Skin, hair and nails β the largest organ you have, the edge where you stop and the world starts, and the reason a drop of oil on a wrist reaches anything at all.
~13 min read
Stretch it flat and it covers close to two square metres; weigh it and you are holding three to five kilograms, heavier than your brain or your liver. Skin is not the wrapping around the body. It is the largest working organ you have.
It renews without stopping β half a kilogram of cells a year β which is why a shortfall in raw materials shows here first.
The organs and structures involved
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Skin is three layers stacked, and almost every practical question resolves into which layer you mean. The epidermis is the top, thinner than a sheet of paper and with no blood vessels of its own: keratinocytes are born at its base and travel upward over about four weeks, flattening and finally losing the nucleus and every organelle, so what reaches the surface is not a living cell but a flattened protein plate. The barrier keeping you alive is built of dead ones. The dermis beneath is the thick one β collagen and elastin, and everything that bleeds, feels or sweats.
Diagram to come
One vertical cross-section through skin, drawn tall enough that the three layers are honestly proportioned β a very thin epidermis, a dermis several times deeper, a subcutaneous layer deeper still. Inside the epidermis, show the keratinocyte journey as a column of cells changing shape from bottom to top: round and nucleated at the base, progressively flatter and paler, finally flat plates with no nucleus at the surface, with a few shedding off the top edge. In the dermis show collagen bundles, a capillary loop reaching up to but not entering the epidermis, a nerve ending, one full hair follicle running at a slant from surface into dermis with a sebaceous gland opening into it and a small arrector pili muscle, and one coiled sweat gland whose duct opens separately on the surface. A thin bracket at the very top marks the stratum corneum only, so the reader can see how little of the whole it is.
Hair follicles are in-foldings of epidermis running into the dermis, each with a sebaceous gland opening into it β dense on face, scalp and upper back, nearly absent from palms and soles, which never feel oily. The part that does the sealing is the top of the epidermis: the stratum corneum, 15 to 20 cell layers deep and 10 to 20 micrometres thick over most of the body β against around 100 for printer paper, and several hundred on palms and soles.
The standard model is bricks and mortar. The bricks are corneocytes β flattened dead keratinocytes in a cross-linked protein envelope, packed with keratin. The mortar is a lipid matrix filling every gap: roughly half ceramides, a quarter cholesterol, free fatty acids for the rest. The bricks give toughness; the mortar seals. Strip the lipids with a strong detergent and the wall still leaks β which is why a harsh wash leaves skin tight.
On top sits the acid mantle β a film of sebum, sweat and the breakdown products of shed cells, holding the surface at pH 4.5 to 5.5. The enzymes that loosen cells for shedding work at that pH, and resident microbes prefer it while transient ones often do not. Plain soap runs at pH 9 to 10; healthy skin rebuilds the mantle within hours.
A barrier that let things through would not be a barrier, and yours is very good. Yet a drop of lavender rubbed into a wrist can be measured in blood within minutes. The space between those two facts is where every dilution rule comes from.
Three ways across. The main road is between the bricks, dissolving into the lipid mortar and winding through it β a molecule crossing twenty micrometres of stratum corneum travels several hundred as it threads between flattened cells. The second is straight through the bricks, crossing lipid and then a protein-and-water interior; few manage both. The third is down a follicle, but those openings are a thousandth of the surface: a shunt, not a bulk route.
Three properties predict what crosses the mortar.
Size
The ceiling is around 500 daltons; above it almost nothing crosses intact skin. Oil constituents sit far below β a monoterpene 136, linalool and linalyl acetate 154 to 196, sesquiterpenes 204, santalols 220.
Fat-friendliness
The mortar is lipid, so a molecule must dissolve into fat to enter β and out of it again for the watery dermis, so something extremely greasy gets in and stays. What crosses best is moderately fat-soluble, which describes most oil constituents.
Charge
A charged molecule will not enter a lipid sheet at all β which is why mineral salts stay on the surface, and why water, tiny at 18 daltons but polar, barely crosses. You do not swell in the bath.
The sentence every dilution rule rests on
Oil constituents cross skin because they are small, fat-soluble and uncharged β the same properties that let them reach a cell once inside. Anything that raises the rate of crossing raises the dose: heat, water, a covering, thinner skin, a broken surface, a larger area. Dose is what you dilute to control.
