Your Face and Your Body Don't Absorb the Same. The 42× Number Everyone Quotes Isn't the Reason.

Someone wrote in last month asking whether it was fine to finish a half-used body lotion on her face. Her reasoning was that face skin is thinner, so a body product would be, if anything, too weak for it. She had it backwards in two separate ways, and untangling why took us into a 1967 paper we could not open and a study published last October that undercuts the whole premise.

The number she was half-remembering is 42. It circulates as proof that where you put a product matters enormously: some parts of you absorb 42 times more than others. The number is real. It comes from an actual experiment on actual people. It just doesn't mean what it gets used to mean, and the explanation attached to it (thin skin absorbs more) falls apart against the data it is supposed to explain.

Below is what the 1967 experiment actually measured, and what happened when somebody put the same question to a real cosmetic ingredient in 2025.

Where the 42 actually comes from

Feldmann and Maibach, Journal of Investigative Dermatology, February 1967, three pages long. They painted radiolabelled cortisol on a run of anatomical sites in men and measured what came out in urine. That paper is the origin of every regional absorption chart you have ever seen in a skincare article.

We could not read it. PubMed carries no abstract for the record, only a title and a DOI. There is no open-access copy anywhere we could find, and the publisher's paywall did not budge. So the numbers below are the ones reproduced by the World Health Organization's International Programme on Chemical Safety in Environmental Health Criteria 235, Dermal Absorption, Figure 12 on page 76, which describes itself as adapted from the 1967 work. That is the honest attribution, and it is the one we are using.

One compound, hydrocortisone, measured under conditions nobody applies a moisturiser in.

Thickness does not produce that ranking

The shorthand everyone reaches for is that face skin is thin and body skin is thick, so the face takes things in faster. Look at the table again. The palm carries the thickest stratum corneum on the human body and sits at 0.83, which is to say roughly level with the forearm. The sole of the foot carries a stratum corneum of the same order and sits at 0.14. Two sites built from nearly the same tissue, six-fold apart.

The 0.83 and the 0.14 cannot both fall out of a thickness model.

It gets worse for the shorthand when you go and measure the layers. Sandby-Møller and colleagues took biopsies from 71 people in 2003 and reported stratum corneum and viable epidermis separately. Dorsal forearm: 18.3 µm of stratum corneum, 56.6 µm of cellular epidermis. Shoulder: 11.0 and 70.3. Buttock: 14.9 and 81.5.

Read those down the column and the two layers rank the sites in opposite orders. The forearm, the site everyone uses as the low-absorption reference, has the thickest stratum corneum and the thinnest living epidermis of the three. Whichever layer you decide is "the barrier," you get a different league table, and neither of them matches the absorption league table.

Also worth saying plainly: that paper never measured the face. It measured forearm, shoulder and buttock. The claim "facial skin is thinner, therefore it absorbs more" is usually sourced to work that did not look at faces.

What does a lot of the explaining is holes

In 2008 Otberg's group at the Charité took six men with pronounced chest hair, clipped it to half a millimetre, and applied caffeine at 10 µg per square centimetre over 25 cm² of chest. On one side they sealed every follicular opening with a microdrop of varnish-and-wax mixture. On the other side they placed identical drops beside the openings, so both arms of the experiment had the same 8% of epidermis covered. Only the route differed.

With follicles open, caffeine showed up in blood at the five-minute draw, 3.75 ng/mL. With them sealed, nothing was detectable until twenty minutes. Peak concentration fell from 11.75 ng/mL at one hour to 6.65 ng/mL at two hours. The authors' own summary sentence is that absorption "took at least four times longer" when the follicles were closed.

A re-analysis by Liu and colleagues in 2011 put a rate constant on it: the follicular route runs roughly ten times faster than transport through the stratum corneum, starts with no lag where the stratum corneum takes about ten minutes to get going, and finishes inside half an hour. In that window it accounted for somewhere between 10.5% and 33.8% of everything absorbed.

Follicle density is wildly regional. Jönsson and colleagues counted it at nine sites in 2017 using the same cyanoacrylate stripping method Otberg used, and got 285.0 ± 84.1 follicles per cm² on the forehead against 15.6 ± 3.6 on the calf. Eighteen-fold. The forearm came in at 40.3. That gradient tracks the WHO table (forehead 6, scalp 3.5, ankle 0.42) far better than any thickness measurement does.

Then somebody measured an actual cosmetic ingredient

Hiller and colleagues published in Archives of Toxicology in late 2025 what appears to be the first modern in vivo comparison of a common cosmetic preservative across body sites. They applied 2-phenoxyethanol at 0.2 mg/cm² to seven regions and tracked blood and urine for 48 hours.

Total recovery ran from 23.4% on the thigh to 42.9% on the backs of the hands. Against the forearm at 26.2%, that is a spread of 0.89× to 1.64×.

Not 42, and not 13.

Time to peak, meanwhile, ran 0.8 hours at the neck and forehead against 2.9 hours at the palms — a 3.6-fold spread going the other way, because the palms were the slowest site of the seven while sitting near the top for total amount. The authors put it flatly: penetration velocity did not seem to influence the overall absorbed amount. Rate and extent are two different questions and body site answers them differently.

Two caveats we are not going to bury. The study ran on two volunteers, and two of its six authors work for BASF, which manufactures 2-phenoxyethanol. Both facts are disclosed in the paper. It still deserves attention, because nobody else has done this measurement on a cosmetic ingredient.

WHO's own summary table in the same document hints at the same thing. Its coarse penetration index puts genitals at 40× arms for hydrocortisone and 12× arms for pesticides. Same sites, different chemistry, index collapses by more than three-fold.

What our screening found

The practical takeaway

Back to the reader with the leftover body lotion. Using it on her face is unlikely to be a dosing problem, because the amount that gets through depends far more on the molecule than on the postcode. The real objections are boring ones: body lotions are built heavier because nobody minds a film on a shin, and they carry more fragrance because nobody smells their own forearm all day. Six declared allergens on a face, twice daily, is a different proposition from six on a pair of hands.

Going the other way, face serum onto the body, is mostly an arithmetic problem. You need roughly ten times the surface area covered, and a 30 ml bottle was not priced for legs.

Where site genuinely earns caution is the short list that keeps coming out high on every measure: forehead and jaw, scalp, underarm, groin. High follicle density, thin or occluded skin, or both. If you are going to be careful with fragrance load or with a strong active anywhere, be careful there, and stop worrying about your shins.

Feet run the other way entirely. The least permeable site on the chart is why foot creams reach for keratolytics at strengths nobody would put on a face — urea sits third on GOUNBAL's list of 33, ahead of everything except water and glycerin. That is not aggression. That is what it takes to get anything to happen down there.

We wrote about the related question of what order to apply things in here, and about how much sunscreen actually stays on skin across formats here. Our scoring method, including the parts of it we are unhappy with, is on our awards page.