Sweat changes the math

Most of what we know about chemicals moving from fabric into skin comes from testing on dry skin. Reasonable enough as a baseline. Except that's not how anyone wears activewear.

A 2025 study looked at what happens when sweat enters the picture and found it can increase PFAS transfer through skin dramatically, by up to roughly 3,000 times compared with dry contact. Not triple. Three thousand times. Moisture opens the door, and a hard workout holds it open for an hour.

Now stack the conditions of a typical gym session. The fabric is worn tight, so contact is constant. The body is warm, so pores are open. The garment is engineered to sit against your skin, which is your largest organ, while both of you are damp. Every variable that makes dermal transfer easier is present at once, in exactly the category of clothing where testing keeps finding things. Consumer-watchdog labs found fluorine indicators, the marker of PFAS finishes, in about two-thirds of tested sports bras, and the Center for Environmental Health's 2022 testing found BPA exposure from polyester-spandex activewear at up to 22 times California's safe dermal limit.

It's an unsettling overlap, and pretending otherwise would be silly. But the CEH data contained its own exit: BPA showed up only in polyester-spandex blends, and not at all in pure cotton, wool or linen. For the hour you sweat hardest, the fibre line on the tag is doing real work. Cotton for yoga, wool for a winter run, and save the technical synthetics for when their performance genuinely earns the spot. Scanning a garment with Lynen before it goes in the gym bag makes that sorting take about four seconds.

You can't renegotiate your skin's permeability. You can decide what's pressed against it for the sweatiest 45 minutes of your day, and that's the 45 minutes where the choice counts most.