Oily Scalp in Late Summer: Sebum Tracks Skin Temperature, Not How Dirty Your Hair Is

Late August, and your hair is greasy by four in the afternoon. It wasn't doing this in March. Same shampoo, same head, same wash schedule.

The internet's answer is that you're washing too much and your scalp is "overcompensating." We went to check that, and also to check the number everyone quotes about heat and oil, and came back with less than we expected in one hand and something fairly solid in the other.

The number everybody quotes, which we couldn't read

If you search for scalp oil and temperature you will land, within about two clicks, on a claim that sebum excretion rises roughly 10% for every 1°C of skin temperature. It's on brand blogs, in trichology course notes, in trade articles.

It traces back to work out of Leeds in the early 1970s, Cunliffe and Burton and Shuster, published in the British Journal of Dermatology. We found the records. Both papers are real.

Neither is readable. There is no open copy anywhere, and we couldn't get the text, which means we never saw the actual figure, the actual conditions, or what "10%" was measured against.

So we're not going to repeat it. It might be perfectly accurate.

We just can't stand behind a number we have only ever seen quoted by people who, as far as we can tell, also only saw it quoted. That chain has to stop somewhere, and this is as good a place as any for it to stop.

What a controlled trial actually measured

Here's what we could read. In 2025 a randomised crossover trial put 30 healthy adults through two hours at 22°C and two hours at 32°C, in random order, measuring sebum at baseline, 60 minutes and 120 minutes.

Sebum was higher in the warm room at both points: about 19.0 μg/cm² more at 60 minutes, 19.1 μg/cm² at 120 minutes. Participants also rated their own skin oilier and stickier. The confidence intervals are wide, the sample is 30 people, and the study was funded and co-authored by Estée Lauder's China R&D group, which you should factor in.

Older work points the same way. A 1990 Sebutape study reported that ambient temperature influences sebum excretion rate, and made a useful distinction: more sebum arrived at the surface without more follicles switching on. Existing glands worked harder.

Two hours, in a room.

That is the whole timescale worth holding on to here. Not a season, not a lifestyle, not a wash habit you picked up in your twenties. An afternoon in a warm room was enough to move the number.

Nobody has measured this on a scalp

This is the gap that annoyed us most.

The scalp is where people actually complain about oil. It is also, as far as we can find, the one site nobody has measured across seasons.

Scalp is the body site where people actually complain about oil, and it is the site nobody seems to have measured seasonally.

There are good seasonal sebum datasets from Korea. One tracked five facial sites across a full year and found summer the highest-secreting season. Another measured 100 Korean men aged 20 to 59 in both summer and winter and found sebum significantly higher in summer. A third took 20 women outdoors for 90 minutes in July and found sebum, hydration, water loss and self-rated greasiness all up against an indoor control.

Every one of those measured a forehead, a nose, a cheek. Not one measured a scalp.

We also couldn't find a primary source for scalp sebaceous gland density, the "the scalp has the highest gland concentration on the body" line that gets repeated as freely as the 10% figure. The sources asserting it don't cite anything we could follow.

So when we say your scalp is oilier in August because it's warm, we are extrapolating from foreheads. It's a reasonable extrapolation, sebaceous glands being sebaceous glands, and it is still an extrapolation.

One more complication, because it cuts against the tidy story: an 11-person study on sleeping humidity found sebum went up after a night at under 30% relative humidity, apparently as a response to dryness. Heat raises oil, and so, sometimes, does dry air. "Humid summer equals greasy" is too simple.

One more thing we can't cleanly measure, while we're listing them. There is a second reason people notice their scalp in August, and it isn't the grease itself. It's the flaking that sometimes arrives alongside it.

Malassezia, the yeast implicated in dandruff and seborrhoeic dermatitis, lives on 75 to 98% of healthy adults. It is not an infection you caught. It metabolises sebum lipids using its own lipases, splitting triglycerides into free fatty acids, and those free fatty acids are what irritate. More sebum means more substrate.

But a 2025 review is careful about this, and so are we. Sebum excess is listed as one contributing factor among several, alongside humidity, cold, barrier condition and individual susceptibility. The chain from warm weather to more oil to more yeast to visible flaking is plausible at every link and demonstrated at none of them as a single sequence.

So if your scalp is flaking rather than just greasy, oil control is not the lever. That is a different product category and a different piece of evidence.

"You're washing too much" doesn't survive its own trial

The rebound theory says frequent shampooing strips the scalp, the scalp panics, and you end up oilier than when you started. It is repeated by people who sell you fewer washes and by people who sell you more.

The best test of it we found ran in two parts. A survey of 1,500 Asian men and women found satisfaction with hair and scalp peaked at five to six washes a week. Then a five-week double-blind controlled study of 60 people in Xi'an compared daily washing against refraining for seven days.

Daily washing produced significantly lower scalp surface sebum and lower oxidised-lipid markers than the seven-day refrain. The authors wrote that concerns about overcleaning were "unfounded both objectively and subjectively." Procter & Gamble scientists ran it, with two academic dermatologists as co-authors, so read it with that in mind.

There's a real methodological caveat too, and it matters: comparing surface lipid after one day against surface lipid after seven days partly measures accumulation time, not secretion rate. The study doesn't cleanly prove your glands don't adapt. It does show that the visible outcome, oil sitting on your scalp, goes the opposite direction from the folklore.

What our screening found

The practical takeaway

Wash on the frequency that makes your scalp comfortable, and stop treating a number of washes per week as a moral position. The one controlled trial on this points at five to six, and at daily being fine.

If August is the month your scalp changes, the lever with actual evidence behind it is temperature, not cleansing frequency. Rinse cooler. Dry your hair before it sits under a hot scalp for three hours. These are small effects and we're not going to pretend otherwise.

Where a scaler earns its place is texture and buildup, not oil control, because nothing in the literature we read shows a product changing how much sebum a gland makes. Our screening criteria treat that distinction as the whole point, and it's also why we won't tell you a shampoo "balances" anything.