In genuinely hot conditions, the body can sweat up to two litres an hour. Most people also apply their antiperspirant at exactly the wrong time of day to deal with it. Professor Caitriona Ryan joined Sean Moncrieff on Newstalk this week to explain why we sweat, why it smells, and what's actually normal in weather like this.

Sweating is the body's main way of cooling itself down, driven by millions of sweat glands across almost the entire skin surface. Antiperspirants are most effective when applied at night to dry skin, not after a morning shower.

Why Do We Sweat in the First Place?

Sweating is thermoregulation, the body's way of cooling itself down when its internal temperature rises. Every mammal sweats to some degree. Sweat glands cover almost all of the skin, not just the underarms, though that's the area most people associate with sweating because of the visible patches and odour it can cause there. In reality, only around 1% of total sweat output comes from the underarms. The rest evaporates from the rest of the body, largely without being noticed.

How Much Do We Actually Sweat in Hot Weather?

Under normal conditions, the body loses about half a litre to a litre of fluid a day through what dermatologists call insensible losses, sweat so gradual it isn't noticed.

In genuinely hot conditions, that figure can climb to two litres an hour. That scale of fluid loss is why rehydration matters more in a heatwave than most people account for.

Why Does Sweat Smell, If It's Mostly Water?

Sweat itself is roughly 99% water, with some salt. It doesn't smell. The odour associated with sweating comes from bacteria on the skin breaking down that moisture, not from the sweat itself.

"It's not the sweat, it's the bacteria that's in the skin and that area that causes the smell." - Professor Caitriona Ryan, Institute of Dermatologists

What's the Difference Between an Antiperspirant and a Deodorant?

They're not the same product, even though many formulas combine both functions. A deodorant only masks odour. An antiperspirant, usually formulated with aluminium salts, actually reduces sweating by temporarily blocking the sweat duct openings so less moisture reaches the skin's surface.

Why Does Antiperspirant Work Better at Night?

Sweat glands are less active overnight, so applying antiperspirant to clean, dry skin before bed allows the aluminium salts to form a more effective plug in the duct.

That protection holds through a morning shower, since the reaction has already taken place inside the duct rather than sitting on the surface where it could be washed off. Applying it after a morning shower is less effective for a second reason too: showering itself can trigger a small amount of sweating, which can undo some of the product's effect straight away.

Is It Ever a Problem to Sweat Too Much?

For some people, yes. Hyperhidrosis is a condition where sweating goes well beyond what's needed for thermoregulation, sometimes across the whole body, sometimes isolated to the underarms, hands, feet, scalp, or face. It can be genuinely disruptive, requiring frequent clothing changes or interfering with everyday tasks.

Dermatologists treat it with botulinum toxin injections for the underarms, face, or scalp, effective for roughly 9 to 12 months per treatment, or with iontophoresis, a device that uses a mild electric current, for the hands and feet. This is a medical treatment for a diagnosed condition, assessed and managed by a dermatologist rather than something to self-treat.

Should Children Be Sweating as Much as Adults in This Weather?

Not in the same way, and that's actually a risk factor worth knowing. Children are born with the same number of sweat glands as adults, but those glands aren't fully functional until puberty.

Combined with a higher body surface area relative to their body mass, this means children can struggle to regulate their temperature as effectively as adults in hot weather, and can overheat more easily once the air temperature approaches their body temperature. Keeping children cool in a heatwave matters more than it might for an adult managing the same conditions.

One Important Note: Not All Summer Rashes Are Heat Rash

Sweat-related skin reactions and sun-related skin reactions are often confused for one another, but they're managed differently. For a full breakdown of heat rash, sunburn, and chafing, see our guide: Sunburn, Heat Rash and Chafing: Professor Caitriona Ryan's Guide to Irish Summer Skin Problems. If the rash appears a few days into a holiday rather than immediately, it may be polymorphic light eruption rather than heat rash, covered in detail here: What Is Polymorphic Light Eruption?

Frequently Asked Questions

Is it safe to use aluminium-based antiperspirant?

Yes. Past concerns linking aluminium in antiperspirants to Alzheimer's disease or cancer have not been supported by evidence. It remains safe for regular use.

Can I use antiperspirant anywhere other than my underarms?

Yes, on troublesome areas like the groin, as long as it doesn't irritate the skin there. Whole-body use isn't advisable, since the body needs to sweat elsewhere to regulate its temperature properly.

If I don't sweat much during exercise, does that mean something's wrong?

No. Sweat that evaporates quickly is often simply not noticed. It doesn't mean the body isn't cooling itself effectively.

Is sweating a lot during exercise linked to a fast metabolism?

No, this is a common misconception. How much visible sweat someone produces during exercise isn't a reliable marker of metabolic rate.

Can talcum powder help with sweaty areas?

It can help absorb moisture in areas prone to sweat-related irritation, such as underneath the breasts, and may reduce associated rashes.