What Thai Humidity Actually Does to Your Workout

Here's a number that should change how you plan your next run. In a 2012 climate-chamber study, eight men cycled to exhaustion at the same 30°C on four separate days. The only thing that changed was the humidity. At 24% relative humidity they lasted 68 minutes on average. At 80%, the kind of sticky air most of Thailand knows well, they lasted 46.

Same temperature. Same bikes. Same people. About a third of their endurance gone, just from water in the air.

I live in Hua Hin, and I've come to think that most fitness advice written for cool, dry places quietly fails here. So this is the piece I wanted to read: what humid heat actually does inside your body, what the research says helps, and where the marketing (electrolyte tablets for a 20-minute jog, anyone?) gets ahead of the evidence.

Why humidity matters more than the thermometer

Your muscles are inefficient engines. Most of the energy they burn leaves as heat rather than movement, and during hard exercise that heat builds quickly. Your body can shed some of it by radiation and by warming the air around you, but once the air gets close to skin temperature, one route does nearly all the work: evaporating sweat.

Evaporation is the key word. Sweat that drips off your chin cools you very little. The cooling happens when liquid on your skin turns into vapor, because that change of state pulls heat out of you. How fast it can happen depends on the difference between the water vapor pressure at your skin and in the air around you. In dry air the gap is large and sweat disappears almost as fast as you make it. In humid air the gap shrinks, sweat pools and runs, and you get soaked without getting much cooler.

That's why the chamber study above is so striking. The researchers also reported each condition as wet-bulb globe temperature (WBGT), a heat-stress index that combines temperature, humidity and radiant heat. With the thermometer fixed at about 30°C, WBGT climbed from 19.6°C in the driest trial to 28.4°C in the most humid one. The drop in endurance wasn't significant at 40% humidity, but it was clear at 60% and 80%.

The practical lesson is simple: in Thailand, a 30°C morning and a 30°C afternoon can be completely different training days. Check the humidity or a heat index, not just the temperature.

What happens in your body, step by step

Your blood gets pulled two ways

When you start running in humid heat, your heart has two customers. Working muscles want blood for oxygen and fuel. Your skin wants blood too, because warm blood flowing near the surface is how heat travels from your core to your sweat glands. Your total blood volume can't grow in a few minutes, so heart rate climbs to keep both supplied.

You start losing plasma

Sweat is made from fluid drawn out of your bloodstream. As you lose it, plasma volume shrinks, each heartbeat moves a little less blood, and heart rate drifts higher even at the same pace. Physiologists call this cardiovascular drift. It's why an easy run in the heat can feel like a tempo run by the last few kilometers.

Core temperature rises and your brain steps in

When heat production outpaces heat loss, core temperature keeps climbing. Effort starts to feel harder well before anything dangerous happens, and your pace slows almost on its own. In the ice slurry trial discussed below, men running in 34°C heat after drinking cold water reached exhaustion at an average rectal temperature of about 39°C (39.05°C). In conditions like these, rising core temperature is one of the main things that sets the limit.

Salt leaves with the water

Sweat carries sodium, and the amount varies a lot between people. In a dataset of 1,303 athletes tested between 2000 and 2017, endurance athletes sweated an average of 1.28 liters an hour and lost about 52 mmol of sodium an hour. That's roughly 1.2 grams of sodium, or about 3 grams of salt, every hour. The spread was wide: the standard deviation for sodium loss was about 28 mmol an hour, so some people lose half the average and others far more.

Heat acclimation: the adaptation you get for free

Here's the good news. Your body adapts to heat, and it adapts fast. Train in it repeatedly and, over days to a couple of weeks, plasma volume expands, you start sweating earlier, your sweat gets less salty, and resting core temperature and heart rate come down.

An updated meta-analysis published in 2024 pooled 135 studies on heat adaptation. Exercise performance and capacity in the heat improved with a moderate-to-large effect (Hedges' g = 0.7). Resting core temperature fell (g = −0.6), resting heart rate fell (g = −0.5) and sweat rate rose (g = 0.4). The review also found moderate or larger changes in sweat sodium and chloride concentration and in resting plasma volume.

Two details from that review are worth knowing. The most common programs lasted 8–14 days. And the authors noted that the performance effect was smaller than an earlier 2016 analysis had suggested, which is a healthy correction in a field that loves big numbers. They also found passive heat exposure, like sitting in hot water or a sauna, to be a practical alternative to training in the heat.

If you've lived in Thailand for a while and exercise outdoors, you're probably already partly adapted. Visitors flying in from a northern winter are not, and the first week or two is the time to slow down, not to chase personal bests.

Hydration: two ways to get it wrong

Drinking too little

The American College of Sports Medicine's position stand on exercise and fluid replacement recommends preventing excessive dehydration, defined as losing more than 2% of body weight from water deficit, because it compromises performance. For a 70 kg person, that's 1.4 kg.

Drinking too much

The opposite mistake is less familiar and more dangerous. In a study of 488 runners at the 2002 Boston Marathon, 13% finished with low blood sodium, a condition called hyponatremia. Those affected tended to gain weight during the race, take four hours or longer to finish, and have a body mass index at either extreme. The researchers named excessive fluid intake as the single most important factor. If you weigh more after exercise than before, you drank more than you sweated.

