Sebum and scalp types

Key takeaways

• Scalp oil glands sit inside the hair follicles Zouboulis 2016. The scalp has roughly 100,000 follicles over about 650 cm2, which together release sebum in amounts estimated at around 1-3 mg per head per hour Gao 2019.
• Oil output is set by androgens, age and sex. People who cannot respond to androgens make essentially no sebum. Men make more than women, and output falls with age, especially in women after menopause Imperato-McGinley 1993, Pochi 1979, Pouradier 2017.
• Washing more often does not make your scalp produce more oil, and washing less does not "train" it to produce less. The one direct test found no change in how fast hair re-greased when washing went from once to three times a week Shaw 1979. No controlled study supports "scalp training".
• After a wash, the scalp re-greases on a curve and levels off after about 2-3 days. The face refills in hours because its follicles hold a larger oil reservoir Pouradier 2017, Saint-Leger 1982, Rode 2000.
• Oil creeps down the hair. In straight-haired Chinese men, sebum reached about 6 cm down the hair 24 hours after washing in men who called their scalp "not greasy" and about 8 cm in men who called it "greasy" Gao 2019 (industry). Greasy-looking hair is partly about how far the oil spreads, not just how much is made.
• "Sebum can't travel down curly hair" is plausible but unmeasured. We found no peer-reviewed study comparing sebum spread on straight and curly hair. Meanwhile, African American subjects had the highest scalp oil levels of seven groups measured Pouradier 2017.
• A dry, flaky or itchy scalp is usually not an oil shortage. Dry-scalp flaking, dandruff and seborrheic dermatitis sit on one continuum driven by sebum, Malassezia yeast and individual sensitivity, with a weakened scalp barrier Ro and Dawson 2005, Harding 2002. In a large survey, people who washed more often reported less scalp dryness Punyani 2021.
• The famous "daily washing is best" study is weak and industry-funded: paid for entirely by Procter & Gamble, with age groups that differed hugely, no control arm, an anti-dandruff shampoo, and only straight or low-texture hair in China Punyani 2021.
• Coily scalps still need regular cleansing. In 60 South African women, dandruff and itch returned to their peak about a week after washing Fajuyigbe 2024 (industry).
• There is no agreed definition of an oily, balanced or dry scalp. Studies use their own device cut-offs, and the device's units are not true micrograms per cm2 Crowther 2016, Guo 2026. At home, how many days your roots take to look greasy is a more useful signal.

The oil gland and sebum

Sebaceous glands are usually attached to a hair follicle and empty into it. Sebum is a holocrine secretion: gland cells fill with lipid and then break apart completely into the follicle duct. Human sebum is roughly 57.5% triglycerides and free fatty acids, 26% wax esters, 12% squalene and 4.5% cholesterol and its esters. Squalene and wax esters are found nowhere else in the body Picardo 2009.

Once on the scalp, sebum changes: microbes split triglycerides into free fatty acids, whose share of scalp and hair lipids roughly doubled from about 19% on day 1 after washing to about 39% on day 10 in a 1977 study of 67 people Gloor and Kohler 1977. Free fatty acids and oxidized lipids are the irritating breakdown products linked to dandruff. See washing frequency.

Androgens are the master switch

• No androgen signal, no sebum. People with complete androgen insensitivity had no measurable forehead sebum, the same as young children Imperato-McGinley 1993.
• Men with inherited type 2 5-alpha-reductase deficiency (very low DHT) had normal sebum, and men taking finasteride for prostate enlargement did not make less sebum Imperato-McGinley 1993. Sebaceous glands mainly use the type 1 enzyme, which finasteride inhibits poorly Thiboutot 1995. So hair-loss drugs should not be expected to make a scalp much less oily.
• Scalp sebaceous glands are a main site of androgen receptors and of enzymes that break down DHT, which decline with age. This was a tissue and cell-line study without clinical outcomes Ikuno 2026.
• Oral isotretinoin reduced scalp sebaceous activity in a 1983 method paper Simpson 1983. Isotretinoin is prescription-only and can cause birth defects.

Over a lifetime, sebum is low in childhood, rises under androgens, peaks in the late teens and then stays fairly stable for decades. In men it stays roughly level until about 80; in women it falls after menopause Pochi 1979. On the forehead, sebaceous wax-ester output was highest at ages 15-35 and fell by roughly 23% per decade in men and 32% per decade in women in 276 people aged 15-97 Jacobsen 1985. The two older studies disagree for men, probably because they measured different things. On the scalp, men produced more than women and older people less than younger in all seven ethnic groups studied Pouradier 2017.

