100 Fun Facts About Human Skin

Skin is the body's heaviest organ, so which is the heaviest organ inside the body?

The liver, at about 1.5 kg, leads the internal organs, but the skin easily outweighs it: at 3.5 to 10 kg it makes up around a seventh of an adult's body weight.

Spread out flat, an average adult's skin would roughly cover what?

A single bed: adult skin covers 1.5 to 2 square metres, about one mattress top, and doctors work out each patient's figure from height and weight to set chemotherapy doses.

What are the three primary layers of human skin, from outside in?

Epidermis, dermis, hypodermis: a thin barrier that is shed constantly, a tough middle layer that carries the nerves and sweat glands, and a bed of fat that cushions muscle and bone.

The prefix 'epi' in epidermis comes from Greek and means what?

Over or upon: Greek epi, 'upon', plus derma, 'skin', makes the epidermis the layer on top, just as an epicentre is the point on the surface directly above an earthquake's focus.

With no blood supply of its own, where does the epidermis get almost all of its oxygen?

The surrounding air: oxygen diffuses straight in and supplies the epidermis almost entirely, so the blood beneath adds little, although the cells' food still has to seep up from capillaries below.

Fingertips wrinkle in the bath because nerves make what constrict beneath the skin?

Blood vessels: a nerve signal narrows them, the fingertip's pulp shrinks and the skin above puckers, which is why fingers with nerve damage stay smooth however long they soak.

Which outermost layer of the epidermis is a sheet of dead, flattened cells?

Stratum corneum, literally the 'horny layer': its dead, scale-like cells flake away constantly and are replaced from below, so the surface you touch is never living tissue.

Which cell type makes up roughly nine in every ten cells of the epidermis?

Keratinocytes: as they are pushed up towards the surface they fill with keratin, the tough protein of hair and nails, then die and harden into the skin's outer shield.

Roughly how long does the skin take to replace its whole outer layer with new cells?

A month or two: estimates run from four to about seven weeks for a new cell to rise to the surface, die and flake off, so the skin you see today was not there last season.

Where on the human body is the skin at its thinnest?

Around the eyelids: the skin there has no layer of fat beneath it, which helps make it the thinnest on the body.

Which skin condition, confirmed by Michael Jackson's autopsy, leaves white patches where the skin has lost its pigment?

Vitiligo: Jackson told interviewers that the disease, and not bleaching, had lightened his skin, and the doctor who performed his autopsy confirmed the diagnosis to a Los Angeles jury in 2013.

The English word 'skin' is borrowed from Old Norse 'skinn', which originally meant what?

Animal hide: the Anglo-Saxons' own word for skin was 'hide', so the two have since traded places, with the Norse loan covering people and the native word left to cattle.

Which part of a fingernail takes its name from the Latin for 'little skin'?

The cuticle: Latin cuticula is the diminutive of cutis, 'skin', the same word that gives doctors 'cutaneous' for anything to do with the skin.

Which of these animals, like humans, cools itself mainly by sweating all over its body?

The horse: cooling by sweat is rare among mammals, but horses pour it out, and a soap-like protein in it whips up the white lather you see on a hard-ridden horse.

Which group of bacteria, among the commonest on healthy skin, includes the drug-resistant superbug MRSA?

Staphylococcus: MRSA is short for methicillin-resistant Staphylococcus aureus, and like its cousins it usually lives harmlessly on the skin, causing infection only if it gets inside the body.

Which animal's skin is so like ours in thickness and hair follicles that wound dressings are tested on it?

Pig: its skin matches ours far better than a rat's or rabbit's, so studies of healing, drug patches and sunburn that cannot be run on people are often run on pigs.

The pigment melanin takes its name from a Greek word meaning what?

Black or dark: Greek melas also lies behind melancholy, literally 'black bile', the fluid that ancient doctors blamed for a gloomy temper.

Which form of melanin is largely responsible for the red or yellow tint of some skin and hair?

Pheomelanin: it gives almost no protection from ultraviolet light, so redheads, whose pigment cells make mostly this form, tend to burn and freckle where others tan.

The skin builds its pigment melanin from which amino acid, named after the Greek word for cheese?

Tyrosine, which Justus von Liebig first got out of cheese in the 1840s: the enzyme that starts turning it into melanin is tyrosinase, and faults in its gene cause one form of albinism.

Melanocytes, the pigment-making cells, sit in which layer of the skin, right where epidermis meets dermis?

The stratum basale, the bottom row of the epidermis: each melanocyte begins in the embryo's neural crest, travels to the skin and then hands its pigment to the ordinary skin cells around it.

