Why Do We Sweat, and What Actually Causes Body Odour? Professor Caitriona Ryan Explains
Professor Caitriona Ryan, Consultant Dermatologist and Co-founder of the Institute of Dermatologists, joined Anna Geary on RTÉ Radio 1's Supercharged this summer to explain the science behind sweat and body odour, and what actually works to manage both.
Sweating is a normal, necessary cooling system, not a problem to fix. Most people sweat up to a litre a day, more in heat or during exercise. Body odour itself doesn't come from sweat directly. It comes from bacteria on the skin breaking sweat down, which is why the right approach is about the skin's surface, not stopping sweat altogether.
What Does Sweating Actually Do?
Sweating is how the body regulates its core temperature. When that temperature rises, whether from heat, exercise, hormones or stress, sweat glands activate to cool the body back down.
"It is a really sophisticated cooling system that our bodies have," says Professor Caitriona Ryan. "Sweating is really necessary, especially when our core temperature goes up."
On an average day, most people sweat up to around a litre. Athletes training in hot conditions can lose far more, up to two litres an hour, which is why hydration (and for extreme exertion, isotonic replacement) matters more than most people realise. For everyday sweating, plain water is enough. Salt or isotonic replacement is really only relevant for endurance athletes and extreme heat.
What Actually Causes Body Odour?
This is where Professor Caitriona Ryan says most people get the science wrong.
There are two types of sweat gland. Eccrine glands are spread across the entire body, roughly two million of them, and produce the watery sweat responsible for cooling. Apocrine glands are concentrated in the armpits and groin, become active at puberty, and produce a thicker secretion.
Crucially, sweat itself has no smell, from either type of gland. Body odour happens when bacteria naturally present on the skin, particularly Corynebacterium, break that secretion down. The odour is generated on the skin's surface, not inside the sweat gland itself.
"Sweat itself is odourless, no matter where it comes from," says Professor Caitriona Ryan. "The body odour is actually the bacteria in those areas causing breakdown of the sweat."
It's also why young children never have body odour. Their apocrine glands simply aren't active yet. That changes at puberty, which is when body odour typically begins.
How Should Parents Talk to Teenagers About Body Odour?
Professor Caitriona Ryan is seeing puberty, and apocrine gland activity, start earlier than it used to. Where 11 or 12 was typical, some children now show early signs of body odour or sweat gland activity at eight or nine.
Her advice for parents:
- Have the conversation in private, never in front of siblings or friends
- Avoid words like "stinking" or flippant comments, even in jest
- Reassure the child that this is a completely normal, healthy part of puberty, not something to be ashamed of
- Introduce gentle daily cleansing of the armpits and groin, in the shower, without vigorous scrubbing (which can break down the skin barrier and make things worse)
- Explain the difference between an antiperspirant and a deodorant, and let them choose their own as part of the conversation
"This is a really important thing to get right, to take any shame away," says Professor Caitriona Ryan. "It's a completely normal process."
Antiperspirant or Deodorant: What's the Difference?
They're often used interchangeably, but they do different jobs.
An antiperspirant contains aluminium chloride, which works by blocking the sweat duct itself, reducing the amount of sweat that reaches the skin. Less sweat reaching the surface means less for bacteria to break down.
A deodorant doesn't reduce sweat. It masks the odour that bacteria produce once sweat is already on the skin.
Which one to reach for depends on the problem. Heavy sweating calls for an antiperspirant. If sweating itself isn't the issue but odour is, a deodorant is usually enough, alongside daily washing either way.
Why Nighttime Is the Best Time to Apply Antiperspirant
One detail from the interview worth repeating: apply antiperspirant before bed, not in the morning.
Sweat glands are far less active overnight, so the aluminium chloride has time to work into the duct without being washed straight back out by daytime sweating. Applied in the morning, much of the product's effect can be lost within the first hour, especially with the small bursts of sweating that come from everyday activity.
Is Aluminium in Antiperspirants Actually Safe?
Yes. Professor Caitriona Ryan is clear on this point: large-scale meta-analysis data has looked specifically at aluminium in antiperspirants and cancer risk, and found no evidence of an increased risk. The concern that circulated some years ago has not held up under proper review.
What About Whole-Body Deodorants?
Professor Caitriona Ryan is not in favour of them, and the reasoning is straightforward. Eccrine sweat glands across the rest of the body don't cause odour and don't interact problematically with bacteria the way apocrine glands do. Body odour is really only an issue in the armpits, groin and feet.
Blocking sweat everywhere else interferes with the body's overall cooling system for no real benefit. Antiperspirant and odour control are best kept localised to where the actual problem is.
When Sweating Becomes a Medical Problem: Hyperhidrosis
Hyperhidrosis, the medical term for excessive sweating, affects around one in 20 people, and Professor Caitriona Ryan says its impact is often underestimated. She describes patients who need to bring a change of clothes to work, who struggle to hold a pen, or who avoid shaking hands because of how much their palms sweat. It can genuinely limit career choices, social confidence and even wardrobe options.
Treatment depends on where the problem is and how severe it is:
- Medical-strength antiperspirants, applied at night, are the first step for many people
- Iontophoresis, a treatment for hands and feet where a mild electric current is passed through water to temporarily reduce sweat gland activity, when topical treatment isn't enough
- Botulinum toxin injections into isolated, severely affected areas such as the underarms, hands or feet, for cases that don't respond to the above
- Oral medications such as oxybutynin or glycopyrrolate, for more generalised sweating that can't be treated with a topical or localised approach
Professor Caitriona Ryan sees this often in the lead-up to weddings, with brides seeking treatment for underarm sweating well in advance of the big day, so it has time to take effect before they're in their dress.
The right starting point depends on where the sweating is and how it's affecting day-to-day life, and that's worth assessing properly rather than guessing at over-the-counter fixes.
Ready to Find Out If You're a Candidate for Treatment?
A hyperhidrosis consultation is available to assess whether you're a suitable candidate for treatment. If you're deemed suitable and choose to proceed on the same day, the consultation fee is waived and applied toward the treatment cost of €800.
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Frequently Asked Questions
Does sweat actually smell?
No. Sweat itself is odourless. Body odour is caused by bacteria on the skin breaking sweat down, not by the sweat itself.
Why don't young children have body odour?
Because the apocrine sweat glands responsible for body odour only become active at puberty. Only the eccrine glands, which don't cause odour, are active before then.
Should I use an antiperspirant or a deodorant?
An antiperspirant reduces how much you sweat by blocking the sweat duct. A deodorant masks odour but doesn't reduce sweat. Use an antiperspirant if heavy sweating is the issue, and a deodorant if odour is the main concern.
Is it true that antiperspirants are best applied at night?
Yes. Sweat glands are less active overnight, so the antiperspirant has time to work into the duct rather than being washed out by daytime sweating.
Is excessive sweating (hyperhidrosis) treatable?
Yes. Options range from medical-strength antiperspirants and iontophoresis to botulinum toxin injections and oral medication, depending on where the sweating occurs and how severe it is.
This article is based on Professor Caitriona Ryan's interview on RTÉ Radio 1's Supercharged with Anna Geary, broadcast 23 August 2026.
