How Humidity Changes the Way You Sweat

If you've ever gone for a run on a humid summer day, you've probably noticed something strange. Sweat pours off your body, your shirt gets soaked, yet somehow you still feel unbearably hot.

Meanwhile, on a dry day at the exact same temperature, you might sweat less, feel cooler, and perform better.

So what is actually happening?

The answer is that sweat itself does not cool you down. Sweat only cools you down when it evaporates.

Humidity changes how easily sweat evaporates, and that has a massive effect on how your body regulates heat.

Sweat Is Your Body's Air Conditioner

When your core temperature rises, your brain activates sweat glands distributed across your body. Sweat is secreted onto the skin surface, and as it evaporates, it removes heat from your body. This process is called evaporative cooling.¹

In cool weather, your body can also lose heat through radiation, convection, and conduction. But as air temperature rises, evaporation becomes the primary way humans dissipate heat. In very hot conditions, it is often the only effective cooling mechanism available.²

This is why sweating is so important during exercise in the summer.

But there is a catch.

The amount of sweat on your skin does not determine how much cooling you get.

The amount of sweat that evaporates does.

Humidity Slows Evaporation

Humidity is simply a measure of how much water vapor is already in the air.

When humidity is low, the air can easily absorb more water. Sweat evaporates rapidly and efficiently cools your body.

When humidity is high, the air is already filled with moisture. Sweat evaporates much more slowly because there is less room for additional water vapor.¹

Researchers describe this as a reduction in the environment's "evaporative capacity." As humidity rises, the environment becomes less capable of accepting evaporated sweat, causing sweating efficiency to decline.³

This is why humid days feel so oppressive. Your body is producing sweat. The sweat is sitting on your skin. But much of it is no longer providing cooling.

As exercise physiologist Ollie Jay explains, "it isn't sweat that cools us down, it is when sweat evaporates."⁴

Your Body Often Sweats More In Humid Conditions

Most people assume humidity makes them sweat more because sweat cannot evaporate.

That is partially true.

However, your body also actively increases sweat production when it senses that cooling is becoming less effective.

Several laboratory studies have shown that sweat rates rise as environmental heat stress increases, including conditions with elevated humidity.³⁻⁵

A 2025 study had trained cyclists perform exercise in 33°C (91°F) heat under four different humidity conditions. Researchers found that increasing humidity significantly reduced evaporative heat loss and increased thermal strain, despite the athletes producing large amounts of sweat. The higher the humidity, the less efficient sweating became.¹

Similarly, a 2021 review on exercise in the heat concluded that humid environments impair sweat evaporation, elevate body temperature, and increase cardiovascular strain during exercise.⁶

In other words, your body often responds to poor evaporation by sweating harder.

Unfortunately, more sweat does not necessarily mean more cooling.

An Example At The Same Temperature

Imagine two runners performing the exact same workout at 90°F.

• Runner A trains in 90°F with 20% relative humidity.

• Runner B trains in 90°F with 80% relative humidity.

Runner B will likely feel much hotter.

Their skin will remain wetter because sweat evaporates more slowly. Their body temperature will rise faster. Their heart rate will typically be higher. And they may actually sweat more overall in an attempt to compensate for the reduced cooling efficiency.¹ ³ ⁶

Meanwhile, Runner A's sweat evaporates quickly, allowing their body to remove heat more efficiently. The temperature is identical. The cooling power of sweat is not.

Real Sweat Rate Differences Can Be Large

Sweat rates vary tremendously between people.

Fitness level, body size, heat acclimation, exercise intensity, clothing, airflow, and genetics all play a role.⁷

That makes it difficult to say exactly how much sweat humidity adds for every individual.

However, controlled laboratory studies show that the effect can be substantial.

One study examining environmental conditions during exercise found that whole body sweat rate increased by approximately 0.3 liters per hour when moving from cooler to moderately hot environments, and by roughly 0.7 liters per hour in hotter conditions.⁵

Other reviews report sweat rates commonly ranging from 0.5 to 2 liters per hour during exercise, with highly trained athletes occasionally exceeding 3 liters per hour in extreme heat.⁷ ⁸

In humid environments, sweat losses can become especially large because the body continues producing sweat even though a smaller fraction of it evaporates effectively.³

That means you may lose far more water than expected while receiving less cooling benefit from each drop.

Why Humidity Feels Worse Than Dry Heat

This explains one of the most common summer experiences. You can walk outside into 95°F Arizona heat and think:

"Wow, it's hot, but manageable."

Then you travel somewhere with 95°F temperatures and 80% humidity and suddenly feel exhausted within minutes. Your body is not imagining it.

The humid environment physically reduces the ability of sweat to evaporate and remove heat.¹ ⁴

The result is:

• Higher skin temperatures• More sweat accumulating on the skin• Greater cardiovascular strain• Faster dehydration• Higher risk of heat exhaustion and heat illness¹ ⁶

Humidity does not stop sweating. It stops sweating from doing its job.

The Bottom Line

Sweat is your body's most important cooling system during exercise in the heat.

But sweat itself is not what cools you.

Evaporation is.

On dry days, sweat evaporates quickly and efficiently removes heat.

On humid days, evaporation slows dramatically. Your body often responds by producing even more sweat, but less of that sweat actually cools you down.

The result is a frustrating combination:

You sweat more.

You lose more fluids.

And you often feel hotter anyway.

That is why hydration becomes even more important when humidity rises.

Because sometimes the sweatiest days are also the days when sweat works the least.

Sources

1. Bright FM, et al. Elevated Humidity Impairs Evaporative Heat Loss and Self-Paced Exercise Performance in the Heat. Scandinavian Journal of Medicine & Science in Sports. 2025.

2. Périard JD, et al. Exercise under Heat Stress: Thermoregulation, Hydration, Performance Implications and Mitigation Strategies. Physiological Reviews. 2021.

3. Muhamed AMC, et al. The Effects of a Systematic Increase in Relative Humidity on Thermoregulatory and Circulatory Responses During Prolonged Running Exercise in the Heat. Temperature. 2016.

4. Jay O. Dew Point and Sticky Sweat: Why Humidity Is So Uncomfortable. The Guardian. January 2024.

5. Foster J, et al. Delineating the Impacts of Air Temperature and Humidity for Athletic Training and Competition. Experimental Physiology. 2022.

6. Xu Y, et al. Effects of Hot and Humid Environments on Thermoregulation and Aerobic Endurance Capacity. Frontiers in Physiology. 2022.

7. de Korver R, et al. Humans Exercising in the Heat: A Review on Sweat Models and Sweat Rate Prediction. Temperature. 2025.

8. Marriott BM, et al. Water Requirements During Exercise in the Heat. National Academies Press. 1993.