That onion smell on your towel is almost certainly caused by bacteria feeding on the sweat, skin cells, and oils you leave behind every time you dry off. Specific sulfur-containing compounds produced by these bacteria are chemically similar to the compounds that give onions their smell. The good news: this is a solvable problem once you understand what’s driving it.
The Bacteria Behind the Smell
Your skin, especially in areas like the armpits and groin, hosts bacteria that thrive on the proteins and oils in sweat. A group called corynebacteria are among the strongest contributors to body odor. These microbes break down sweat components into sulfur-containing molecules called thioalcohols, which produce that distinctive onion-like or musky scent. Another species, Staphylococcus hominis, imports a specific sweat precursor into its cells and converts it into a compound called 3M3SH, one of the most pungent molecules in human body odor.
When you towel off after a shower, you transfer these bacteria along with residual sweat, skin flakes, and sebum (the oily substance your skin produces) directly into the fabric. A damp towel hanging in a warm bathroom is an ideal breeding ground. The bacteria multiply, continue breaking down the organic material trapped in the fibers, and release volatile sulfur compounds you detect as an onion or allium-type smell. The longer the towel stays damp, the worse it gets.
Your Washing Machine May Be Making It Worse
Even freshly washed towels can come out smelling funky if your washing machine itself harbors bacteria. Biofilms, thin layers of bacteria embedded in a slimy matrix, build up on the rubber seals, detergent dispensers, and drum surfaces of washing machines over time. These biofilms act as a reservoir: bacteria from dirty laundry colonize the machine, and then recontaminate the next load. Research published in the journal Antibiotics confirmed that biofilm formation in both washing machines and fabrics is directly responsible for persistent laundry malodor, with bacteria producing volatile organic compounds that users can smell.
Front-loading machines are particularly prone to this because the door seal traps moisture. If you’ve noticed the onion smell appears on towels that seemed fine before washing, the machine is a likely culprit. Sweat residue, skin particles, and detergent buildup all accumulate in these biofilms, creating a self-sustaining cycle of contamination.
Detergent and Fabric Softener Buildup
Using too much detergent or regularly adding fabric softener to towel loads creates a waxy residue that coats the fibers. This residue traps sebum and bacteria inside the fabric where water and soap can’t reach them effectively during washing. Over time, the towel may look clean but still carry a microbial load that reactivates the moment it gets damp again. Fabric softener is especially problematic for towels because it’s specifically designed to coat fibers, reducing both absorbency and the ability of wash water to flush out trapped material.
Fabric Type Matters
If your towels are microfiber rather than cotton, they may hold onto bacteria more stubbornly. A study comparing bacterial contamination across fabric types found that microfiber towels harbored significantly greater numbers of bacteria than cotton towels. The dense, tightly woven structure of microfiber that makes it so absorbent also creates more surface area for bacteria to cling to. Cotton towels aren’t immune to the problem, but they tend to release bacteria more readily during washing.
Your Diet Can Amplify the Smell
What you eat changes the chemical composition of your sweat, which in turn changes what ends up on your towel. Foods high in sulfur compounds are the biggest offenders. Garlic, onions, cruciferous vegetables like broccoli and cabbage, and strong spices all contain volatile compounds that get absorbed into your bloodstream and released through your sweat glands. If you eat a lot of these foods, the sulfur content of your sweat goes up, giving bacteria on your towel even more raw material to convert into onion-smelling thioalcohols. This doesn’t mean you should stop eating broccoli, but it helps explain why the problem might be worse for you than for someone else in your household.
How to Get Rid of the Smell
The core strategy is to deny bacteria the conditions they need: moisture, warmth, and food.
- Dry towels quickly. Spread your towel on a bar (not folded over a hook) so air circulates on both sides. If your bathroom is humid and poorly ventilated, move the towel to a drier room or use a fan.
- Wash towels every 2 to 3 uses. The longer you wait, the more bacteria establish themselves deep in the fibers.
- Use less detergent. More soap doesn’t mean cleaner towels. Excess detergent leaves residue that traps the very bacteria and oils you’re trying to remove.
- Skip fabric softener on towels. It coats fibers and seals in odor-causing material.
- Dry towels completely in the dryer. Research shows that a hot drying cycle at around 93°C (200°F) provides an additional reduction in bacterial counts beyond what washing achieves. If you air-dry, make sure towels are bone dry before folding and storing them.
For towels that already smell, a reset wash helps. Run them through a cycle with white vinegar (about one cup in the drum, no detergent). Follow that with a second wash using a small amount of detergent and no softener. The vinegar dissolves the buildup that’s trapping bacteria and oils in the fabric. Baking soda (half a cup added to the wash) works similarly by raising the pH enough to break down greasy residue.
Clean Your Washing Machine
If the smell returns quickly after washing, your machine’s biofilm is recontaminating your towels. Run an empty hot cycle with a washing machine cleaner or two cups of white vinegar. Pull back the rubber gasket on front-loaders and wipe it down, since this is where the worst buildup hides. Leave the door open between loads so the drum dries out. Doing this once a month prevents the biofilm from re-establishing. Interestingly, research found that cold-water wash formulas with proper detergent reduced bacterial counts just as effectively as hot-water cycles, so the machine’s cleanliness and your drying method matter more than water temperature alone.