Page 215 - Industrial Ventilation Design Guidebook

I 76 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

illustrates that as conditions deviate from neutral the body activates mechanisms
to stabilize body temperature. These efforts all result in small but noticeable and
measurable increases in metabolism and physiological effort.

Body Temperature
To maintain proper body temperatures (T), metabolic energy (M) must be
continuously transferred to the environment.
Energy balance:

where

The body temperature limits for health in terms of internal or core tempera-
ture are fairly limited. The limits are basically related to the function of ner-
vous tissue. Body temperatures around 28 °C or less can result in cardiac
fibrillation and arrest. Temperatures of 43 °C and greater can result in heat
stroke, brain damage, and death. Often, too high a temperature causes irre-
versible shape changes to the protein molecules of nervous tissue. That is,
cooling overheated tissue to normal temperatures may not restore its original
function.
Metabolism
Metabolism is often characterized by a convenient, relative, and dimen-
sionless quantity called the met unit:

The metabolism of a resting person is then 1 met. Some met levels of vari-
ous activities are listed in Table 5.2.
Metabolic energy is often normalized in terms of energy per unit area of
2
skin (1 met = 58.2 W/m ):

BH TABLE 5.2 Met Level of Various Activities

Reclining -0.8 met Walking 5 km/h ~3 met
Seated and quiet -1.0 met Standing and heavy activity ~3 met
(heavy work, garage work)
Standing -1.2 met Basketball -5-8 met
Standing and light activity -1.6 met Max -10-12 met
(shopping, lab, light industry)
Standing and medium activity -2 met
(house work, machine work)