In São Paulo, October is when the bill arrives. Seasons here run opposite to the northern hemisphere, so October is spring tipping into summer. You leave the house at seven with clean hair tied back, you spend the day at 31 °C (88 °F) and high humidity, and by five in the afternoon the roots are damp, the nape itches, and there is a smell you notice the moment you take the elastic out. The following week, you buy another shampoo.
That is the argument I want to make here: the October problem is almost never the shampoo. It is what happens underneath tied-back hair, with two very different fluids that we treat as if they were one.
Sweat and sebum are not the same thing, and that matters in practice
Your scalp has two separate production lines running at the same time. Eccrine sweat glands produce sweat, whose job is to cool the skin by evaporation. Sebaceous glands, attached to every follicle, produce sebum, whose job is to maintain the skin’s lipid barrier.
| Sweat | Sebum | |
|---|---|---|
| Composition | roughly 99% water, with salt, urea, lactate and amino acids | a mix of fats: triglycerides, free fatty acids, squalene, waxes |
| What triggers it | temperature, physical effort, acute stress | hormones and genetics, produced continuously |
| Does water alone remove it? | yes, it is water-soluble | no, it needs a surfactant |
| What it leaves behind as it dries | salt, which is hygroscopic and irritating | a film that oxidises and glues dead cells down |
Read the third row again, because that is where the useful part of this article comes from. Sweat rinses out. Sebum does not come off without shampoo.
Heat makes both worse, but by different routes. You sweat more because the body needs to cool itself. And sebum becomes more fluid as skin temperature rises, so the same amount spreads faster and the roots feel heavy earlier in the day.
What actually creates the itch and the odour
Neither sweat nor sebum, on its own, bothers you that much. What bothers you is the combination with occlusion.
Hair that is tied up or damp for hours keeps the surface of the scalp wet and unventilated. That raises the hydration of the outermost layer of skin and creates exactly the environment in which two normal residents of the scalp multiply beyond their usual numbers: the bacteria of the skin flora, and Malassezia, a lipophilic yeast, meaning one that feeds on fat.
Malassezia does not digest sebum whole. It breaks down triglycerides and releases free fatty acids, among them oleic acid, which is irritating to the skin of anyone predisposed. That is where the late-afternoon itch comes from, and that is where the flaking that shows up after a few hot days comes from.
The odour has a different origin: it is the product of bacterial action on the components of sweat, plus the oxidation of sebum lipids in contact with air and light. A scalp that has been damp and airless for eight hours smells for chemical reasons, not for lack of hygiene.
Add to that the point that interests me most in a reader who already has hair loss: a tight elastic every single day, pulling on the same spot at the front hairline and at the nape, over skin that is damp and macerated. Constant tension on inflamed skin is the recipe for folliculitis and, repeated over years, for traction alopecia.
Changing shampoo every week is what costs the most time
The common belief is that an oily summer scalp calls for a stronger shampoo, and that if it has not sorted itself out in ten days the product was no good. Both halves are wrong.
A deep-cleansing shampoo used every day removes sebum and removes barrier lipids along with it. The skin responds by drying out, flaking and inflaming, and an inflamed scalp produces more sebum. So you switch products, thinking it got worse by coincidence.
And an antifungal active such as zinc pyrithione or ketoconazole needs contact time, and a few weeks, to show what it can do. Four weeks is the honest minimum for judging it. Anyone who switches every ten days never gives any product the chance to work, and ends the summer with five half-empty bottles and the same symptom.
The adjustment that solves most cases
Start with what costs nothing:
Rinse without shampoo on heavy-sweat days. Lukewarm water, three or four minutes, working the scalp with your fingertips. That removes the salt left by sweat, which is the most irritating part. Conditioner from mid-length to ends afterwards, and the day is handled without washing twice.
Dry the roots, not just the lengths. Hairdryer on cool or low, held away from the head, fingers opening up the roots. A dry scalp does not stew.
Never tie hair up while it is damp. This is the one gesture that creates the entire environment described above, and it is the easiest to give up.
Move the hairstyle around. Alternate where the elastic sits, use a wider fastening, keep it looser. Tension does not have to hurt to do damage.
Two short shampoo applications, not one long scrub. The first emulsifies the sebum, the second cleans.
Dry shampoo replaces nothing in hot weather. It absorbs oil. It does not remove salt, it does not remove sweat, and on a humid day it only adds another layer on top of what is already occluded.
If flaking and itching persist, use an antifungal shampoo two or three times a week, left on for three to five minutes before rinsing, alternating with a gentle shampoo on the other days. Change one thing at a time and give it four weeks.
If hair loss increased at the same time, heat is not the obvious culprit
There is literature describing seasonal variation in the proportion of hairs in the resting phase, with increased shedding some months after the peak. But the gap between cause and shedding is two to four months, which makes last week’s heat a poor suspect for this week’s hair loss.
What heat does do, and quickly, is aggravate inflammation on the scalp and increase traction because the hair is tied back. Those two things do weigh on hair loss, and both are treatable.
When it is a case for a dermatologist
See a doctor, without going to a salon first, if there is a painful pustule, a sore that will not close, discharge, a strong odour localised to one single spot, thick plaques that do not change after six weeks of a correct routine, or a visible bald patch with smooth, shiny skin in the middle. Nothing in this article replaces a medical appointment, and hair therapy is an aesthetic service that works on the scalp as support, not in place of medical treatment.
If you would like to talk about your case
The hair assessment looks at the scalp with a trichoscope, a device that magnifies the image on screen while you follow the assessment together with the professional. It is possible to tell dry flaking from oily flaking, to see whether there is inflammation around the follicles, whether there is thinning along the line where the elastic sits, and whether the shedding you have noticed is related to what is happening on the skin.
It costs R$200 (Brazilian reais), credited in full towards the protocol, if there is a protocol. We measure again during or at the last session and compare with the image from the first day. At no extra cost.
We are in Tatuapé, in the East Zone of São Paulo, near the Carrão and Tatuapé metro stations, Tuesday to Saturday, 9am to 6pm, by appointment. Tuesday is the day reserved for scalp therapy. English is spoken here: mention it when you book.