Light and dark skin hold similar numbers of pigment cells, so what mainly sets their colour apart?

Packaging of eumelanin in melanosomes: dark skin carries its pigment in many larger granules spread singly through each cell, while light skin bundles small ones in clusters of two or three.

Freckles form not because there are more melanocytes but because existing ones do what?

UV-B exposure ramps up production, which is why freckles darken in summer.

Freckling is linked to variants of which gene, also associated with red hair?

MC1R: the gene builds a switch on pigment cells that tells them to make dark, protective melanin, and the variants common in redheads weaken it, so the cells turn out a pale red-yellow pigment instead.

Strong evolutionary pressure for dark skin acted on early Homo about how long ago?

1.2 million years ago: our ancestors had shed their fur to cool by sweating, and dark pigment is thought to have spread to shield the folate in their blood, which strong ultraviolet light breaks down.

The Fitzpatrick scale, developed in 1975, classifies human skin by what?

Its response to UV light: Thomas Fitzpatrick drew it up to set safe starting doses of ultraviolet therapy for psoriasis, and it runs from Type I, which always burns, to Type VI, which never does.

Which nutrient does the skin synthesise for itself when exposed to ultraviolet B in sunlight?

Vitamin D: too little softens growing bones into rickets, and because weak winter sunlight cannot make enough, Britain's NHS advises everyone to consider a supplement through autumn and winter.

What is the medical name for the reflex that produces goose bumps?

Piloerection: a tiny muscle at the root of each hair tightens and pulls it upright, which helps a furry animal keep warm or look bigger but on our sparse body hair leaves only bumps.

Which division of the nervous system fires goose bumps when you are cold or frightened?

Sympathetic: the same nerves that speed the heart in a fright also stand each hair on end, and a threatened porcupine raises its quills by exactly this reflex.

Which naturalist called blushing 'the most peculiar and the most human of all expressions'?

Charles Darwin gave blushing a chapter of The Expression of the Emotions in Man and Animals in 1872, arguing that monkeys redden with passion but that no animal could be shown to blush.

At what stage of life do the ridges of a person's fingerprints take shape?

Before birth: the ridges begin to form around the third month of pregnancy and are complete by the sixth, and the pattern they settle into then stays the same for the rest of a person's life.

How do the fingerprints of identical twins compare?

Ridge patterns are shaped by conditions in the womb as well as genes.

Which Australian marsupial has fingerprints so like a human's that even a microscope struggles to tell them apart?

The koala: it climbs by gripping branches, while its close relatives the wombats have no ridges, so its prints evolved separately from ours, which points to gripping as what fingerprints are for.

Which Victorian, who also coined the word 'eugenics', wrote the 1892 book 'Finger Prints'?

Francis Galton used statistics to show that two people's prints would practically never match, and that same year a bloody thumbprint on a door in Argentina solved a double murder, considered the first cracked by a fingerprint.

Scotland Yard's first fingerprint files used a system devised by police in which country?

India: Edward Henry, police chief of Bengal, worked the system out with two Indian officers, Azizul Haque and Hem Chandra Bose, and brought it to London in 1901, when he set up Scotland Yard's fingerprint bureau.

The very rare condition of being born without any fingerprints is called what?

Adermatoglyphia: fingerprint ridges are dermatoglyphs, Greek for 'skin carvings', and a Swiss doctor nicknamed their absence immigration delay disease after a patient was held up at the United States border.

Which type of skin gland, found almost all over the body, does most of the work of cooling?

Eccrine: these glands open straight onto the skin and pour out watery sweat that cools as it evaporates, while the other kind empties into hair follicles in the armpits and groin and only switches on at puberty.

Anatomists class the glands that make which of these as modified sweat glands?

Breast milk: the mammary glands are thought to have evolved from sweat glands of the scent-making kind found in the armpit, and the glands that make earwax are modified from the same stock.

Oily sebaceous glands cover almost all of the skin but are missing from which areas?

Palms and soles: most of these glands open into hair follicles, and the thick skin there is hairless, while the face and scalp carry the most of all.

Into which layer of the skin is tattoo ink deposited to make it permanent?

The dermis: ink left in the layer above flakes away with the dead surface cells, but here immune cells called macrophages swallow the pigment and hand it on when they die, holding the design in place for life.

Which preserved body bears the oldest tattoos yet found?

Ötzi the Iceman: his 61 tattoos are plain lines and crosses, and many sit over joints that his bones show were worn, so researchers think they were meant to ease pain.

