.

What limits you is solubility. Salicylic acid melts at 158 to 161 C and is only sparingly soluble in triglyceride oils, roughly one to two percent at room temperature, more when warm. Castor oil and the more polar cosmetic esters carry it better than sunflower or olive. That is enough for a working level, but it means you must dissolve it, verify it stays dissolved after a week at room temperature, and reject any batch that throws crystals. A recrystallised acid is both gritty and unevenly dosed, and the sensation that follows is the subject of balm stings or burns.

Read that table as a boundary, not a menu. A UK or EU foot balm can hold up to 2 percent salicylic acid as a cosmetic, and at that level it will soften and gradually thin a callus over weeks, which is a real if modest effect. It cannot be sold as a callus remover, because the moment you say that you have described a medicinal product regardless of what is in the jar. The distinction is worked through in cosmetic versus drug claims.

Lactic acid and the alpha hydroxy acids: inert, or a sting

Lactic acid has a pKa of about 3.86 and glycolic acid about 3.83, and every efficacy study behind them used an aqueous formulation at a controlled pH between roughly 3 and 4, where the free acid fraction is defined and measurable. In an oil base there is no pH to control and no way to state one.

Worse, the material itself will not stay put. Lactic acid is supplied as an 80 to 90 percent aqueous solution, which is a water phase by any sensible definition, insoluble in oils, denser than the base and strongly hygroscopic. Stirred into a melt it disperses, then coalesces as the balm sets, and the droplets sink. What the user then applies is not a two percent acid, it is a patch of concentrated acid on one part of the skin and nothing on the rest. That is a burn risk rather than an exfoliation, and adding water to a wax base introduces every microbiological question that anhydrous formulation was designed to avoid.

There is also a specific regulatory trap. The EU set consumer limits for these acids in 2024: lactic acid up to 10 percent in leave-on face and hand products with the pH not below 3.5, and glycolic acid up to 4 percent with the pH not below 3.8. A product with no water phase cannot demonstrate a pH, so there is no way to show the condition attached to the permission has been met, and a safety assessor writing your report has nothing to sign against. If you are selling into that market, read this alongside safety assessment and the CPSR and selling balms in the UK and EU.

Grit, settling and what milling actually buys

If you do suspend a powder, particle size decides whether the product feels like a balm or like a scrub. Urea is sold as prills of one to three millimetres for agriculture, as crystalline grades of a few hundred micrometres, as milled grades around 50 to 150 micrometres, and as micronised grades below about 30. Hard particles on skin are generally detectable somewhere around 20 to 30 micrometres, lower on the lip, so only the micronised grades disappear. Urea decomposes at 132 C rather than softening, so nothing on the skin will melt it: whatever goes into the pot is what the customer feels for the life of the product. The same problem, with a different powder, is set out in allantoin.

Grinding it yourself is a poor answer. A domestic grinder gives a wide, uncontrolled distribution, warms the powder and exposes a hygroscopic solid to room air, so it cakes while you work. Settling is the second issue: urea has a density of about 1.32 g per cubic centimetre against roughly 0.92 for a finished balm, so it sinks steadily in a fluid melt until the wax network arrests it. The countermeasures are the ones in pigment sinking to the bottom: a firmer base, a faster set and a cooler pour.

Where a foot product stops being a cosmetic

In the UK and the EU a cosmetic is defined by function: cleaning, perfuming, changing appearance, protecting, keeping in good condition, or correcting body odours. Removing a callus is not on that list, and neither is softening a nail plate so it can be taken off. A product can fall outside the cosmetic definition two ways. By presentation, if you describe it as treating a condition, which is a decision you make in your own copy. Or by function, if it acts by a pharmacological, immunological or metabolic mechanism, in which case it is a medicine, or by a physical mechanism aimed at a medical purpose, in which case it may be a medical device instead. The MHRA publishes a borderline guide for exactly these cases, and it applies whatever the ingredient list says.

Practically: a 5 to 10 percent urea foot cream is a cosmetic in ordinary use, a 40 percent urea nail preparation is not, and between them lies a band where your own wording decides. The US route differs in structure and lands in the same place, because intended use determines whether you are inside a monograph and therefore inside drug labelling and facility registration. Both sides are mapped in cosmetic versus drug claims.

What occlusion alone achieves on a cracked heel

The evidence for urea creams at 5 to 25 percent in dry, scaling skin is reasonable: repeated controlled trials show higher hydration and less scaling against untreated skin and against bland bases. The evidence specific to heel fissures is much thinner, mostly small studies and clinical practice, and it consistently involves more than a cream.

What an occlusive balm does is well characterised and worth stating plainly. A heavy anhydrous film cuts water loss substantially, with petrolatum the benchmark at around 98 percent reduction in laboratory measurement and most botanical butters and waxes well below that, as covered in measuring occlusivity. Hydrated keratin is pliable and dry keratin is brittle, so raising the water content of a callus makes it far less likely to split under load. That is genuinely useful: it is why an overnight balm and a cotton sock reduce fissure pain within a few days.

What occlusion does not do is remove callus. The mass of hardened tissue is unchanged; it is simply softer. Reducing it is mechanical work, a file or a professional debridement, plus attention to footwear and standing time. A balm's honest role is to keep the skin pliable between those sessions and to stop the fissure reopening, and that is the position taken in the foot and heel balm formula. The occlusive materials that do the work are covered in lanolin and petrolatum and mineral oil, and the underlying mechanism in occlusive, emollient and humectant.

The decision rule

Ask what you are actually trying to change. If the answer is water loss, an anhydrous balm is already the right format and it needs no active at all; adding urea to it is a marketing decision, not a formulation one. If the answer is the thickness of the callus, you need keratolysis, and keratolysis needs water, which means an emulsion, a preservative, a controlled pH, a stability programme and a regulatory route that is probably not "cosmetic". Those are two different products and there is no clever way to make one of them do both jobs.

The single exception is salicylic acid at 1 to 2 percent, fully dissolved in a castor-rich or ester-rich oil phase, in a market where that level is permitted and where you are not tempted to describe what it does. Everything else on the keratolytic shelf either fails to dissolve, hydrolyses when it finally gets wet, or drags your product across a regulatory line you did not intend to approach.

Reviewed and updated 6 September 2026. Spotted an error? Tell us and we will fix and log it.