Why "100% Cotton" Can Legally Contain Plastic

Go grab a 100% cotton t-shirt right now. Look at the seam. Pull on the thread. Hold a lighter to a snippet of it — if it melts into a bead instead of charring like ash, it's polyester. Almost certainly is. And the label on that shirt is still telling you the truth, according to the rulebook.

I went and read the rulebook. Here's what "100%" actually means.

The actual rule

In the US, the Textile Fiber Products Identification Act — 16 CFR Part 303 — governs what has to appear on a clothing label. The key bit: the fibre content percentage describes the body of the garment. Not the whole garment. The body.

Everything else is a separate conversation the label isn't having.

What legally doesn't have to be counted

The list of exemptions is longer than you'd guess. None of these have to appear in the percentage:

• Sewing thread — the stuff holding the whole garment together
• Trim — as long as it stays under 15% of the surface area
• Decoration and embroidery — same 15% cap
• Elastic in "minor proportion for holding, reinforcing, or similar structural purposes" — waistbands, sock cuffs, the stretchy bit at the bottom of a hoodie
• Labels and tags themselves
• Interfacing and other internal stiffening

So the elastic in your waistband, the rubber thread woven through your gym socks, the polyester thread joining every seam, and the small embroidered logo on the chest — none of that has to show up in the percentage.

The 5% "other fibres" rule

There's also a separate rule worth knowing. Any fibre making up under 5% of the fabric weight can just get bucketed as "other fibres." So a 95% cotton, 5% elastane garment can legally read as "95% cotton, 5% other fibres" on the tag. The specific fibre doesn't have to be named.

Helpful, right?

Being fair — the rule isn't crazy

This is where I have to steelman, because I went into this expecting villainy and came out understanding the logic.

Cotton sewing thread isn't strong enough to survive the lifespan of most garments — it would snap before the shirt wore out. So polyester thread is genuinely the right tool for the job. A 100% cotton shirt stitched with 100% cotton thread would unravel.

And making a brand weigh and list every microgram of every component — every label, every interfacing, every speck of embroidery floss — would be a compliance nightmare that nobody would actually read. The exclusions exist for sensible reasons. They're not a loophole someone snuck in.

But — once you know, you read everything differently

The point isn't to be outraged. The point is to know what the percentage actually covers. Once you do:

• A "100% cotton" shirt has polyester running through every single seam
• A "100% linen" dress has elastic in the waist
• A "100% wool" coat has poly thread in every buttonhole
• A "100% silk" blouse has polyester interfacing in the collar

The base fabric is what the brand is promising you. Everything that holds that fabric together is a separate conversation the label isn't having.

Why this is worse online

In a physical shop, you can flip the tag over, pinch the seam, read the small print on the inside. You can find out a lot about what's really in there.

On a product page, you mostly just see "Material: 100% cotton" in the bullet list, and you're done. The exemptions are still in effect — you just have even less ability to spot what's actually in there. The gap between what "100%" suggests and what "100%" legally means is the gap that quietly trips most people up online.

What to actually do with this

I'm not telling you to never buy a "100% cotton" shirt — most of those shirts genuinely are mostly cotton, and the polyester thread is doing useful work. The move is:

• Stop reading "100%" as if it means the whole garment
• Treat it as a claim about the body fabric — usually true, sometimes the most important number
• Assume there's some plastic in the structure unless the brand specifically calls out cotton or hemp thread (a few heritage and slow-fashion brands do)
• For anything that touches your skin all day — base layers, underwear, sleepwear — that polyester thread matters less than the 95% of fabric that's actually cotton

The headline gives you the story. The fine print gives you the truth. The label isn't lying. It's just answering exactly one question, and the question is narrower than it looks.