Sunburn is an inflammatory response triggered by UV damage to what in skin cells?

DNA and RNA: ultraviolet light damages these nucleic acids inside skin cells, and cells too badly hit to repair themselves self-destruct, which is why sunburnt skin later peels.

Window glass blocks the sun's burning UVB rays, but which of its rays pass straight through and age the skin?

UVA: it makes up about 95 percent of the ultraviolet light that reaches the ground and sinks deeper into the skin than the burning kind, so wrinkles can build up even behind a window.

Red Vet Pet, the red petroleum jelly that led to Coppertone, protected whom from the sun in the 1940s?

US soldiers in the Pacific: the red jelly was unpleasant to wear, so the airman and pharmacist Benjamin Green turned it into the friendlier lotion that became Coppertone.

Which of these skin cancers is the most aggressive and the likeliest to spread beyond the skin?

Melanoma often shows up as a new or changing mole and can spread quickly beyond the skin, while the far commoner basal-cell cancer grows slowly and rarely spreads, so an odd mole is worth having checked early.

Which two structures of normal skin are missing from scar tissue?

Sweat glands and hair follicles: scar tissue is dense collagen laid down without either, so a scar stays bald, and survivors of large burns can overheat because so much of their skin cannot sweat.

Which drug, long sold as Accutane or Roaccutane, is the most effective medication for severe acne?

Isotretinoin shrinks the skin's oil glands and can clear acne for good in some patients, but it causes severe birth defects, so anyone who could become pregnant must use contraception throughout the course.

Malassezia, the scalp dweller most often blamed for dandruff, is what kind of organism?

A yeast: Malassezia feeds on the scalp's natural oils, and its by-products are thought to irritate susceptible skin into flaking, which is why anti-dandruff shampoos often contain an antifungal such as ketoconazole.

In 1801 which European city turned a whole hospital over to skin diseases, founding dermatology's first great school?

Paris: the Hôpital Saint-Louis, founded by Henry IV two centuries earlier, was given over to skin diseases that year, and its physician Jean-Louis Alibert sorted what he saw there into families and species on a 'Tree of Dermatoses'.

Calluses form in response to repeated friction; what forms instead if the friction is too much, too fast?

A blister: the rubbing shears the upper layers of skin apart and clear fluid fills the gap, and the roof of loose skin is worth leaving on because it shields the raw layer beneath while it heals.

Faults in the gene for filaggrin, a protein that seals the skin's outer barrier, strongly raise the risk of which common condition?

Eczema: filaggrin bundles the fibres inside the outermost skin cells, flattening them into a tight barrier, and then breaks down into the skin's own moisturiser, so a faulty copy leaves skin dry and poorly sealed.

Nails and hair seem to keep growing after death, but what actually creates that illusion?

Skin drying and tightening as the body dehydrates leaves more of each nail and hair exposed, so they only look longer. Nothing is actually growing.

Which mammal of the southern United States is one of the few creatures able to give people leprosy?

Armadillo handling or meat can give people leprosy, and 15 to 20% of those in the south-central United States carry the bacterium. They are thought to have caught it from early European explorers.

Eating very large amounts of which vegetable over a long period can harmlessly tint a person's skin?

Carrots are rich in carotenoids, pigments that build up in the outer skin and turn it yellow-orange. The harmless condition, carotenemia, fades once the diet changes.

Fresh sweat from the armpit glands is odourless, so what turns it into body odour?

Skin bacteria break the oily armpit secretion down into smelly compounds. Deodorants fight back with antibacterial ingredients and perfume to mask what is left.

Folk belief blames toads, but which virus really causes the common warts on hands and feet?

Human papillomavirus, or HPV, infects only people, so toads cannot carry it. Toads simply have bumps on their skin that look like warts.

Blackheads are clogged pores open at the surface; what turns them dark, rather than trapped dirt?

Oxidation of the pigment in the plug, where the open pore exposes it, turns a blackhead dark. A whitehead stays pale because skin closes over the pore.

Shaving does not thicken hair, so why does regrowth feel coarser than hair that was never cut?

It has a blunt tip, because the razor slices off the naturally tapered end that uncut hair keeps. The stubble itself grows back no thicker or darker.

Which painful rash forms a stripe on one side of the body when dormant chickenpox virus reawakens?

Shingles follows one nerve to the skin, which is why it forms a belt that never crosses the body's midline. The virus can lie dormant in nerve cells for decades after chickenpox.

'Toasted skin syndrome', a net-like rash from gentle heat, is linked to heavy use of which device?

