Why does anxious sweat smell worse?

This week, Phil Sansom has really worked up a sweat in his efforts to answer listener Margaret...

Margaret - Why, why, why can I work in the yard and be covered in sweat for hours, and only stink a little; but reveal one personal thing to a group of friends, and immediately stink to high heaven?

Phil - I have a rubbish sense of smell and probably a bad sense of shame too; I’ve never noticed this. But Margaret’s touched on some genuine science, and I have two experts here to explain between them: Angela Ballard from the International Hyperhidrosis Society, and University of Cambridge physiologist Christof Schwiening. Here’s Angela first.

Angela - The sweat your body produces when you’re on stage, interviewing, or running late is, indeed, smellier than the sweat produced when you’re working in your garden or exercising. Your body has two different kinds of sweat glands.

Christof - When you get hot exercising, the sweat you produce comes from eccrine sweat glands distributed throughout your body.

Angela - This type of sweat begins after a slight warm-up period and tends to be odorless because it is composed mostly of water.

Christof - Whilst the liquid contains a natural antimicrobial, it provides the moisture that allows bacteria on your skin and in your clothes to grow, producing a 'little stink'. The amount of stink will depend upon what bacteria are present and the sensitivity of your nose to those smells. But a word of warning: others may detect a stronger smell from you under those conditions because they are not adapted to your smell.

Angela - Apocrine glands, on the other hand, are found mostly in your underarm area, genital area, and on your feet. These glands produce a thick, viscous fluid that’s full of proteins and lipids, and respond immediately to stress – no warm-up period required.

Christof - Now fresh apocrine sweat doesn’t actually smell that bad, but it’s more oily than eccrine sweat, and critically it wets regions that are rich in bacteria and odorants. And when you get embarrassed the apocrine sweat wets these regions, and all of a sudden the pong is released.

Angela - And the bacteria that naturally inhabit the surface of your skin love to feed and grow where there are proteins and lipids, and where you have bacterial growth, you have odour.

Christof - Your nose, like everyone else's, is sensitive to it because it gets to smell it, if you like, 'for the first time'. But most people would struggle to detect a 'major' stink under these circumstances. Now it could be that your bacterial population is particularly 'pongy', or it could just be that your nose is more sensitive than most other people’s.

Phil - Thanks to Christof and Angela for helping with the answer. Now don’t run away, because next week’s question comes from listener Charlie…

Charlie - Humans have adrenaline for our fight or flight situation. Do bugs have this too?