The Science of Comfort

Why we feel comfortable, how can it influence sustainable design, and how hot you can survive

"I'm freezing."

"Are you kidding me? I'm sweating!"

Sound familiar? Fighting over the thermostat is all too common. But why? Shouldn't we all just be comfy in the same room or car?

No. For several reasons. Understanding why humans feel comfortable under different conditions can help guide sustainable design. After all, houses, buildings, and vehicles are all designed to keep us comfy.

What Is Comfortable?

Human comfort is influenced by a combination of physical, physiological, and psychological factors. While temperature is central, comfort is shaped by many environmental variables that interact with human perception and regulation mechanisms.

Maintaining a stable core body temperature (98.6°F) is a key biological goal. The body uses mechanisms like sweating, shivering, changing blood flow, and altering posture or behavior to achieve this. If the core temperature drifts too far from this set point, it leads to discomfort, stress, and eventually danger.

Human body temperature normally ranges from 97 to 99°F. Hypothermia occurs when it drops below 95°F, and heat stroke when it rises above 104°F. The body's ability to regulate core temperature within a few degrees despite being exposed to external temperature variations of over a hundred degrees is incredible.

However, this is only part of the story. You can maintain a stable core temperature and still feel uncomfortable, like when you get a blast of cold air in your face on a wintry day, even though your organs are fine.

The skin is the primary sensory interface with the environment. Specialized nerve endings called thermoreceptors detect local changes in temperature and send signals to the brain.

Humans are especially sensitive to rapid changes or imbalances, not just steady-state temperatures. If one part of your body is cold (like a hand in ice water) and another is hot (hand in hot water), you don’t average it out. You experience the discomfort of contrast and imbalance. Thermal comfort depends not only on the core body temperature but also on the uniformity of skin temperature across the body.

Comfort is ultimately about thermal energy balance. The body must be in homeostasis, where the rate at which the body gains or loses heat matches the body’s metabolic heat production. If you're gaining more heat than you're shedding, you'll probably feel warm even if your core temperature hasn't changed yet.

So, what factors affect your thermal energy balance and, therefore, comfort?

1. Metabolic Rate

Let's start with an obvious question: If most of us have a body temperature of 98°F but seldom experience air temperatures anywhere near that high, especially in winter or cold climates, how do we stay warm?

Metabolism.

The food you eat, and stored glucose and body fat, combine with the oxygen you breathe in a series of chemical reactions in your body called respiration.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

These reactions provide the energy for you to live and move. Additionally, your body is constantly building up (anabolism) and breaking down (catabolism) tissues and substances. The sum of all of these reactions occurring in the body is called metabolism.

About 95% of metabolism is aerobic cellular respiration, which occurs in mitochondria. The rest of the metabolism supports growth, repair, signaling, storage, and defense. Respiration is about 40% efficient. That means about 60% of all the energy that we consume turns into thermal energy, which is how we maintain our core body temperature higher than the environment.

Physical activity increases your metabolic rate and, therefore, the amount of heat your body produces. That's why, when you walk outside on a 50°F day in a T-shirt, you feel cold, but if you start running, you'll start to feel warm.

Your level of thermal comfort depends on your level of activity. When you're exercising, you'll feel comfortable at a lower temperature than if you're sedentary because your body is turning more glucose and fat into heat.

Higher metabolism = you feel warmer.

Besides exercise, several other factors can influence metabolic rate, including age, body composition and size, hormones, temperature, diet, sleep, substances, illness, and genetics. I'll briefly describe the most influential factors, which should all be prefaced with "all else being equal..."

Metabolic rate declines with age, so younger people will tend to feel warmer. Muscle mass increases metabolism, so larger and more muscular people tend to generate more heat. Thyroid hormones increase metabolic rate, so having low thyroid hormones (T3, T4) will make you feel colder. Diet-Induced Thermogenesis (DIT), also called the thermic effect of food, is metabolism stimulated by digesting, absorbing, and storing nutrients in food. Protein has a very high thermic effect, where 20-30% of its calories are used in digestion. So, higher protein diets will make you feel warmer. Exposure to cold temperatures activates thermogenesis, especially in brown fat, increasing metabolism and making you feel warmer.

2. Air Temperature

Air temperature is the most obvious factor affecting how comfortable we feel. When the air is cold, we lose heat faster to the surrounding air molecules and may start to shiver to generate warmth. If it's too hot, we sweat to cool down. Temperature is the main factor that influences how we feel, and it's what HVAC systems are designed to control, but it's not the only factor.

3. Humidity

Relative humidity (RH) is the percentage of water vapor in the air compared to how much water vapor the air can hold at that temperature. It plays a big role in how warm or cold you feel. When your internal temperature gets too high, your sweat glands release water and salts onto the skin. As the sweat evaporates, it removes heat from the body, helping to lower your core temperature and maintain thermal balance.

Moisture is always evaporating from your skin, even if you aren't dripping with sweat. When you're perspiring just a little, and that moisture can be fully evaporated into the air at the same rate your sweat glands are bringing it to the surface of your skin, your body is still cooling down by evaporative cooling, even though droplets of sweat aren't forming.