Find your own number

The simplest test costs nothing. Weigh yourself in minimal dry clothing before and after an hour of training, and note how much you drank. Each kilogram lost is about a liter of sweat, and the ACSM stand itself points to body weight changes as the practical way to estimate sweat rate. Try it on a humid morning and again on a hotter afternoon, and you'll know your range. The target is to finish within about 2% of your starting weight, without ending up heavier.

Salt, sports drinks and electrolyte hype

Electrolyte marketing implies everyone needs a fizzy tablet before a short workout. The physiology says otherwise. For most sessions under an hour, ordinary meals and water replace what you lose. Sodium matters most when sessions run long, sweat rates are high, or you train twice a day in the heat. The ACSM's stated goal after exercise is to replace any fluid and electrolyte deficit, and how big that deficit is depends on your own numbers from the weighing test.

Thai food happens to be good at this. A bowl of khao tom or jok, a plate seasoned with nam pla, a side of salted egg: plenty of everyday Thai meals are salty enough to help you top up after a long, sweaty session. Coconut water, the tropical favorite, is rich in potassium but much lower in sodium than sweat, so it's a pleasant drink rather than a complete replacement after a long effort.

To be straight about our own product: Primaldew contains a pinch of Himalayan salt, mostly for taste, and a scoop doesn't come close to covering a training session's sweat losses. (Our look at Himalayan salt explains why the "84 minerals" story doesn't hold up either.)

Cooling tricks: what works and what's mostly theater

Pre-cooling means lowering your body temperature before you start, so you have more room before heat limits you. Two methods have the most research behind them.

Ice slurry. In a 2010 trial, 10 men drank 7.5 g per kg of body weight of an ice slurry (about half a liter for a 70 kg person) or the same amount of cold water, then ran to exhaustion at about 34°C and 55% humidity. After the slurry they ran 50.2 minutes on average, compared with 40.7 minutes after cold water, about 19% longer. Interestingly, they also stopped at a higher core temperature (39.36°C versus 39.05°C), which suggests the cold drink did more than just buy time.

Cold water immersion. A 2018 meta-analysis of 22 studies found that sitting in cold water before exercise improved endurance performance with a moderate effect (g = 0.53) and lowered peak core temperature. Ice ingestion showed a smaller effect (g = 0.2) whose confidence interval crossed zero, so the pooled evidence couldn't confirm it helps.

So the results conflict. One well-controlled trial showed a big benefit from slurry, while the pooled data favor immersion. For everyday training, a cold shower before you head out and cold drinks during a long session are easy wins. Cooling vests and specialty gear are mostly worth it for races and long events, not your Tuesday run.

What the research can't tell you yet

Heat research has real limits, and it's worth knowing them before you take any single number too seriously.

• Small, narrow groups. The humidity study had 8 participants and the slurry trial 10, all men. Women and older adults are underrepresented across much of this field.
• Labs aren't roads. Climate chambers use fixed fans, fixed workloads and no sun. Outdoor training adds radiant heat, wind and hills.
• Time to exhaustion isn't a race. Most trials ask people to keep going until they can't, which is more sensitive to small changes than a race over a fixed distance.
• Long-term data are thin. The 2024 review noted that longer adaptation programs may work better, but the data to prove it are lacking.

A practical plan for training in Thai heat

1. Train early or late. Coastal mornings before the sun gets high and evenings after it drops usually mean less radiant heat, even when humidity stays high.
2. Check humidity or a heat index, not just the temperature, and adjust pace on sticky days rather than forcing a target time.
3. Give yourself one to two weeks of easier training after arriving from a cooler climate or after a long break.
4. Do the weighing test to learn your sweat rate. Aim to finish within about 2% of your starting weight, and never heavier.
5. After long or very sweaty sessions, get sodium from food or a salted drink, not just plain water.
6. Pre-cool before important efforts: cold shower, cold towel, cold drink.
7. Stop and cool down if you feel dizzy, confused, nauseous or unusually unwell, and get medical help if it doesn't pass quickly.

Some people should talk to a doctor before pushing hard in the heat, including anyone who is pregnant, takes diuretics or blood pressure medication, or has a heart or kidney condition. If you want to see what else goes into a greens blend and why, our ingredients page lays it out.

FAQ

How long does it take to get used to training in Thai heat?

Most heat-adaptation programs studied in research last 8–14 days, and the main changes (more plasma volume, earlier sweating, lower heart rate) appear within that window. Keep intensity down for the first week or two and build from there.

Should I drink water or a sports drink when exercising in the heat?

For most sessions under an hour, water plus normal meals is generally enough. For longer or very sweaty sessions, adding sodium makes sense. Drinking large amounts of plain water over a long event, beyond what you sweat out, is how hyponatremia happens.

Is coconut water good for rehydration?

It's a pleasant fluid that's high in potassium, but it contains much less sodium than you lose in sweat. After a long session, pair it with salty food rather than relying on it alone.

Why do I feel so much worse running in Thailand than back home?

Mostly humidity. Moist air slows the evaporation of sweat, which is your main way of cooling in the heat, so core temperature and heart rate rise faster at the same pace. Give yourself time to adapt and judge your runs by effort, not pace.

This article is general information, not medical advice. If you're pregnant, taking medication or managing a health condition, check with your doctor before adding a new supplement.