The carrier is the most misunderstood part. A neat drop on warm skin is largely gone within minutes β most of it leaves as vapour rather than going in. A carrier does not evaporate, so it holds the constituents against the surface and spreads the same drops over a larger area: concentration at any one point falls. That reduces irritation while total crossing stays as high or higher. Diluting is not weakening.
Each of those rate-raisers is met in an ordinary day. Water swells the corneocytes and loosens the mortar, so skin out of a bath is more permeable. Occlusion β a plaster, tight clothing, a thick balm β can multiply absorption several times. And broken skin has no barrier at all, which is why nothing goes on it.
Thin skin is not thick skin, either. Stratum corneum thickness varies roughly tenfold across one body β eyelids and face at the thin end, palms and soles at the other, the inner forearm between, which is why patch tests go there. A face and a forearm at the same percentage are not the same exposure. Children are the same argument at another scale: a newborn carries roughly three times the skin area per kilogram, so childrenβs dilutions run at a half or a quarter of an adultβs. Safety holds the figures.
Diagram to come
A wide, heavily magnified slice of stratum corneum only, drawn as a brick wall: flattened overlapping corneocytes as the bricks, a visibly continuous lipid matrix as the mortar, about 15 courses deep, with living epidermis as a thin band along the bottom. Three routes traced from the surface downward β a long winding path threading only through the mortar between bricks, drawn thick to mark it as the main one; a short straight path punching directly through bricks, drawn thin and faint; and a third running down the side of a hair follicle that cuts through the wall. Along the right edge, four small icons with arrows showing what increases the rate: a heat symbol, a water droplet, a covering or plaster, and a larger applied area. A size key at the bottom places a monoterpene at 136 daltons and a marker at the 500-dalton ceiling on one line.
Most of what is known about skin permeation comes from laboratory work in a diffusion cell β a disc of skin clamped between the test substance and a chamber of fluid sampled over hours. That is how the rules above were found, and it is skin in a dish rather than a person.
No blood flow carries arrivals away, no core temperature is held, nothing lives underneath. What such measurements give is a ranking β which molecules cross more readily β and it has held up. What they cannot give is what any amount does inside a living body.
Hold that line firmly. A result measured on cells or tissue in a laboratory is not a result measured in a person and must never be read as one. Knowing a constituent crosses skin tells you it arrives; it tells you nothing about what follows.
Five things, and they are not a warm-up before the real advice. They are the real advice, and each outranks anything in a bottle.
An oil on skin short of sleep, water and protein is decoration on a deficit.
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Skin is rebuilt continuously out of materials arriving through the blood. Vitamin A is the signal telling keratinocytes how to mature as they rise. Vitamin C is a required co-factor for the enzymes that cross-link collagen; without it the fibres are made but never stabilised, which is why the dermis holds one of the bodyβs highest vitamin C concentrations. Zinc drives cell division, so the fastest-renewing tissues feel a shortfall first.
Vitamin E is fat-soluble, so it sits inside the lipid mortar and inside cell membranes, taking the hit from oxidation before the membrane lipid does β and it and vitamin C regenerate each other. Biotin works in fatty-acid synthesis, which is literally how the mortarβs lipids get made.
Linoleic acid is a direct building block of the ceramides sealing the barrier and your body cannot make it, while the longer-chain fats in GLA and the marine omega-3s change every membrane they enter. Collagen peptides deserve a note: eaten collagen is digested like any other protein, and the argument for the peptide form is that certain di- and tripeptides survive digestion intact. That is suggestive rather than settled, and adequate protein with enough vitamin C does the same job for less.
Most arrive together in a daily foundation β Microplex VMz, xEO Mega and TerraGreens β and which of those is sold differs across the 65 markets. Nutrition and the Body Systems maps the set against all nine.
| Strength | Where it belongs | Per 10 mL | Per 30 mL |
|---|---|---|---|
| 0.5β1% | Face, sensitive skin, children over two | 1β2 drops | 3β6 drops |
| 1β2% | Daily body use on an adult | 2β4 drops | 6β12 drops |
| 2β3% | A small area, short run of days, adult | 4β6 drops | 12β18 drops |
Those counts assume about 20 drops to the millilitre; a thick oil such as myrrh drops larger. For children under two, ask a healthcare practitioner. Patch test before anything goes on regularly: mix at final strength, a coin-sized amount inside a forearm, unrinsed, checked at 24 and 48 hours. If skin reacts, reach for carrier oil rather than the tap.