The rate at which water evaporates into the air determines the rate of cooling of your body. Changing the phase of water from a liquid to a gas—in other words, evaporating it—requires roughly 680 watt-hours per liter. So, for every liter of water that evaporates from your skin, you are cooling down by 680 Wh, which is a significant amount of cooling.

The rate at which your sweat evaporates from your skin is partially determined by the humidity already in the air. The higher the RH of the air, the lower the evaporation rate of sweat and therefore, the lower your body's rate of cooling. At 100% RH, the air is already saturated with moisture, and your body is unable to evaporate any sweat into the air, which means sweating is completely ineffective.

The lower the RH of the air, the more effectively your body cools itself. This applies at all temperatures, even if it's not hot. All else being equal, you will feel cooler at lower relative humidities. 95°F and 90% RH will feel much hotter than 95°F and 20% RH, just as 50°F and 20% RH will feel cooler than 50°F and 90% RH.

Humans generally prefer RH between 30% and 60%.

4. Air Movement

It's a common misconception that fans make the air colder. Technically, fans make the air ever so slightly warmer, but the air moving over your skin increases convective heat loss and increases the evaporation rate of skin moisture, making you cooler. Whether the air movement is from fans or wind, greater air movement makes you feel colder.

The one exception is when the air temperature is above your body temperature, and you don't sweat. In that case, the hot wind will heat you up like a convection oven.

5. Radiant Heat

Radiation is one of the three primary methods of heat transfer. Radiation is unique, however, in that it does not require a medium. Heat can be transferred through a vacuum. In fact, that's how the radiation from the sun can travel through space to reach Earth.

Everything in the universe above absolute zero is constantly emitting electromagnetic radiation. Most things around you emit this radiation in the infrared part of the electromagnetic spectrum, which we feel as heat. Since it's invisible to our eyes, this heat transfer is not necessarily intuitive.

If you are sitting in a room where the air, the walls, and all surfaces around you are at 70°F, you might feel comfortable. The radiant energy flowing into your body from the surroundings balances with the energy leaving your body, leaving you feeling comfy. But if you're in that same room and everything is the same temperature, but it's a cold winter day and there's a cold glass window in line of sight of your body, there will be a net transfer of radiant energy out of your body and into the cold window, making you feel cold.

If instead of a cold window, there was a hot woodstove near you, you would feel much warmer even if the temperature of the rest of the room was the same.

The sun is a powerful source of radiation, with over 1000 watts of energy per square meter hitting the ground in sunny places. That's why when you're outside in the sun, you feel much warmer than when you're in the shade—even if the air in the sunny area is the same temperature as the air in the shade.

6. Clothing Insulation

Clothing acts as insulation, trapping a layer of warm air next to the skin and helping regulate body temperature. The insulating ability of clothing is measured in “clo” units—1 clo is roughly equal to what you'd get from wearing a typical business suit. The more clo units you're wearing, the colder the air can be while still feeling comfortable. At 60°F, wearing a t-shirt and shorts might make you feel cold, but in a sweater and pants, you'd likely be comfortable. Dressing appropriately for the conditions is one of the simplest ways to stay comfortable.

7. Psychological & Cultural Expectations

Interestingly, comfort is not purely physical. It’s also shaped by what we expect. If you’re from the hot and humid Philippines, 80°F may feel fine, but if you're from Fairbanks, Alaska, 80°F would feel sweltering.

Similarly, bright sunlight or cozy furnishings can make a space feel warmer, even if the temperature hasn’t changed. People also tend to tolerate temperatures better if they feel in control, like being able to open a window or adjust a fan. Cultural norms, personal habits, the color of lighting, and even visuals (seeing a fire or a snowy scene) can influence how comfortable we feel.

Modeling Comfort

We're comfortable when we aren't thinking about how we're uncomfortably hot or cold. Engineers model comfort using heat transfer equations and concepts like the Predicted Mean Vote (PMV) and Standard Effective Temperature (SET) when designing spaces to be comfortable.

Design and Lifestyle

By understanding these 7 factors that affect human comfort, we can design homes and buildings to achieve comfort while reducing energy input. For example, we can use awnings over south-facing windows that keep the summer sun out but let the winter sun in.

In our daily lives, we can use these principles to maintain comfort while using less energy. Instead of keeping our house at 72°F in the winter, we can keep it at 68°F and wear a thick sweater. Or, instead of keeping our house at 72°F in the summer, we can keep it at 78°F and use fans, which are a lot less energy-intensive than cooling the whole house.

We can keep warm in the winter even if we keep our homes a little bit cooler by increasing our metabolic rate. That means being more active. I keep my house pretty cold in the winter to save on electricity. When I get cold, I do some push-ups, pull-ups, or burpees. This increases my metabolic rate and warms me up. I feel comfortable, save electricity, and get some extra exercise in.

Finally, we can adjust our psychological expectations of what it means to be comfortable (AKA, deal with it, princess). Humans evolved to survive and thrive in a wide range of temperatures, without sophisticated climate controls. Being comfortable 100% of the time is not only unrealistic but also may not be healthy. In fact, there is ample evidence that shows being hot or cold acts as a hormetic stressor, leaving the body healthier and more resilient overall. If you deliberately put yourself in situations where you're either very hot or very cold, you'll expand your resilience and not complain or even notice when the temperature strays from a 2°F window.

