Merino, cotton, polyester: which t-shirt to really wear for summer hiking

The technical t-shirt range today offers three main material families: cotton (often organic, sometimes blended), polyester (and its derivatives polyamide, recycled), and merino. The marketing claims are all similar: breathable, lightweight, odor-resistant, quick-drying, comfortable. In practice, each material has real strengths and measurable weaknesses. This article examines the three through seven objective criteria to help choose based on use rather than storytelling.

The three materials on the table: technical definitions

Cotton: natural fiber, low technicality

Cotton is a plant fiber from the cotton plant's fruit. Its internal structure is hollow and highly hydrophilic: the fiber absorbs up to 27 times its weight in water. Cotton fabric has a soft touch, a typical weight of 140 to 180 g/m² in t-shirts, and moderate wear resistance. It is mainly used in casual clothing.

Polyester: synthetic fiber, chemical technicality

Polyester is a synthetic fiber derived from petroleum (or recycled PET bottles). Its structure is smooth, tubular, hydrophobic. Modern technical versions are often structured to create a capillary effect that transports sweat to the outside of the fabric. Typical weight for a technical t-shirt, 100 to 150 g/m². High abrasion durability.

Merino: ultra-fine animal fiber, biological technicality

Merino is a wool from a sheep breed selected for the fineness of its fleece. Fiber diameter between 16.5 and 19.5 microns for technical textile use. The fiber is made of keratin, structurally wavy (crimp), and hygroscopic (absorbs water vapor inside the fiber). Typical weight for a summer t-shirt, 140 to 170 g/m². Average durability, specific care required.

Criterion 1: sweat management during effort

Cotton absorbs moisture on the surface, retains it, saturates quickly, and becomes heavy. Immediate wet fabric sensation, very slow evaporation, sudden cooling when stopping. For prolonged effort in the mountains, cotton does not offer a satisfactory technical solution and becomes problematic on exposed ridges.

Polyester wicks sweat by capillarity to the outside of the fabric. Effective management of surface moisture, but the fabric remains wet on the outer surface as long as the environment does not allow evaporation. "Wet but not heavy" sensation.

Merino wool absorbs water vapor inside the fiber (up to 35% of its weight), retains it, then releases it through gradual evaporation. Prolonged dry skin sensation, fabric that does not suddenly become heavy. Superior performance to cotton, different from polyester: slower in surface moisture wicking, but without the wet surface feeling.

Criterion 1 verdict: polyester and merino tied for short effort. Merino leads for long effort thanks to the absence of a wet feeling. Cotton far behind.

Criterion 2: drying speed after washing or rain

Cotton dries very slowly: 8 to 12 hours in open air, longer in humid conditions. Impractical for washing in the evening in a hut if the next day's stage starts at dawn.

Polyester dries very quickly: 1 to 3 hours in open air. The best-performing material on this criterion.

Merino dries moderately: 4 to 6 hours in open air under normal conditions, longer in high humidity. Enough for washing in the evening and use in the morning.

Criterion 2 verdict: polyester clearly ahead. Merino acceptable. Cotton disqualified.

Criterion 3: body odor management

Sweat itself has no odor. Odor appears when bacteria metabolize the organic compounds in sweat. Not all textiles offer the same hospitality to them.

Cotton is relatively neutral in odor for short use, but its persistent moisture creates a favorable environment for bacterial growth over several days.

Polyester is the material most prone to odors. Its smooth, oleophilic structure particularly welcomes bacteria responsible for isovaleric acid. Strong odor after 24 hours of effort.

Merino is the most odor-resistant material. The keratin surface is biochemically hostile to bacteria, and internal moisture absorption limits proliferation. Can be worn for several days without strong odor, a decisive difference in CAS hut dormitories.

Criterion 3 verdict: merino clearly ahead. Cotton acceptable for short use. Polyester behind.

Criterion 4: comfort directly on the skin

Cotton is soft to the touch, gentle on the skin, suitable for sensitive skin. No tingling, no irritation. The historical reference for comfortable t-shirts.

Modern polyester is okay on the skin but retains a slightly plastic or slippery feel. Some finishes cause micro-irritations under prolonged effort, especially in friction areas (backpack, sports bra).

