Why do my armpits smell like onions?

Not musty, not sweaty: onions. Sometimes fried onions, sometimes curry, sometimes something closer to weed. People describe it in very specific words, and they are usually right. Armpit smell really can smell like onion, and there's a clear reason why.

Where the onion smell comes from

Fresh sweat has almost no smell. Under your arms, though, glands release a thicker sweat that carries odourless sulphur-containing substances. A few kinds of skin bacteria break those substances down, and what they release smells strongly.

In 2004, two Swiss research teams identified one of the main culprits, a sulphur compound called 3-methyl-3-sulfanylhexan-1-ol. One of its two forms smells of sweat and onion (News-Medical summary of the research). Later work at the Universities of York and Oxford showed that only a small number of armpit bacteria produce it (ESRF summary).

Onions and garlic get their smell from sulphur compounds too. That's why your nose files the two together.

What makes it stronger

• Warmth. A warm commute, a heated office or a busy day brings out more of the thicker sweat, so more for the bacteria to work on.
• Stress. Nervous sweat comes from the same glands, which is why the smell can arrive in a meeting rather than at the gym.
• Food and drink. The NHS suggests avoiding lots of strong-smelling or spicy food and too much alcohol if body odour bothers you. Garlic, onions and some spices can show up in your sweat.
• Hormones. Puberty, pregnancy and perimenopause can all change how you smell. More on that in menopause body odour.
• Clothes. Synthetic fabrics trap the smell and release it again as soon as you warm up.

Curry, weed, vinegar: is it all the same thing?

Mostly, yes. Armpit smell is a mix of compounds, and the balance of bacteria on your skin decides which ones lead. Sulphur compounds are behind the onion-like notes, and people often describe the same family of smells as curry or weed. Fatty acids in sweat, broken down the same way, give a sour, vinegary or cheesy note. Everyone's mix is a little different, which is why two people with strong body odour can describe it so differently.

What matters more is change. If your smell has recently become different from how it usually is, that's worth mentioning to a GP (see below).

What helps

The routine is the same as for smell that comes back after a shower, which we cover in why your armpits still smell after a shower. In short:

1. Wash your underarms with real attention, then dry them completely.
2. Wear cotton, wool or silk where you can, and loose rather than tight.
3. Wash tops inside out, and retire the ones that hold on to smell.
4. Notice your own pattern: certain meals, stressful days, warm rooms. It tells you when you need to be ready.
5. Use a deodorant made for strong odour, rather than one that mostly adds fragrance on top.

When to see a GP

The NHS suggests seeing a GP if body odour hasn't improved after trying things yourself and it affects your self-esteem, if it smells different from how it usually does, or if you suddenly sweat much more than usual (NHS: body odour).

Where MARL fits

Our mum made MARL for our sister, whose smell came back even after a shower. It's a balm with only seven ingredients and no baking soda. Alum stone and Nigella do the odour work, from the first morning, with no detox period.