Merino or synthetic base layers, and what a lab found when it measured the smell

Gear & Tech Merino wins the week-long wear, synthetic wins the overnight dry, and the lab reason merino stays fresh is not the bacteria story you have been told. By Prakash Tamang · Sep 9, 2026 Read the full story

If you are buying one base layer for a long trek, buy merino. If you are buying three, make one of them synthetic. That is the honest answer, and the reason people usually give for it — that wool kills the bacteria which cause the smell — is not what the… Travel Himalaya Nepal · News

What the odour study actually found

Polyester goes sour faster than wool, and that part of the folklore is true. A team led by Rachel McQueen tested nine knit fabrics in cotton, wool and polyester after wear, and had a trained odour panel rate them. Polyester rated high for odour intensity… Travel Himalaya Nepal · News

Where synthetic genuinely beats merino

Drying, and it is not close. This is the reason we never tell anyone to go all-merino. Polyester holds very little water in the fibre itself, so a synthetic shirt rinsed in a lodge sink at four in the afternoon is wearable by breakfast. A merino shirt of the… Travel Himalaya Nepal · News

How fine the wool actually needs to be

Merino is graded by fibre diameter in microns, and that number tells you more than the brand on the label does. Woolmark puts merino between under 17.5 microns and around 22 to 24 microns, and identifies fibres below roughly 19.5 microns as the ones used for… Travel Himalaya Nepal · News

What we tell trekkers to pack

Two tops, one of each. A merino shirt you walk in and keep wearing, and a synthetic one you rinse and dry overnight so there is always a clean dry option. That combination solves both problems at once and weighs less than the three or four shirts most people… Travel Himalaya Nepal · News

The shirt you wear for eight days without anyone minding

Best overall Icebreaker Merino 200 Oasis Long Sleeve Crew Merino at 200-weight is the one true trekking base layer — warm damp, warm dry, and it does not smell; it is also three times the price of synthetic and it will eventually hole at the… ↑ Swipe up Merino at 200-weight is the one true trekking base layer — warm damp, warm dry, and it does not smell; it is also three times the price of synthetic and it will eventually hole at the shoulder where the pack rubs. Not for: A four-day trek where you can wash and change. Synthetic is cheaper, tougher and dries faster; merino earns its price on long trips without laundry. Check price on Amazon Read the full review If you buy through that link we earn a commission. It costs you nothing and it does not change what we recommend — the rental and advice-only entries in our gear guide earn us nothing at all.

The one that dries by dinner when merino does not

Best synthetic Under Armour Tech 2.0 Long-Sleeve Dries in an hour and costs a fraction of merino, which makes it the better buy on a short trek — it will smell by day three and nothing you do changes that. ↑ Swipe up Dries in an hour and costs a fraction of merino, which makes it the better buy on a short trek — it will smell by day three and nothing you do changes that. Not for: A fortnight without laundry. That is exactly the case merino is for. Check price on Amazon Read the full review If you buy through that link we earn a commission. It costs you nothing and it does not change what we recommend — the rental and advice-only entries in our gear guide earn us nothing at all.

What you change into at the lodge and sleep in

Best sleeping layer Smartwool Classic Thermal Merino Base Layer Bottom The single best sleep-warmth upgrade above 4,000 m and it doubles under trousers on a cold pass; at 250-weight it is too warm to walk in on any ordinary day. ↑ Swipe up The single best sleep-warmth upgrade above 4,000 m and it doubles under trousers on a cold pass; at 250-weight it is too warm to walk in on any ordinary day. Not for: Low-altitude treks. Below 3,000 m you will never take them out of the bag. Check price on Amazon Read the full review If you buy through that link we earn a commission. It costs you nothing and it does not change what we recommend — the rental and advice-only entries in our gear guide earn us nothing at all.

The three pairs that do a whole trek

Best overall Darn Tough Hiker Micro Crew Cushion Take three pairs and you can wash one, dry one and wear one for a fortnight — the price per pair is high enough that people talk themselves into cheaper socks and regret it around day four. ↑ Swipe up Take three pairs and you can wash one, dry one and wear one for a fortnight — the price per pair is high enough that people talk themselves into cheaper socks and regret it around day four. Not for: Nothing, honestly. If the budget is tight, buy two pairs of these rather than five of anything else. Check price on Amazon Read the full review If you buy through that link we earn a commission. It costs you nothing and it does not change what we recommend — the rental and advice-only entries in our gear guide earn us nothing at all.

The part nobody wants to hear about cost

A merino base layer costs several times what a synthetic one does, and the honest position is that a lot of trekkers do not need it. If you are doing one trek in your life, on a teahouse route with a sink and an afternoon of sun, two synthetic shirts and a… Travel Himalaya Nepal · News

Questions we get asked about base layers

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Can I just wear a cotton T-shirt?

No, and this is the one place we are absolutely rigid. Cotton holds water against your skin and stays cold once it is wet, whether that is sweat on a climb or rain. Both merino and synthetic are enormous improvements over it. If you take one thing from this… Travel Himalaya Nepal · News

Do I need a heavyweight or a lightweight?

Lightweight, almost always. You regulate temperature with the layers over the top, not by starting with a thick shirt you cannot take off. A heavy base layer is comfortable for the first hour of the morning and too warm for the rest of the day. Travel Himalaya Nepal · News

Is it worth buying base layers in Kathmandu?

For a spare synthetic top, yes, and Thamel is full of them at a fraction of what you would pay at home. For merino, be careful — the labelling is unreliable and you cannot verify a micron count on a market stall. Bring the merino, buy the spare there. Travel Himalaya Nepal · News

How many days can I really wear merino?

In our experience several, and most people are surprised by it. But it is not infinite, the study above explains why, and your fellow trekkers in a small dining room will notice before you do. Base layers are the first thing on and the last thing you think… Travel Himalaya Nepal · News

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