A laptop resting on the thighs can reach 43 to 47 °C, enough to cause the rash without burning. It was once seen mostly in elderly people who sat close to open fires.

Argyria, a condition that turns the skin blue-grey, comes from long exposure to which metal?

Silver compounds lodged in the skin darken in sunlight, as they do in photographic film, so sun-exposed areas turn bluest. Swallowing colloidal remedies is a classic cause.

Which protein, the body's most abundant, gives skin its strength and breaks down as wrinkles form?

Collagen makes up 25 to 35% of the body's protein. Boiled out of animal skin and bones, it becomes gelatin, the stuff that sets a jelly.

Despite its name, ringworm, the itchy circular rash, is caused by what kind of organism?

A fungus causes ringworm, and about 40 types can do it. The same infection on the feet is known as athlete's foot.

Which component of blood sticks to a fresh cut within minutes, forming the plug that becomes a scab?

Platelets stick to the wound and are glued together by a mesh of the protein fibrin. A moist wound heals faster, because a dry one forms a bigger, tougher scab.

The word 'tattoo' entered English in the 1700s, borrowed from the languages of which region?

Polynesia supplied the word as 'tatau', used in both Tahitian and Samoan. Captain Cook's voyages to the South Seas carried it home.

Mehndi, the temporary skin art of South Asian weddings, is drawn with a paste of which plant?

Henna leaves hold a dye called lawsone that binds to proteins in the outer skin. Whole leaves will not stain: they must be crushed into a paste to release it.

Which chemical, released by mast cells in the skin, raises the itchy weals of an allergic rash?

Histamine makes fluid leak into the upper skin, which raises the weals of hives. That is why the standard treatment is an antihistamine.

Demodex, which live unnoticed at the roots of many adults' eyelashes, are what kind of tiny animal?

Mites of two species live on human faces, eating skin cells and oils and crawling about at night. Older people are far more likely to carry them.

Antiperspirants cut sweating by plugging the ducts in the skin with salts of which metal?

Aluminium salts react with the electrolytes in sweat to form a gel plug in each duct. They also make the pores contract slightly.

Which body system, named from the Latin for 'a covering', is made up of the skin, hair and nails?

Integumentary parts vary from animal to animal: a flat nail is a primate speciality, while other species run to claws, hooves, scales and feathers.

Human hair and fingernails are built from the same tough protein as which of these animal structures?

Rhino horn is a solid mass of that protein with no bone core. A deer's antler, by contrast, is true bone, and an elephant's tusk is an overgrown tooth.

A sunburn that reddens the skin without blistering counts as which degree of burn?

First-degree burns reach only the surface layer of the skin, so they heal within days and leave no scar. Blisters are the mark of a second-degree burn.

Which long-term immune condition makes skin cells renew within days, piling up as scaly patches?

Psoriasis replaces skin cells every 3 to 5 days, far faster than they can be shed. It cannot be caught, though many people wrongly believe it is contagious.

Which inherited condition leaves skin and hair without pigment and usually impairs eyesight as well?

Albinism weakens eyesight because the growing eye needs pigment: without it, nerve fibres to the brain are misrouted. Pale eyes are also painfully sensitive to light.

A spreading red 'bull's-eye' rash around a tick bite is the classic early sign of which infection?

Lyme disease is named after Lyme, Connecticut, where a puzzling cluster of cases was identified in 1975. The rash usually neither itches nor hurts, so it is easy to miss.

Urushiol, an oil that also hardens into East Asian lacquer, gives which plant its notorious itchy rash?

Poison ivy carries an oil named after urushi, the Japanese word for the lacquer tree, whose sap holds the same irritant. Most people who touch it react.

A sunburn on the French Riviera in the 1920s helped which fashion designer make tanned skin chic?

Coco Chanel caught the sun by accident, and her status reversed an old code: brown skin had marked the lower classes, who worked outdoors.

Official Soviet portraits often removed the port-wine birthmark from which leader's head?

Mikhail Gorbachev had a port-wine stain, a patch of malformed capillaries that is present from birth. Lasers can fade such marks without harming the skin above.

Which bacterial toxin, considered the deadliest natural substance known, is injected in tiny doses to smooth wrinkles?

Botulinum toxin, sold as Botox, blocks the nerve signal that makes a muscle contract, so the skin above it smooths out. A doctor studying bad sausages first proposed it as a medicine.

Cerumen, which comes in a sticky wet type and a flaky dry type, is the medical name for what?

Earwax is made by glands in the skin of the ear canal, and a single gene decides which type a person has. The dry type is the commoner one in East Asia.