How hot is too hot?

At this point, you're probably dying to know (pun so intended) – "What is the maximum temperature that a human can survive indefinitely?"

That's a fascinating question, and maybe not entirely irrelevant on a warming planet. I will go into the details and math in the "nerds only" section at the bottom. I'll keep it brief here.

In order for a human to survive in hot temperatures, heat loss (evaporative + convective + radiative) must be greater than or equal to metabolic heat production.

Now, we've got to mention wet bulb temperature. Wet bulb temperature is the lowest temperature that air can reach through evaporation alone. It reflects how well sweat can evaporate and cool the body.

To measure it, a thermometer is wrapped in a wet cloth and exposed to moving air. As water evaporates, it cools the thermometer. The drier the air, the more evaporation occurs, and the lower the wet bulb temperature. In humid air, less evaporation occurs, so the wet bulb temperature is closer to the actual air temperature (dry bulb temperature).

Wet bulb temperature is a key measure of heat stress on humans because it combines heat and humidity into a single value. If it gets too high, the body can no longer cool itself effectively, even in shade or with unlimited water, making survival impossible for extended periods.

The maximum wet bulb temperature that a human can survive at is about 92°F.

A human can survive indefinitely at a dry-bulb air temperature of up to 115°F (46°C), provided very low relative humidity (around 10%), no direct sun exposure, a resting state with minimal physical exertion (to keep metabolic heat production low), an adequate and continuous supply of water, and a physical environment that allows sweat to evaporate efficiently—meaning little clothing and good air circulation. Under these circumstances, the body can lose enough heat through evaporation to maintain a stable core temperature. However, if any of these conditions are not met, or if the air temperature rises above this threshold, the body’s ability to shed heat through sweating becomes overwhelmed, and core temperature begins to rise, making long-term survival impossible.

Nerds Only:

What is the highest temperature a human can survive indefinitely, given unlimited water, 10% relative humidity, and no sun exposure?

To survive indefinitely in heat, the body must maintain thermal balance. This means that the amount of heat the body produces must be equal to or less than the amount of heat it can lose to the environment. The key ways the body loses heat are through evaporation (sweating), convection (air movement), and radiation (heat transfer to surrounding surfaces). The most important of these in hot, dry environments is evaporation.

Step 1: Estimate human metabolic heat production

A resting human produces about 80 to 100 watts of heat continuously. For safety, we assume:

Qmet = 100 watts

This is the amount of heat the body needs to shed to avoid overheating.

Step 2: Estimate evaporative cooling from sweat

Sweating cools the body through evaporation. The heat loss from evaporating sweat is given by:

Qevap = m * L

Where:

Qevap = heat loss through evaporation (watts)

m = rate of sweat evaporation (kg/s)

L = latent heat of evaporation of water ≈ 2.43 x 10^6 J/kg

Assume a maximum sustainable sweat evaporation rate:

m = 1.0 L/hour = 1.0 / 3600 = 0.000278 kg/s

Then:

Qevap = 0.000278 * 2.43 x 10^6 ≈ 676 watts

This shows that under ideal conditions, the body can remove up to 676 watts of heat through sweat evaporation—far more than the 100 W it needs to dissipate. But this assumes sweat actually evaporates, which depends on humidity and air temperature.

Step 3: Effect of humidity and vapor pressure

Evaporation rate depends on the difference in water vapor pressure between the skin and the surrounding air. The driving force for evaporation is:

E = he * (Psk - Pa)

Where:

E = evaporative heat loss (W/m^2)

he = evaporative heat transfer coefficient (~16 to 32 W/m^2·kPa)

Psk = vapor pressure at the skin surface (~5.6 kPa at 35°C skin temperature)

Pa = vapor pressure in ambient air

Ambient vapor pressure Pa depends on temperature and humidity:

Pa = RH * Psat(Ta)

Where:

RH = relative humidity (as a decimal, e.g., 0.10 for 10%)

Psat(Ta) = saturation vapor pressure at ambient air temperature Ta

Saturation vapor pressure can be estimated using Tetens' formula:

Psat(T) = 0.6108 * exp((17.27 * T) / (T + 237.3))

Where T is in °C and Psat is in kPa

We want to find the highest temperature Ta where the vapor pressure gradient (Psk - Pa) is still large enough to allow evaporation to remove at least 100 W of heat. At 10% relative humidity, vapor pressure in the air remains low even at high temperatures, so sweat can continue to evaporate effectively.

Step 4: Estimate maximum survivable temperature

According to models used in biophysics and thermal engineering (e.g. ASHRAE), and supported by recent physiological studies, humans can survive indefinitely at rest in dry air up to:

Max survivable temperature (at 10% RH, no sun):

Ta ≈ 115°F (46°C)

Above this temperature, even in very dry air, the rate of sweat evaporation will not be enough to shed all metabolic heat, and core body temperature will begin to rise.