What to look for in a pair of undies

Field Note 01 · Fabric · NZ + AU

A plain English guide to what your undies are made of.

When I started looking into starting with underwear I knew that it had to be cotton. It's always been my personal preference, and I've always known that a lot of what we are offered is essentially plastic. But what I didn't know is everything in between, and that's what I'm trying to help clarify for you.

Should you be scared of your current underwear if it is plastic? No, probably not (with some caveats, see below).

For those of you (like me) who already read labels: on your food, your skincare, the stuff you clean the house with. The one you've probably never read is the label in your underwear, and what I've learnt is that it is worth more of your consideration.

That fabric sits against your vulva for most of your waking day. The skin there isn't like the skin on your arm. It's thinner, more hydrated, and it sits under occlusion, which is a technical way of saying it's covered up and doesn't get much air. Studies comparing absorption through the skin of the vulva with skin on the forearm found it takes up significantly more of what it's exposed to (sources at the bottom).

So here's a quick fabric guide to help you check what you're wearing. Everything sorts into three piles: skip, acceptable, and look for. This guide is about what the fabric actually does, and how to spot it on a label. It isn't meant as a scare piece.

Why the fabric matters here at all

Two things worth thinking about.

Proximity and time. The skin of your vulva is more permeable than most of the skin on your body, and it's in contact with whatever you're wearing for sixteen hours a day. What touches it, and for how long, is a fair question, and not one most of us have ever been prompted to ask.

Breathability. Natural fibres let air and moisture move through them. Most synthetics don't. They hold heat and moisture against the skin, in an area that's already warm and covered. "Breathable" on a synthetic label usually means a finish was added to make up for what the fibre doesn't do on its own.

The hierarchy

Skip

Pure synthetics for everyday wear: polyester, nylon, acrylic. They come from petroleum, and they hold heat and moisture against the skin.

The same thing under a nicer name. Synthetics don't always say "polyester" on the tag. Polyamide is just nylon. "Ice silk", "milk silk", "vegan silk" and most cheap "satin" are polyester or nylon, not silk. Microfibre is finely spun polyester or nylon. Elastane, spandex and Lycra are the same synthetic stretch fibre, and Lycra is just a brand name for it (a little of it is fine, more on that below). If a fabric promises to keep you cool, dry or fresh under a poetic name, assume synthetic until the fibre content says otherwise.

Performance finishes: anything labelled anti-odour, moisture wicking, stain resistant or wrinkle free. Those effects come from a chemical finish applied to the fabric. Fine on a raincoat. Less ideal against this skin, all day.

Don't panic. You don't need to throw out your lacy date-night special pair, or your plastic workout wonders. Keep them for those occasions, just check that the lining is cotton, and think about the length of time that you are wearing them. Do what works for you.

Acceptable, once you know what it is

"Naturally derived" fabrics: Tencel, lyocell, modal, bamboo viscose. The distinction that matters here isn't plant versus not-plant, because cotton is a plant too. It's how the fibre is made. Cotton is a natural fibre: the plant grows the fibre and you spin it into yarn much as it is. These are regenerated fibres: the plant, usually wood pulp or bamboo, is chemically dissolved into a pulp and reformed into a brand-new fibre. It starts as a plant and ends as something rebuilt. Better than straight synthetic, but "naturally derived" is not the same as a natural fibre.

Bamboo is worth its own note. Bamboo lyocell uses a cleaner process than bamboo viscose. Same plant, very different treatment. Most of the soft bamboo undies you'll find are viscose.

Look for

Natural fibres: cotton, wool, silk. For everyday undies, cotton is the obvious one. It breathes on its own, no finish required.

But "natural fibre" is only half the story. Here's the fair objection: cotton can be chemical-heavy too. Conventional cotton is one of the more pesticide-intensive crops in the field, and conventional processing piles more on top: chlorine bleach, heavy dyes, formaldehyde-based finishes. So "natural fibre" and "clean" are two different claims. Which is exactly where the growing-and-processing certification earns its place.

Made with organic cotton, verified by someone independent. This is where the certification names earn their keep, and it pays to know what they actually cover, because the words get used loosely.

There are two governing bodies to look out for (see below). They can both cover the fabric (yarns and dyes), and OEKO-TEX can also cover the threads and other trims, like the elastic around the waist and leg.

GOTS stands for Global Organic Textile Standard. It covers organic growing, without the synthetic pesticides conventional cotton leans on, and the wet processing, restricting the bleach, dyes and finishes used to turn fibre into cloth. That second part is the one people miss. "Organic" on the fibre says nothing about how the fabric was then processed. A supply-chain standard like GOTS is what covers that, which is why "made with GOTS organic cotton" says more than "organic cotton" on its own. GOTS also comes in two grades. The "organic" grade needs at least 95% certified organic fibre. The "made with organic" grade needs at least 70%, and it's the grade most stretch fabrics sit in, because the elastane that lets a pair move counts against that percentage. Neither is a trick, but "made with organic" and "organic" are different claims.

OEKO-TEX isn't an acronym. It's the name of an independent testing association (the International Association for Research and Testing in the Field of Textile and Leather Ecology, based in Zürich). "Öko" is German for ecological, "Tex" for textiles. It tests for substances a fabric isn't allowed to contain. The part that trips people up: it works at two levels. A fabric mill can hold an OEKO-TEX certificate for its cloth. Separately, a whole finished garment can be certified as STANDARD 100, which means every part, thread, elastic and dye, was tested together on the assembled pair. Finished-garment certification is the higher, rarer bar. So the useful question isn't "is it OEKO-TEX?" It's "what was actually tested: the fabric, or the finished pair?"

