Heat rash and chafing complaints both spike during a heatwave, and it's not a coincidence — both are driven by the same underlying conditions: excess moisture trapped against the skin, combined with friction. Most advice focuses on fit ("size down," "avoid loose fabric") or on treating the symptom (anti-chafe balm, talc). Less discussed is that the fabric itself — how it handles moisture and how smooth its fibres actually are — is doing a significant amount of the work, for better or worse.

What's Actually Causing It

Heat rash (miliaria) develops when sweat becomes trapped under the skin, typically because sweat ducts become blocked or because moisture can't evaporate fast enough in hot, humid conditions. Chafing is a mechanical problem — repeated friction between skin and fabric, or skin and skin, that breaks down the skin's surface, and it gets dramatically worse when skin is damp, since wet skin has a higher friction coefficient than dry skin. In both cases, heat is the multiplier: higher temperatures mean more sweat, and more sweat sitting on the skin for longer means more time for both problems to develop.

Why Fabric Surface Structure Matters

Not all fabric touches your skin the same way. Synthetic fibres like polyester have a rougher, more irregular microscopic surface than natural fibres like lyocell — the same rough structure that traps odour-causing bacteria also increases friction against skin during repetitive movement. Combine that with polyester's tendency to hold moisture on the surface rather than absorb it, and you get a fabric that stays both wetter and rougher against the skin for the length of a session — exactly the combination that drives both chafing and heat rash.

Lyocell's fibre structure is smoother and rounder at a microscopic level, which reduces surface friction, and because it absorbs moisture into the fibre itself rather than leaving it pooled on the surface, skin stays comparatively drier during a session — reducing the moisture side of the equation as well as the friction side.

Where Chafing Actually Happens — and Why Seams Matter

Most chafing occurs at repetitive contact points: inner thighs, underarms, and anywhere a seam or waistband sits directly against skin during movement. This is where garment construction matters as much as fibre choice — flat-seam construction and fabric that moves with the body rather than shifting against it reduces the repeated rubbing that causes chafing in the first place. For high-movement sessions, well-fitted performance shorts with minimal seam bulk at friction points make a measurable difference, particularly over a long session in heat where the number of repetitions goes up.

What Actually Helps in Extreme Heat

• Prioritise moisture absorption over pure "wicking." Fabric that pulls moisture away from the surface entirely — like lyocell — leaves less standing moisture against skin than fabric that just spreads it across the surface.
• Pick smoother fibres for high-friction zones. Inner thighs and underarms are where fibre roughness matters most; this is where the difference between polyester and lyocell (or polyester and polyamide) is most noticeable.
• Fit matters as much as fabric. Loose fabric that shifts against skin causes more friction than well-fitted fabric that moves with the body — "loose and airy" isn't automatically better for chafing, even though it feels cooler.
• Wash regularly during a heatwave. Residue from sweat, sunscreen and detergent left in fabric can increase skin irritation risk; this is one period where the usual "wear it two or three times" advice for anti-odour fabric should give way to washing after every session.

None of this replaces general skin-health guidance for heat rash or chafing — if either becomes persistent, painful, or shows signs of infection, that's a question for a doctor or dermatologist, not a fabric choice. But for the everyday version most people deal with during a heatwave, fabric is one of the more overlooked variables, and one of the easier ones to actually fix.

Frequently Asked Questions

What causes heat rash during exercise?

Heat rash develops when sweat becomes trapped under the skin and can't evaporate effectively, often because sweat ducts become blocked in hot, humid conditions. Fabric that holds moisture against the skin rather than absorbing or evaporating it — like polyester — can make this worse, while fabric that absorbs moisture into the fibre, like lyocell, tends to leave less standing sweat on the skin.

Does chafing get worse in humid or hot weather?

Yes. Wet skin has a higher friction coefficient than dry skin, so the more sweat that accumulates against the skin, the worse friction-related chafing tends to become. This is why chafing complaints increase noticeably during heatwaves — higher heat means more sweat, and more sweat means more friction at contact points like the inner thighs and underarms.