Page 231 - Industrial Ventilation Design Guidebook

i 92 CHAPTER S PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

FIGURE 5.12 Perceived ambient humidity by sedentary subjects.

stant skin wettedness levels, the RH sk, swelling, and softening of the skin in-
crease with increasing ambient absolute humidity.
Humans are sensitive to moisture and can reliably describe the humidity
10
of the environment using word scales as demonstrated in Fig. 5.12. The sub-
ject's humidity judgments appear to be functions of the air's dew point, a mea-
sure of absolute humidity, and are relatively unaffected by the ambient
temperature. Further, people are also good at perceiving skin moisture, as il-
lustrated in Fig. 5.13, where perceived skin wettedness is seen to correlate well
with measured skin wettedness.
In situations of prolonged sweating, skin wettedness slowly increases
with time because of accumulating salt on the skin. The increasing salt oc-
curs because the water in perspiration evaporates while the dissolved materi-
als, principally sodium chloride, remain on the surface. The salt lowers the
vapor pressure of the sweat film, decreasing its rate of evaporation per unit
area. The area of the film then naturally increases in order that evaporation
will equal the rate of sweat secretion. It is thought that part of the relief that
bathing brings after a warm day or strenuous activity is that by cleaning the
skin, perspiration can then evaporate more efficiently with reduced skin wet-
tedness.
Clothing can be one of the detractors from acceptability in humid envi-
26
ronments. Measurements by Gwosdow reveal that the friction between
skin and clothing increases abruptly for skin wettedness levels above 25%.
Further, fabrics are perceived to be rougher or to have a coarser texture and
to be less pleasant with increasing skin moisture. This may be one of the rea-
sons that, in the comfort studies cited earlier, the people have rarely indi-
cated they were comfortable when they had skin wettedness levels near and
above 25%.

Low Humidity
Low humidity also affects comfort and health. Comfort complaints about
dry nose, throat, eyes, and skin occur in low-hurnidity conditions, typically
when the dew point is less than 0 °C. Low humidity can lead to drying of the
skin and mucous surfaces. On respiratory surfaces, drying can concentrate
mucus to the extent that ciliary clearance and phagocytic activities are re-