STANDARD 100 also has four product classes, and the limits get stricter the more the item touches skin. Class I is for babies and children under three, with the tightest limits. Class II covers direct skin contact, which is where underwear sits. So "OEKO-TEX certified", with no class and no answer on fabric versus finished pair, doesn't tell you much on its own.

So the honest version of "look for GOTS and OEKO-TEX" is really this: look for organic cotton you can trace, ask what grade and what was tested, and treat a fabric-level certificate as a good sign rather than a finished guarantee.

How to read a label in five seconds

  1. Find the fibre content. "100% cotton", or cotton with a little elastane for stretch. Good sign.
  2. Watch for the performance words. Anti-odour, wicking, stay fresh. That's a finish. Ask why it needs one.
  3. Check the certification is independent. "Eco" and "natural" on the front of a pack mean nothing on their own. GOTS and OEKO-TEX are audited by an outside body. A brand's own green logo isn't.

The honest caveat

A little elastane is what makes a pair stay up and move with you. We all know from our workout wear and our jeans with some stretch in them that a little elastane contributes significantly to comfort. The goal isn't zero synthetic at any cost. It's knowing the trade off and choosing on purpose.

Here's what that trade off actually looks like. The less elastane in the fabric itself, the more elastic trim has to go around the legs and waistband to hold a pair up. So less synthetic in the cloth doesn't automatically mean more comfortable, or that there won't be elastane where you don't really want it. Choose what works for you, and how comfortable you like to be.

With most undies, the main fabric is picked because it's cheap and it has high stretch for comfort. Nobody's asking what it's doing against your skin.

FAQ

Does underwear fabric affect vulva health?
The skin of your vulva is more permeable and more prone to irritation than skin elsewhere on the body, and it spends most of the day covered. Breathable natural fabric lets heat and moisture move away from the skin, where synthetics hold both against it. That's the practical reason fabric choice matters here more than it does for, say, a jacket.

Is synthetic underwear bad for you?
Not necessarily, but cotton is the standard everyday recommendation for vaginal health, because synthetics hold heat and moisture against thin, absorbent skin all day, where natural fibres let it breathe. There's research on this. A 2019 study of 986 women found that the style of underwear predicted nothing. Thongs weren't associated with any urogenital infection. What was associated with yeast infections was wearing underwear without a cotton crotch. The cut didn't matter. The fabric did.

Is bamboo underwear natural?
Usually not in the way people assume. The difference is how the fibre is made. Cotton is a natural fibre: it's not a huge leap to spin the fibre the plant grows. Most soft "bamboo" is bamboo viscose, where the plant is chemically dissolved and reformed into a new fibre. It starts as a plant but ends as something rebuilt. Bamboo lyocell uses a cleaner process. Both beat pure synthetic. Neither is the same as cotton.

Isn't conventional cotton chemical-heavy too?
Yes. Conventional cotton is a pesticide-intensive crop, and conventional processing uses chlorine bleach, heavy dyes and formaldehyde-based finishes. Being a natural fibre isn't the same as being clean. That's what organic growing plus GOTS certification is for: GOTS restricts the chemistry across both the field and the processing. Organic cotton without GOTS only addresses the growing, not what happens to the fabric afterwards.

What does GOTS certified mean?
GOTS stands for Global Organic Textile Standard. It verifies the cotton was grown organically and processed without harmful chemistry, audited by an outside body. It has two label grades: "organic" needs at least 95% certified organic fibre, and "made with organic" needs at least 70%. Stretch fabrics usually sit in the "made with organic" grade, because elastane counts against the organic percentage.

What does OEKO-TEX stand for?
It isn't an acronym. OEKO-TEX is an independent testing association (the International Association for Research and Testing in the Field of Textile and Leather Ecology, based in Zürich). "Öko" is the German for ecological, "Tex" for textiles. STANDARD 100 by OEKO-TEX is the certification you'll see on a label.

What do the OEKO-TEX classes mean?
STANDARD 100 has four product classes, and the limits get stricter the more the item touches skin. Class I is for babies and children under three and has the tightest limits. Class II covers products with direct skin contact, which is where underwear normally sits. Class III and IV cover items with little or no skin contact. "OEKO-TEX certified" without a class doesn't tell you much.

What's the difference between GOTS and OEKO-TEX?
GOTS covers how the fibre is grown and made. OEKO-TEX tests for substances the fabric shouldn't contain. They do different jobs, and both can apply at fabric level or to a finished garment.

Sources

  • Oriba HA, Bucks DAW, Maibach HI. "Percutaneous absorption of hydrocortisone and testosterone on the vulva and forearm: effect of the menopause and site." British Journal of Dermatology, 1996. PubMed
  • Britz MB, Maibach HI, Anjo DM. "Human percutaneous absorption of hydrocortisone: the vulva." Archives of Dermatological Research, 1980. PubMed
  • Farage M, Maibach H. "Vulvar susceptibility to contact irritants and allergens: a review." Archives of Gynecology and Obstetrics, 2005. PubMed
  • Hamlin AA, et al. "Brief versus Thong Hygiene in Obstetrics and Gynecology (B-THONG): A survey study." Journal of Obstetrics and Gynaecology Research, 2019. Wiley
  • Healthline. "8 Underwear Tips for Vaginal Health." healthline.com
  • OEKO-TEX. STANDARD 100 and product classes. oeko-tex.com
  • "Organic cotton production may alleviate the environmental impacts of intensive conventional cotton production." Renewable Agriculture and Food Systems, Cambridge University Press. Cambridge Core

A note on numbers: you'll see "six times more absorbent" quoted a lot online. I've left it out, because I couldn't trace it to the original study. The research shows absorption is significantly higher. That's enough.

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