Merino wool at a glance
- Next-to-skin range
- About 17 to 19.5 microns
- Prickle threshold
- Around 30 microns
- Coarser fibers are felt rather than draped
- Typical blend
- 10 to 15 percent nylon
- Diameter test
- ASTM D2130
What the micron number means
A micron is a thousandth of a millimetre, and it is the diameter of a single wool fiber. It is the spec that decides everything a wearer notices, because a finer fiber bends against skin instead of pressing into it. Around 30 microns is the rough threshold where fibers stop bending and start being felt as prickle, which is why a traditional wool jumper itches and a merino T-shirt does not.
Next-to-skin merino apparel generally sits between about 17 and 19.5 microns. Below that, at 15 to 17, is superfine and ultrafine territory: softer, dearer and more delicate. Above about 21 the yarn is heading towards socks and outerwear, where a little prickle does not matter.
Why it does not smell
Odour on a base layer comes from bacteria feeding on what sweat leaves behind, and wool is markedly worse ground for that than polyester is. Part of it is that the fiber draws moisture into its own structure rather than leaving it on the surface, and part is the chemistry of the fiber itself. The practical version is that a merino top goes three or four days before anyone notices, and a polyester one goes about one.
Why it still works when it is damp
Wool takes a lot of water into the fiber before it feels wet on the outside, and it keeps a usable amount of loft while it does. That is a safety property rather than a marketing one: a soaked wool top is unpleasant, and a soaked cotton one is dangerous. It is also slower to dry than synthetic, for exactly the same reason.
Why it wears out, and why blends exist
The trade for all of that is durability. Fine wool fibers are weak in abrasion, so pure merino thins at the shoulders where pack straps sit, and holes turn up at the elbows long before a polyester top shows any wear at all.
Which is why most merino base layers are not pure merino. Blending in roughly 10 to 15 percent nylon, or spinning the wool around a nylon core, substantially extends the life of the garment while leaving the feel and the odour resistance largely intact. A label reading 100 percent merino is a softer top and a shorter-lived one.
What this means when you're choosing
- A stated micron figure, since merino covers a wide range of fineness
- Nylon in the blend if the top will live under a pack strap
- The gsm alongside it, which is what sets the temperature range
- Flatlock seams and offset shoulders, because wool abrades where it rubs
Merino wool vs Polyester
| Merino wool | Polyester | |
|---|---|---|
| Odour | Resists it for several days | Noticeable after about a day |
| Drying | Slower, because water goes into the fiber | Fast, because water sits on the surface |
| When damp | Keeps a usable amount of warmth | Cools quickly once it is wet |
| Durability | Abrades at shoulders and elbows | Outlasts wool comfortably |
| Price | Roughly twice as much | The affordable default |
Which to pick: Merino for multi-day trips where nothing gets washed, polyester for hard efforts, hard use and a tighter budget. A blend is the honest middle, and it is what most good tops now are.
The common mistake
Reading merino as one material
The word covers everything from 15 micron ultrafine down to yarn coarse enough to itch. A cheap top marked merino can sit well above the comfort threshold, and the figure that tells you is the micron number, not the price.
Common questions
Is merino wool itchy?
At the fineness used for base layers, no. Prickle is a physical effect of stiff fibers pressing into skin rather than an allergy, and it largely disappears below about 30 microns. Next-to-skin merino runs far finer than that.
How many days can you wear a merino base layer?
Three or four before odour becomes obvious is typical, against about one for polyester. Airing it overnight extends that noticeably, which is why it is the usual choice for trips with no way to wash anything.
Why is my merino top blended with nylon?
Because fine wool is weak in abrasion. Ten to fifteen percent nylon, or a nylon core the wool is spun around, extends how long the garment survives pack straps and washing while leaving the feel and the odour resistance intact.
Where this is defined
- ASTM D2130, diameter of wool and other animal fibers by microprojection, ASTM International