Things that did not matter in small studies: scalp and forehead sebum did not vary across the menstrual cycle in 13 women followed for three cycles Birch 2003; and women with female pattern hair loss did not make more sebum than women without it, nor did density correlate with sebum (100 women, forehead measurement) Birch 2006. An oily scalp is not, by itself, a sign of androgen-driven hair loss.

Measuring scalp oil

• The Sebumeter presses a matte tape on the skin and reads how translucent it becomes. A calibration study found the readings do not directly equal micrograms of oil and differ by type of oil Crowther 2016. Recent washing strongly affects scalp readings Simpson 1983.
• After cleansing, surface sebum rises quickly and then plateaus. On the face this casual level returns within about 2 hours Rode 2000; on the scalp it takes about 2-3 days Pouradier 2017, Gao 2019.
• Cut-offs are study-specific. One study classed Chinese men as greasy above a reading of 150 one day after washing Gao 2019. A 2026 study of 63 Chinese women aged 18-25 used over 150 for oily, 70-120 for medium and under 40 for dry, leaving gaps Guo 2026. These are research conveniences from East Asian samples, not validated categories.
• In 1,325 people from seven groups, scalp casual levels were highest in African American subjects, then Caucasian American, Japanese, Chinese, Thai and Caucasian European subjects, and very low in Indian subjects. The authors attribute the gaps to both gland output and hygiene routines Pouradier 2017 (industry; abstract only).

From scalp onto the hair

Sebum spreads onto the hair over the days after washing, leaving a gradient from root to tip Courtois 1986. In 123 Chinese men aged 18-35, sebum fell off in a straight line with distance from the root. It reached about 8.4 cm down the hair 24 hours after washing in men who rated their scalp greasy and about 5.8 cm in those who did not; the greasy group had about twice as much sebum along the hair in total Gao 2019.

• Oily hair looks flat because fibers stick together. As hair re-greases, the spacing between fibers falls, attributed to capillary forces between oily fibers (one subject, industry study) Maes 1979.
• Some sebum on the lengths is good. Hair stripped completely of sebum is harsh, dull and hard to style, which is why shampoos for dry or treated hair add conditioners Draelos 2010.

Does curl stop sebum reaching the ends?

This is one of the most repeated claims in hair care: straight hair gets greasy because oil slides down it, while curly and coily hair is dry because oil cannot travel along the bends.

• No peer-reviewed study measured sebum spread along straight versus curly hair. The only measured spreading data come from straight-haired Chinese men Gao 2019.
• The only direct statement we found is from a 2024 company-sponsored video with no published data, so the claim is at the level of marketing.
• The scalp side does not look dry. African American subjects had the highest scalp oil levels Pouradier 2017, and women with afro-textured hair get dandruff and itch that rebound within a week of washing Fajuyigbe 2024. Oil staying near the scalp would fit both an oily-feeling scalp and dry lengths, but this has not been shown directly.

So it is fair to say oil probably spreads less easily along tight curls, but not to present it as established, and not to tell people with curly hair that their scalp is dry.

Does washing frequency change how much oil you make?

The claim is that washing often strips the scalp so it overproduces oil, and washing less trains it to produce less.

• Direct test, 1979: changing shampoo frequency from once to three times a week had no significant effect on how fast hair re-fatted. One wash removed essentially all surface lipid Shaw 1979 (industry; sample size not given).
• Refilling is not over-production. Surface oil returns because the follicle reservoir empties onto the surface, not because the gland speeds up Rode 2000.
• We found no controlled study of "scalp training". People who wash less may simply get used to the look and feel, and sebum varies with season, age and hormones.

The evidence is one old, small industry study plus indirect data, all pointing the same way.

What washing frequency does change

Washing removes accumulated sebum, its breakdown products, flakes and microbes. The P&G study often quoted as "daily washing is best" Punyani 2021 had two parts:

• A two-week survey of 1,500 Chinese adults aged 18-75 grouped by how often they washed. More frequent washers had less expert-graded flaking and reported less dandruff, itch and dryness. But 68% of daily washers were 18-30, while 93% of weekly-or-less washers were 46-75.
• A trial of 60 habitual infrequent washers (7 days without washing, then 4 weeks of daily washing with a zinc pyrithione anti-dandruff shampoo) lowered scalp and hair sebum, oxidized lipids, flaking and odor. It was not randomized and had no control arm.