Loops, whorls and which third shape make up the three basic patterns of a human fingerprint?

Arches are the rarest of the three, making up about 5% of all prints, while loops account for well over half.

Jaundice turns the skin and the whites of the eyes yellow through a build-up of which pigment?

Bilirubin is a yellow waste product of worn-out red blood cells. Jaundiced newborns are laid under blue light, which changes it into a form the body can flush out.

Which white compound, an ingredient of calamine lotion, also absorbs ultraviolet light in mineral sunscreens?

Zinc oxide paste has protected skin for thousands of years, and the same compound goes into nappy-rash creams and antiseptic ointments.

A melanocytic naevus is the medical name for which very common, usually harmless mark on human skin?

A mole is a cluster of pigment-producing cells. Most appear during the first two decades of life, and only about one baby in 100 is born with one.

Onychophagia is the medical name for which nervous habit, seen in almost half of teenagers?

Nail biting is rare before the age of three but is found in about 20 to 30% of the whole population. The skin beside the nail often gets bitten as well.

Which medical term for hair loss comes from the ancient Greek word for a fox?

Alopecia has two rival origin stories: that foxes with mange lose their fur in patches, or simply that foxes shed their coats twice a year.

Which compound makes chilli peppers burn by triggering the skin's sensors for scalding heat?

Capsaicin causes no real burn: it trips the sensor that fires at scalding temperatures. The bird version of that sensor ignores it, and birds spread the seeds.

The thin film of oils and perspiration on healthy human skin gives its surface which kind of pH?

Slightly acidic skin, at a pH of roughly 4.5 to 6.5, is a hostile surface for many germs. The protective film is known as the acid mantle.

A surge of which hormones at puberty enlarges the skin's oil glands and sets off teenage acne?

Androgens such as testosterone rise in both sexes at puberty, which is why girls get spots too. The enlarged glands pump out extra oil that clogs the pores.

What is the pale crescent at the base of a fingernail called?

The lunula, Latin for 'little moon', is easiest to see on the thumb and is often hidden on the little finger. It is the visible part of the root that makes the nail.

Pacinian corpuscles, onion-layered sensors set deep in human skin, respond chiefly to which kind of stimulus?

Vibration is their speciality, above all a fast buzz of about 250 cycles a second. That sensitivity helps the fingertips tell a rough surface from a smooth one.

Which waxy white coating covers the skin of newborn babies?

Vernix caseosa, the full name, means 'cheesy varnish'. The coating is thought to waterproof and protect a baby's skin during the months spent in the womb.

Dark skin is thought to have evolved partly to shield which B-group nutrient from destruction by sunlight?

Folate is broken down by ultraviolet light, and a shortage in pregnancy is linked to serious birth defects, so shielding it mattered for survival.

Which kind of raised scar, named from the Greek for a crab's claw, spreads over the healthy skin around a wound?

Keloid scars can grow many times larger than the injury that caused them, and something as small as a piercing or a pimple can set one off.

Which condition lets a person raise welts on the skin simply by scratching it lightly?

Dermatographia, literally 'skin writing', affects about 2 to 5% of people. The raised lines appear within minutes of a scratch and normally fade again inside half an hour.

Spoon-shaped fingernails that curve upwards at the edges are a classic sign of a shortage of which mineral?

Iron deficiency is the first thing doctors rule out when nails turn spoon-shaped, although the same shape can come simply from years of wear at work.

A dermatome, one of the bands doctors map across the body, is the area of skin supplied by a single what?

Spinal nerve territories follow the same plan in everyone, yet their exact borders are as individual as fingerprints. Pain confined to one band points to trouble at its root.

Anthropodermic bibliopegy, a practice whose surviving examples date mostly from the 1800s, uses human skin for what?

Binding books in human skin was rare but real: protein tests have confirmed some volumes and exposed others as sheepskin. Harvard removed one such binding in 2024.

Knismesis, the light feathery kind, and gargalesis, the heavier kind, are the two recognised types of what?

Tickling of the heavier kind is the only sort that brings laughter. Most people cannot do it to themselves, because the brain predicts the sensation.

Which metal, released by cheap jewellery and the studs on jeans, is a common cause of allergic skin rashes?

Nickel rashes have been traced to jewellery, stocking suspenders and jeans buttons. A simple wipe test turns pink when an object is leaching the metal.

Which protein lets human skin spring back into place after it has been poked or pinched?

Elastin is among the longest-lived proteins in the body, with a half-life of more than 78 years. Long sun exposure damages it, a process called photoaging.