Twelve hours out of the sun after expressed citrus
Cold-pressed citrus oils β bergamot most of all, then lime, lemon, grapefruit and wild orange β carry furanocoumarins squeezed out of the peel. Those absorb ultraviolet light, and once energised they react with skin cells in a way the oil alone never would, burning or darkening in the shape it was applied in, hours after the sun. So after applying an expressed citrus oil, keep that skin out of direct sunlight and off sunbeds for 12 hours. Distilled citrus oils and bergamot FCF do not carry the risk.
The rest do not move. Nothing in or near the eyes, nothing in the inner ear, nothing on broken skin. The hot oils β oregano, thyme, cinnamon bark, cassia and clove β belong at 0.5% or less, and never in a whole-body leave-on product. Peppermint stays off the face and neck of a child under six. Oils never go into a bath alone: stir them into a tablespoon of carrier first.
What separates these is which family of molecules dominates β which is what decides how each behaves on skin.
Lavender
The one to reach for when you are not certain: 25β45% linalyl acetate, 20β47% linalool, 0.3β10% ocimene β esters and monoterpene alcohols, small and crossing readily. Well diluted, among the gentlest things you can put on skin. Lavender in full.
Frankincense
Ξ±-pinene, limonene and sesquiterpenes β exactly the shape that crosses. The boswellic acids the product data also lists belong to the resin rather than the distillate and are far larger molecules; they are not what reaches skin. Frankincense in full.
Tea tree
Terpinen-4-ol first, with gamma-terpinene and a little 1,8-cineole. High terpinen-4-ol with low 1,8-cineole is the quality standard; a good tea tree differs from a poor one in that ratio. Possible sensitivity. Tea tree in full.
The heavier four
Helichrysum, neryl acetate plus italidiones found in almost nothing else. Geranium, monoterpene alcohols β citronellol, geraniol, linalool. Myrrh, furanoid sesquiterpenes, larger and slower than anything in lavender, which shows in the bottle: it pours thick and drops big. And Hawaiian sandalwood, almost entirely sesquiterpenols, so the aroma lasts on skin for hours rather than minutes. Possible sensitivity from helichrysum and geranium.
Six appear together in Immortelle β frankincense, sandalwood, lavender, myrrh, helichrysum and rose β a pre-diluted Touch roll-on for the appearance of ageing skin. Availability differs across the 65 markets. For making things rather than buying them, the bathroom is the companion lesson.
Skin responds to consistency far more than to intensity, so this follows what the barrier is doing through the day.
Morning
Wash with something mild, or water alone if your skin runs dry β nothing accumulates on a face overnight that needs a detergent. Apply to skin still damp, at 0.5β1% for a face.
Evening, and after a day outdoors
Skin after a long day outdoors is warm and more permeable, so it needs less rather than more: let it cool, then a well-diluted oil in a generous carrier, 1% is plenty. At night, barrier repair and cell division both run while you sleep, so what goes on then has hours of contact rather than minutes β oil onto damp skin before bed, and once a week a facial massage with a teaspoon of fractionated coconut oil and a drop or two in it.
The seasons
Winter is a barrier problem first: cold outside, dry heated air inside, hot showers stripping the mortar twice a day. Change the carrier and the strength with the season, and keep the oils the same.
Photograph to come
Close, quiet, natural light. Two open hands, palms up, a small pool of carrier oil in one and a single drop falling into it from a dark glass bottle held just above β the moment of dilution, not of application. Skin in frame should be ordinary and unretouched, any age. A folded cloth and a bowl of water nearby to place it in a real bathroom rather than a studio. No legible product labels, no packaging, no styling.
Three things belong with a qualified practitioner, not a bottle. Any mole that changes β in size, shape, colour or edge, or that begins to itch, bleed or crust. Anything on skin that does not heal within a few weeks. Anything spreading, especially if you feel unwell with it.
Skin is the one organ you can watch directly, which makes it the easiest place to notice a change early β and early is most of the value. Photograph anything you are watching, with a coin beside it and the date; two photographs a month apart beat any description from memory. Reading Your Own Body sets out how to keep that record without it turning into worry.
This is education about how skin is built and how it works. It is different from professional medical care and does not replace it. Anything that concerns you or persists belongs with someone who can examine you in person.
Keep oils away from eyes and inner ears, and out of reach of children. If you are pregnant, nursing, under medical care or taking medication, speak to your healthcare practitioner before use. Nothing here is intended to diagnose, treat, cure or prevent any disease.
Lesson 10 of 11 Β· Body Systems