Research
Insensible Water Loss Through the Skin
Your body loses water continuously even when you are not sweating at all. About 400 millilitres a day evaporates straight through the skin as pure water vapour, a process called transepidermal water loss (TEWL), or insensible perspiration. Roughly another 400 millilitres leaves through the breath, for a minimal insensible loss of about 800 mL/day in an adult. Unlike sweat, this water carries no salt and involves no sweat-gland activity; it simply diffuses through the skin barrier. This page sets out the verified figures, how they are measured, and why the hands and feet leak the most.
By the Sweat Explained Editorial Team · Published 2026-07-13 · Last reviewed 2026-07-13 · Educational information, not medical advice.
Key statistics at a glance
~400 mL/day
pure water lost through the skin by passive diffusion (insensible perspiration)
Brandis, Fluid Physiology
~400 mL/day
additional insensible loss through the respiratory tract (breathing)
Brandis, Fluid Physiology
~800 mL/day
total minimal insensible water loss in an adult
Brandis; StatPearls 2026
~50%
of baseline whole-body water loss (~30 g/h) passes through the skin
Taylor & Machado-Moreira 2013
80–160 g/h
TEWL from the hands, the highest-loss region of the body
Taylor & Machado-Moreira 2013
What is insensible water loss?
Insensible water loss is the water the body loses without any obvious sweating or sensation, hence "insensible." It happens along two routes: evaporation through the skin and evaporation from the moist lining of the airways as you breathe. According to Kerry Brandis's Fluid Physiology, daily loss through the skin is about 400 mL in an adult, insensible loss from the respiratory tract is also about 400 mL/day in an unstressed adult, and the minimal total insensible loss is about 800 mL/day.
The key distinction from sweat is that this water is solute-free. As Brandis puts it, insensible skin loss "differs from sweating as sweat contains solutes." No sweat glands are switched on; water molecules simply diffuse through the skin and evaporate.
Transepidermal water loss: diffusion, not sweating
The skin route has a technical name: transepidermal water loss (TEWL). Taylor and Machado-Moreira (2013), writing in Extreme Physiology & Medicine, describe it as vapour that "diffuses through the largely impermeable epidermis... and into the boundary layer of air," a passive process that "does not involve the sweat glands." That is what separates it from eccrine sweating, which requires active nerve signals to open the glands.
Their measurements put baseline whole-body water loss at about 30 g/h in adult males, with roughly half of that, about 50 percent, passing through the skin and the remainder through respiration. That baseline (~15 g/h through skin, or about 360 mL/day) lines up closely with the classical ~400 mL/day figure for insensible skin loss.
Why hands and feet lose the most water
TEWL is not spread evenly over the body. Taylor and Machado-Moreira (2013) measured resting transepidermal water loss at many sites and found the hands and feet lose water several times faster per hour than the trunk and limbs. Bars show the midpoint of each measured range; the range itself is in the note.
| Group | Value |
|---|---|
| Hands | 120 g/h (range 80–160 g/h) |
| Feet | 100 g/h (range 50–150 g/h) |
| Head & neck | 57 g/h (range 40–75 g/h) |
| Trunk, arms & legs | 37 g/h (range 15–60 g/h) |
Source: Taylor & Machado-Moreira, Extreme Physiol Med 2013. Chart is an original rendering of the cited data.
How it is measured, and why estimates vary
TEWL is measured with a small probe (an evaporimeter) held against the skin, which reads the water-vapour gradient just above the surface. Summing site measurements over the whole body, Taylor and Machado-Moreira report ranges of 0.02 to 0.07 mg·cm⁻²·min⁻¹, or about 26 to 43 g/h, equivalent to daily whole-body losses of roughly 0.6 to 2.3 litres for a person of 1.8 m² skin surface.
That upper end is well above the classical ~400 mL/day skin figure, and both can be correct: the classical value is a resting baseline, while whole-body evaporimeter surveys capture higher-loss regions and warmer conditions. Insensible loss rises with skin temperature and body-surface area and falls in humid air, because water evaporates more slowly when the surrounding air is already near-saturated.