The authors note all participants had straight or low-texture hair. Other data point the same way for scalp health: lower wash frequency was more common among women with "bad hair days" (p = 0.06, 13 women) Birch 2003; 12 weeks of daily washing improved scaling and itch in 18 people with seborrheic dermatitis or psoriasis (uncontrolled) Kobayashi 2016; and in afro-textured hair, dandruff peaked about a week after a wash Fajuyigbe 2024. Scalp sebum also carries an inflammatory signal (IL-1 alpha) that fell after shampooing in 36 women with afro-textured hair Beach 2012.

The counterweight: repeated shampooing measurably depleted natural moisturizing factor and lipids in the scalp's outer skin layer in a small pilot study Kourbaj 2022, so very harsh or very frequent cleansing can still dry the skin itself.

Dry, flaky and itchy scalps

• Dandruff is a barrier problem as well as a yeast problem. Dandruff scalps had much lower levels of barrier lipids (ceramides, fatty acids, cholesterol) in UK and Thai samples, and reacted more to histamine Harding 2002. A review links dandruff to raised water loss through the scalp and disorganized barrier lipids Turner 2012. Both are industry-authored.
• Dry-scalp flaking, dandruff and seborrheic dermatitis share one cause (sebum, Malassezia metabolism and individual susceptibility) and differ mainly in severity; scalp itch and flaking affect over half of adults Ro and Dawson 2005.
• Sensitive scalps tend to be oilier, not drier. In 62 Chinese women, those with self-reported sensitive scalps had more sebum at the back of the head, more free fatty acids and squalene, higher pH and signs of barrier disruption Ma 2019.
• A truly dry scalp exists in conditions such as atopic dermatitis, where scalp lesions show low barrier function and ceramide levels (industry-funded, uncontrolled) Takagi 2018. See dandruff and seborrheic dermatitis.

What it means for your hair

• Curl pattern: straight and wavy hair shows grease soonest. Curly and coily hair often feels dry along the lengths while the scalp still gets oily, flaky or itchy. Cleanse the scalp regularly (about weekly as a floor for afro-textured hair with any flaking or itch Fajuyigbe 2024) and condition the lengths. See curl pattern.
• Thickness and density: fine hair and high density plausibly make grease more visible. That is inference; the only data are a speculation that thicker hairs re-grease more slowly Gao 2019. See strand thickness and hair density.
• Age and sex: expect oiliness to peak from the late teens to the 30s, to be higher in men and to fall with age, especially after menopause. An "oily scalp" from a 60-year-old woman is more likely wash interval or product buildup than gland output.
• Ancestry: average scalp oil differs a lot between populations Pouradier 2017, but individual variation is large, so go by your own scalp. See hair by ancestry.
• Porosity: damaged lengths lose their own surface lipid layer and rely more on conditioners. See hair porosity.

Myths and facts

• "Washing often makes your scalp produce more oil." Not supported: no effect of once versus three times weekly shampooing on re-greasing Shaw 1979.
• "Wash less and your scalp will adapt and get less oily." Unsupported; output is driven by androgens, age and sex.
• "Curly and coily hair means a dry scalp." False as a rule: African American subjects had the highest scalp oil levels of seven groups Pouradier 2017.
• "Flakes mean a dry scalp, so wash less and oil it." Usually wrong: flaking is mostly on the dandruff spectrum and worsens as washes are spaced out Ro and Dawson 2005, Punyani 2021.
• "Studies show daily washing is best." Overstated: one P&G-funded study in straight-haired Chinese adults Punyani 2021.
• "An oily scalp causes hair loss." Not shown Birch 2006; see hair loss scales.
• "My cycle makes my hair greasy on bad hair days." Not supported: no menstrual variation in scalp sebum in 13 women Birch 2003.

Practical points

1. Classify scalp oiliness by time, not by feel on wash day: how many days after washing do the roots look or feel greasy? Roughly, within 1 day is oily, 2-3 days balanced and rarely even after 4 or more days dry. This mirrors how research measures re-greasing, but no study has validated it as a questionnaire item.
2. Treat flakes and itch as their own category, whatever your oiliness.
3. Wash as often as your scalp needs. Frequent washing will not make the scalp oilier. Straight, fine or oily hair can be washed daily with a mild shampoo; curly and coily hair can be washed less often, but the scalp still benefits from regular cleansing.
4. Separate scalp care from lengths care, especially for curly and coily hair.
5. Reassess after puberty, hormonal changes, pregnancy and menopause.

Bottom line

Scalp oil is mostly hormonal and age-driven, and how often you wash changes how much oil is sitting on the scalp, not how much you make. Flaky or itchy scalps are better treated as a barrier and yeast issue than as dry skin needing less washing. Several of the key studies are industry-funded, small or old, so treat the details as provisional.