What is merino wool?
Merino sheep were bred over centuries, first in Spain and later perfected in Australia and New Zealand, to grow finer, softer fleece than ordinary sheep. That breeding is the whole difference between merino and plain wool.
Merino sheep were so valuable to Spain's economy that exporting them was once a crime punishable by death. Flocks reached Australia in the late 1700s, and Australian and New Zealand merino now supply most of the world's fine wool.
How merino wool is made
Merino is shorn once a year, usually in spring, off sheep bred to grow fleece far finer than a standard breed's, often 18 microns or thinner. The raw fleece is heavy with lanolin grease, suint (dried sweat salts), dirt, and plant matter, so it is scoured in hot water and mild alkali before it can be spun.
The superwash step is where merino picks up chemistry it did not start with. About 75 percent of machine-washable wool runs the chlorine-Hercosett process: a chlorine bath strips the fiber's natural 18-MEA coating, a fatty acid layer that repels water, and etches down its surface scales, then a thin polyamide-epichlorohydrin resin sold as Hercosett 125 is coated over the fiber so it cannot felt in a washing machine. The chlorination throws off AOX (adsorbable organic halogens) in the wastewater, and a plastic film ends up bonded to the fiber.
The animal-welfare flashpoint sits earlier, at the farm. Mulesing cuts strips of skin from a lamb's breech to scar the area so blowflies cannot strike it, and around two-thirds of Australian growers still do it. The Responsible Wool Standard (RWS) bans mulesing outright.
Untreated, non-superwash merino stays closest to its clean starting point. The prickle you might feel is mechanical, set by how fine the fiber is, not by anything added to it.
Not all merino wool is the same
Merino is graded by micron count like other wool, but the whole range runs finer. Ultrafine merino is 15 to 17 microns, the softest, most next-to-skin grade. Fine merino sits around 17 to 19 microns, comfortable for most people. Above 20 microns, merino starts to feel like standard wool.
Is merino wool safe to wear?
Merino's fineness is what keeps it off your nerves. A 2017 review in Acta Dermato-Venereologica found wool is not a true allergen. What people call an allergic reaction is prickle, stiff fiber ends pressing on the skin's nerve endings, and fibers under roughly 22 microns mostly sit below the threshold where that fires. Ultrafine merino at 15 to 18 microns lands well under it, which is why dermatologists often clear it for eczema-prone skin.
The chemistry to watch sits on top of the fiber, not inside it. Superwash merino carries chlorine-based residue and a thin Hercosett polymer film that can bother reactive skin more than the wool itself would. Leftover lanolin in modern scoured merino is low, so true lanolin sensitivity is rarer than its reputation. Untreated merino breathes, buffers temperature, and resists odor, so it stays comfortable across a full day of wear.
- Below the prickle threshold Fine enough (15 to 18 microns in the best grades) that most wearers, including many with sensitive or eczema-prone skin, never feel the poke coarser wool causes.
- Temperature regulating Insulates in the cold and releases heat and moisture in the warm, so one base layer works across a wider range than most synthetics.
- Odor-resistant The fiber's structure resists holding onto sweat odor, so a merino layer can be worn several times between washes.
- Not a true allergen Dermatology research has debunked wool as an allergen; discomfort is mechanical prickle, not an immune reaction.
Merino's ultrafine, low-lanolin fiber gives it one of the cleanest hazard bases of any animal fiber, which lands it near 90, high in the green band. Non-superwash, RWS-certified merino sits at the top of that; the chlorine-Hercosett superwash finish and its AOX byproducts are the main thing that pulls a given piece down. See the full method.
Doing this check on every product page yourself is the tedious part. The Toxome Chrome extension reads the composition for you while you shop, so you see whether something is merino wool (and what else is in it) before you buy, not after it arrives.
How to care for merino wool
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Merino Wool, answered
The fiber is not. The added risk is the superwash finish: a chlorine bath that strips the fiber's natural 18-MEA layer, plus a Hercosett polymer coating, which leaves chlorine-based residue and a plastic film behind. Untreated, OEKO-TEX or RWS certified merino carries little.
Effectively, for most people. A 2017 dermatology review found wool is not a true allergen, and merino at 15 to 18 microns sits below the thickness that causes prickle, so it is often recommended even for eczema-prone skin.
Superwash merino goes through the chlorine-Hercosett process: chlorine etches the fiber's scales and a thin polyamide resin coats it so it survives a washing machine. It is not banned or acutely dangerous, but it leaves chlorine-based residue on the fiber and a plastic film, worth skipping if you chose wool for its natural, unprocessed qualities.
Some of it. Mulesing cuts skin from a lamb's breech to prevent flystrike, and around two-thirds of Australian growers still practice it. Look for a non-mulesed claim or Responsible Wool Standard (RWS) certification, which bans it.
- Debunking the Myth of Wool Allergy · Acta Dermato-Venereologica
- Superwash wool - is it really that super? · Bergstrand Insights
- What Is the Responsible Wool Standard? · Darn Tough
The health score reflects wearer health only and mirrors the Toxome app. This guide is educational and is not medical advice.