Why insensible loss matters
For a healthy adult, insensible loss is a quiet but real part of the daily fluid budget, roughly 600 to 800 mL/day, or about 30 to 50 percent of total water loss, according to StatPearls. It is one reason fluid needs continue even on a cool, sedentary day when little visible sweat is produced.
TEWL also matters as a marker of skin-barrier health: a damaged barrier lets more water escape, so higher TEWL readings are used in dermatology research to gauge how well the skin is holding moisture. Insensible losses climb sharply when the skin barrier is compromised, for example across burns or in premature infants, whose thin skin loses water rapidly. Those are clinical situations that call for professional care and are outside the everyday range described here.
Methodology and limitations
This page combines a peer-reviewed physiology study (Taylor & Machado-Moreira, Extreme Physiology & Medicine 2013) for transepidermal and regional water-loss measurements with established physiology references (Kerry Brandis's Fluid Physiology text; StatPearls, NCBI Bookshelf 2026) for the classical route-by-route daily figures. Each number was traced to its source and confirmed.
Limitations: 'insensible water loss' is, by definition, difficult to measure precisely, and reported totals vary with the method used. The classical ~400 mL/day skin and ~400 mL/day respiratory figures are resting baselines for an average adult; whole-body evaporimeter surveys yield wider ranges (about 0.6–2.3 L/day) because they include high-loss regions and warmer conditions. Values depend on skin temperature, humidity, body-surface area, and skin-barrier integrity, and rise dramatically with burns or in neonates. Nothing here is a diagnosis or medical advice.
Frequently asked questions
- How much water does skin lose without sweating?
- About 400 mL a day of pure water evaporates through the skin of an average adult by passive diffusion (transepidermal water loss, or insensible perspiration) with no sweat-gland activity involved (Brandis, Fluid Physiology).
- What is the difference between insensible loss and sweat?
- Insensible loss is solute-free water that diffuses through the skin and evaporates without any sweating sensation. Sweat is produced by active sweat glands and contains salts and other solutes. Insensible loss happens constantly; sweating switches on when the body needs to cool.
- How much total water is lost insensibly each day?
- Roughly 800 mL/day in an adult: about 400 mL through the skin and about 400 mL from breathing. StatPearls estimates insensible loss at 600–800 mL/day, around 30–50 percent of total daily water loss.
- Why do the hands and feet lose the most water?
- Resting transepidermal water loss is highest at the hands (about 80–160 g/h) and feet (about 50–150 g/h), several times the rate of the trunk and limbs (about 15–60 g/h), reflecting differences in skin structure at those sites (Taylor & Machado-Moreira 2013).
- Does humidity change insensible water loss?
- Yes. Water evaporates more slowly when the surrounding air is already near-saturated, so insensible loss falls in humid conditions and rises when the air is dry or the skin is warm.
- Can insensible water loss become a health concern?
- In everyday life it is a normal, small part of the fluid budget. It rises sharply when the skin barrier is damaged, for example across burns or in premature infants, which are clinical situations that need professional care.
Sources
Primary peer-reviewed studies and official sources first, then reviews and institutional framing (secondary).
- Taylor NAS, Machado-Moreira CA. Regional variations in transepidermal water loss, eccrine sweat gland density, sweat secretion rates and electrolyte composition in resting and exercising humans. Extreme Physiol Med. 2013;2:4. Full text
- McNeil-Masuka J, Boyer TJ. Insensible Fluid Loss. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026. Bookshelf ID NBK544219. Full text
- Brandis K. Fluid Physiology: 3.2 Insensible Water Loss. anaesthesiaMCQ.com. Skin ~400 mL/day, respiratory ~400 mL/day, total ~800 mL/day. (secondary) Reference text
How to cite this page
Sweat Explained. Insensible Water Loss Through the Skin. Published 2026-07-13; last reviewed 2026-07-13. Available at: https://sweatexplained.com/research/insensible-water-loss-through-skin
Please cite the original studies for the underlying figures. Journalists are welcome to link to this page; the charts are original renderings of the cited data.