Optimizing the Measurement of Skin Wettedness in Exercising Humans

Purpose Skin wettedness is the proportion of the skin that is wet at any given time. Skin wettedness is an important contributor to the decision to initiate thermoregulatory behavior, particularly during instances in which sweat builds up on the skin. The direct measurement of skin wettedness involves measuring the partial pressure of water (P H2O ) at a distance of 1–2 mm from the skin. During situations involving movement (e.g., exercise) the close proximity of the humidity sensor to the skin risks supersaturation, which occurs when a drop of sweat enters the humidity sensor. This artificially saturates the sensor independent of an increase P H2O on the skin. Thus, raising humidity sensors off the skin as far as possible, but without compromising the measurement, is advantageous to avert the risk of supersaturation. Some evidence suggests that raising the sensors 6 mm off of the skin results in skin P H2O responses that are comparable to 2 mm. However, this has never been experimentally tested. Therefore, we tested the hypothesis that raising sensors 6 mm above a surface saturated with water resembles the P H2O responses observed at 2 mm. Methods Following a 5 min baseline period on a dry surface, humidity sensors (hydrochron iButtons) were placed on a paper towel saturated with water for 60 min. The paper towel was placed on a water perfused mat set to elicit 32.1±0.4°C (T32) or 34.3±0.2°C (T34). These temperatures mimic the skin temperatures observed during exercise in a 20–28°C environment. During each testing period, five humidity sensors were placed at a distance of 2, 4, 6, 8, and 10 mm above the mat. For a given humidity sensor, the height was randomly assigned and all heights were measured during each testing period to eliminate any between trial variability. Each height was measured on five occasions. All testing was conducted in a 27±1°C, 25±6% relative humidity environment. The humidity sensors measure relative humidity and temperature, which were later converted to P H2O . Data are presented as mean±SD. All comparisons were made to 2 mm, the standard height of measurement. Results In T32, P H2O increased in all heights within the first 5 min and plateaued until 25 min (P≥0.41), where after P H2O returned toward baseline levels. P H2O was lower (P<0.01) than 2 mm at 30 min in 8 mm (−1.0±0.9 mmHg) and 10 mm (−1.3±0.6 mmHg). For 6mm, P H2O was lower than 2 mm at 35 (−1.4±0.8 mmHg, P<0.01) and 40 min (by −0.9±0.9 mmHg, P<0.01). The 4 mm height was not different from 2 mm at any time (P≥0.56). P H2O for all heights were not different to 2 mm from 45–65 min (P≥0.11). In T34, P H2O increased within 5 min and plateaued through 15 min (P≥0.90), where after P H2O returned toward baseline levels. For 10 mm, P H2O was lower than 2 mm at 20 min (by −0.4±0.5 mmHg, P<0.01). For 8 mm, P H2O was lower than 2 mm at 25 min (by −1.0±0.6 mmHg, P<0.01). 4 mm (by −0.7±0.8 mmHg, P<0.01) and 6 mm (by −1.1±1.7 mmHg, P<0.01) were lower than 2 mm at 30 min. P H2O for all heights were not different to 2 mm from 35–65 min (all P≥0.22). Conclusion Compared to 2 mm, at temperatures of 32°C and 34°C, raising a humidity sensor 6 mm off the surface underestimates P H2O during dynamic changes in P H2O . Thus, to minimize the risks of supersaturation and measurement bias, P H2O during exercise should likely be measured at a height of 4 mm above the surface of the skin. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .