If you’ve ever skipped a laundry day and been forced to hit the gym in your backup sports kit, you’ll know that when it comes to stinkiness, not all fabrics are created equal. But exactly why that is has proved elusive – until now.

“Although we know that polyester is smellier after being worn next to sweaty armpits compared to cotton T-shirts, we haven’t really known why,” said Rachel McQueen, a clothing and textiles scientist in the Faculty of Agricultural, Life & Environmental Sciences at the University of Alberta and first author of a new paper investigating the phenomenon, in a statement this week.

“Now we have a better understanding of how odorants transfer and are selectively absorbed by various fibre types in sweat.”

To study the stink, McQueen and her colleagues set out to mimic the effects of exercise on various types of fabric. First, they soaked them in a bottle filled with a solution of simulated sweat, which they shook vigorously for a couple of minutes before setting aside for half an hour. The fabric was then removed from the liquid, dehydrated a bit, and left again to give the odor particles time to percolate. 

Next, they needed to actually measure how whiffy the fabric had got. Just letting each of the researchers have a sniff and score them out of ten wouldn’t be good enough, so the team turned to a technique called mass spectrometry – a way of measuring the mass-to-charge ratio of ions in a given sample. If that doesn’t mean much to you, don’t worry: you can think of it as a high-tech science-nose, capable of picking out the odorants in the air in real-time.

The results showed a clear pattern. Fabrics made from cellulose – that is, natural fibers made from plants, like cotton, linen, or hemp – absorbed and released lower amounts of the smelly odor compounds. Synthetic fibers like polyester, on the other hand, as well as wool, took in more of the stink – and released more too.

To understand why this is, we need to look at the constituent ingredients of sweat. Obviously, it’s mostly water, but it also contains oily compounds, McQueen pointed out – in fact, that’s where the odor is formed.

And, depending on the particular chemistry of the fibers, these oils can interact differently. “While water-loving cellulosic fibres such as cotton and viscose absorb more of the water from sweat than polyester does, polyester doesn’t want to absorb the water,” McQueen explained. “It’s more oil-loving, and it absorbs more of the odorants, which don’t dissolve in water, and more of the oily compounds, which could also later break down and become smelly.”

The smellier fabrics weren’t all bad news, though. For nylon and wool at least, that bigger release of odor particles didn’t actually last that long – sure, they were stinkier than their cotton brethren at first, but after 24 hours the smell had dissipated a lot. “That tells us that while polyester still needs to be washed, for nylon and wool garments, people might be able to freshen them by just airing them out rather than laundering every time,” McQueen pointed out.

But while most of the fabrics had at least some redeeming quality, smell-wise, there was one clear loser in the sniff test: polyester.  

“Basically, if you’re concerned about smelly clothes, then keep away from polyester,” McQueen said. “Even with some of the anti-odor claims on some clothing labels, you might want to be cautious. If the anti-odor property is due to an antimicrobial, it may not be as effective as you think, because there’s another mechanism in play, which is all about the fibre chemistry and the interaction with odorants.”

The paper is published in the Textile Research Journal.