You may have heard the term transepidermal water loss (TEWL), but do you know why and when it’s happening, and why you should care?

What is transepidermal water loss (TEWL)?

Transepidermal water loss is the loss of water that exits through your skin. It gives us a way to measure how readily water passes through the skin barrier.

This is one reason your skin may feel dry at certain times. TEWL can be affected by internal factors, such as your skin’s circadian rhythm, and external factors, such as the environment.

Researchers have found daily variation in TEWL, but how much water is lost and when it happens is not the same for every skin function or every part of the body.

It varies by skin function and time of day

For example, in a study of eight healthy women, researchers measured several skin properties on the cheek and inner forearm every four hours over 48 hours. The results show just how differently skin functions can change throughout the day.

• On the face, sebum production was highest around midday and lowest around midnight, with a smaller secondary peak around 4 a.m. TEWL, which measures water loss through the skin, showed two peaks, around 8 a.m. and 4 p.m., before reaching its lowest levels between about 8 p.m. and midnight. Facial skin pH stayed relatively stable during the day and then dropped during the night, reaching its lowest point around 4 a.m.
• The forearm showed a different pattern. TEWL also peaked around 8 a.m. and 4 p.m., but its 12-hour rhythm was stronger than its 24-hour rhythm. Skin temperature reached its lowest point around midday and rose to peaks around 4 p.m. and 4 a.m. Skin capacitance, a measure related to water content in the outer skin, showed three peaks, around noon, 8 p.m., and 4 a.m., with the largest peak around 8 p.m.

Interestingly, another human study found a different pattern. Researchers measured the transepidermal water loss every two hours on the forehead, forearm, upper back, and shin and found that TEWL was lower in the morning and higher in the evening and at night, indicating greater skin permeability during those hours.

The difference between these studies is important. Skin does have daily rhythms, but there isn’t necessarily one simple pattern that applies to every skin function or every part of the body. The timing and strength of a rhythm can vary depending on what is being measured, where it is measured, and how frequently it is measured.

Your environment adds another layer. Temperature and humidity can affect water loss through the skin and influence skin hydration and barrier function. Let’s take a look at both.

Temperature and TEWL

Temperature has a direct effect on TEWL. In experimental studies, raising skin temperature increased TEWL substantially. One study found that increasing skin temperature by 7–8°C approximately doubled TEWL in healthy skin.

Humidity and TEWL

Humidity is more complicated. Very dry air can increase water loss from the skin, but increasing humidity does not simply make TEWL rise or fall in a straight line.

In one study, increasing relative humidity from extremely dry conditions of 2–3% to 30–50% caused TEWL to increase two- to threefold. At approximately 73–77% relative humidity, however, TEWL fell back to near its original level. The researchers proposed that humidity changes both the water content and permeability of the outer skin layer, while also changing the difference in water vapor pressure between the skin and surrounding air.

A systematic review of human studies found that the effects of temperature, humidity, season, and climate on TEWL are not consistent across studies. The authors concluded that environmental conditions clearly matter, but the relationship is more complex than simply saying that hot or dry weather increases water loss.

TEWL and Ayurveda

Ayurveda looks at the qualities of the skin through the doshas and their underlying elements. Kapha is composed of water and earth and is associated with moisture, softness, stability, and unctuousness. Pitta is composed of fire and water and carries qualities of heat, sharpness, and oiliness. Vata is composed of space and air and is associated with dryness, lightness, and movement.

These qualities offer an Ayurvedic way to understand why skin can behave differently in terms of hydration, moisture, dryness, oiliness, and heat. A person with a kapha-dominant constitution may naturally have softer, more hydrated, or oilier skin. Pitta-dominant skin may have more oiliness along with warmth and a tendency toward redness or inflammation. Vata-dominant skin tends toward dryness and roughness and may have a greater need for both hydration and moisture.

That doesn’t mean everyone should have the same amount of kapha, pitta, and vata. Your prakruti, or individual constitution, determines your natural baseline. What matters from an Ayurvedic perspective is understanding your own constitution and recognizing when your skin’s current condition has moved away from that baseline, or vikruti.

This is particularly relevant when we talk about water loss. Kapha’s water and earth qualities are associated with moisture and stability, while vata’s dry and light qualities are associated with depletion and dryness. Pitta contains both fire and water, so its relationship with skin moisture can be more nuanced. When pitta is aggravated and its fire quality becomes excessive, that heat may diminish the moist, unctuous quality associated with its water element. This can contribute to dryness even in someone whose natural constitution is pitta-dominant and whose skin may otherwise tend toward oiliness or moisture.

Changes in your environment, diet, sleep, stress, and daily routine can influence these qualities and may be reflected in how your skin feels and behaves.

One observational study examining skin hydration in people with different Ayurvedic constitutions found higher skin hydration in kapha-dominant individuals and lower hydration in vata- and pitta-dominant individuals. The researchers related these differences to traditional Ayurvedic characteristics of the doshas, including kapha’s moist qualities and vata’s dry qualities.

TEWL gives us a modern way to measure one aspect of what is happening at the skin barrier. Ayurveda gives us a different framework for observing the qualities that may accompany changes in the skin, including hydration, moisture, dryness, oiliness, and heat.

Learn more about your skin’s circadian rhythm in the Organic Skincare Hub, including what happens during the 24-hour daily rhythm and what factors, besides TEWL, influence it.

This post is not intended as medical advice. It is merely provided as an educational resource. Consult your dermatologist or healthcare provider for personalized medical advice. 

Image by Maria Clara Alvarenga, Pexels