Extra-fine merino (micron count under 19 microns) is extremely soft, comparable to cotton, with a slightly drier and more "lively" feel. Standard merino (22 microns and above) can itch on sensitive skin. The 140 g/m² at 17.5 microns used on the Bjork MC 140 and Finn MC 140 falls into the non-itching category.

Criterion 4 verdict: cotton and extra-fine merino are tied. Polyester slightly behind.

Criterion 5: durability and longevity

Cotton holds up well to wear but can deform and fade quickly depending on the weave quality. Its real lifespan under intensive use rarely exceeds 2 to 3 seasons.

Polyester is the most abrasion-resistant. It keeps its shape, colors, and structure for many years. Negative point: it releases plastic microfibers with every wash, a documented environmental problem.

Merino is more fragile than polyester against snags (brambles, poorly adjusted backpack, rings) but retains its shape, thermal regulation, and anti-odor properties for many seasons if properly cared for. Quality 140 g/m² t-shirts have a real lifespan of 5 to 8 years with regular use.

Criterion 5 verdict: polyester leads in raw mechanical strength. Merino leads in maintaining technical properties. Cotton falls behind.

Criterion 6: environmental impact

Conventional cotton is one of the most water- and pesticide-intensive crops in the world. Organic cotton significantly reduces these impacts but remains very water-intensive. Biodegradable at end of life.

Virgin polyester comes from petroleum, emits CO2 during production, releases microplastics with every wash, and is not biodegradable. Recycled polyester reduces the production footprint but retains the other two issues.

Merino is a renewable, biodegradable fiber. Sheep farming has a real carbon impact (enteric methane, land use) but is limited if practices are extensive. The mulesing-free label (which Fjork guarantees compliance with via AWTA certification, the Australian Wool Testing Authority) excludes problematic animal welfare farming practices. Fjork Merino is an independent brand based in Sion.

Criterion 6 verdict: mulesing-free merino and organic cotton are roughly equivalent according to the selected indicators. Polyester falls behind in the long term (microplastics, end of life).

Criterion 7: price relative to actual use

Cotton is the cheapest to buy: 20 to 50 francs for a decent t-shirt. Based on a lifespan of 2 to 3 seasons in technical use, the annual cost is 7 to 20 francs.

Technical polyester costs between 40 and 100 francs depending on the brand. Lifespan of 3 to 5 seasons, annual cost of 8 to 25 francs.

Technical merino is the most expensive to buy: 80 to 150 francs for a quality 140 g/m² t-shirt. Lifespan of 5 to 8 seasons, annual cost of 10 to 25 francs.

Criterion 7 verdict: for real intensive and long-term use, the three materials fall within a very close annual cost range. Cotton loses its price advantage over time, merino loses its price disadvantage over time.

Summary table and verdict by usage profile

Visual summary of the 7 criteria:

• Long effort sweating: merino > polyester > cotton
• Drying: polyester > merino > cotton
• Anti-odor: merino > cotton > polyester
• Skin comfort: cotton = extra-fine merino > polyester
• Technical durability: polyester = merino > cotton
• Environmental impact: mulesing-free merino ≈ organic cotton > polyester
• Real annual cost: equivalent materials over time

Urban use and short outings profile: organic cotton remains a legitimate choice. Comfort, low price, no odor problem during the day. Technical polyester and merino are oversized for this use.

Day hike and short sports outings profile (under 3h): technical polyester is a good price-performance compromise. Merino offers superior comfort but is less economically justified for this use.

Alpine day hike in heat profile (over 3h, over 25°C): merino takes the lead by managing sweat without a wet feeling. Cotton disqualified.

Multi-day trek profile, Haute Route, hut-to-hut hiking: merino is the most coherent material for this usage format. The anti-odor and strategic weight reduction of the pack more than compensate for the purchase price and drying time. Cotton disqualified, polyester acceptable but dominated.

Short intense dry sport profile (track running, air-conditioned gym): technical polyester remains relevant. Merino offers a different comfort but not necessarily superior in this format.

The choice is therefore not ideological. It’s a matter of material-use suitability. For most summer outdoor activities in self-sufficiency in Switzerland and the Alps, 140 g/m² merino is the most technically coherent answer. For urban use, organic cotton remains valid. For short intense sports, polyester keeps its place.

Discover the Bjork MC 140 women’s | Discover the Finn MC 140 men’s