Survival in Hot Environments

13.4 Survival in Hot Environments

The same considerations apply to survival in hot environments as do for determining the upper lethal limit of animals. However, one additional factor needs

of sweating. The rate of sweat evaporation may be either environmentally or physiologically controlled. If the skin surface is wet, the rate of water loss

sweating is given by Eq. (12.16) with

If the skin surface is not wet, latent heat loss is controlled by sweat rate. Control of sweat rate is still not entirely understood, but apparently it involves sensing of surface heat flux (Kerslake 1972). Thus, changes in metabolic rate or external environment can cause changes in

Survival in Hot Environments 217

- maximum sweat rate

0.0 1.0 2.0 3.0 4.0 5.0 6.0 Air Vapor Pressure

13.4. Latent heat loss from the skin of heat-stressed humans as a function of vapor pressure and total vapor conductance. Skin temperature is assumed to be 35 C.

F IG URE

clothing and boundary layer conductance, and atmospheric vapor pres- sure. Boundary layer conductance for vapor transport is given by Eq. 7.33. In a 2.5

wind, boundary layer conductance,

2.5 mol

1.4 0.17 0.8 - The rightmost line in Fig. 13.4 therefore corresponds to the evaporation

rate wet

without clothing.

Example 3.2. What clothing conductance would allow skin to remain dry in a heat-stressed person if the vapor pressure of the air is 1.5

Solution. Consulting Fig. 13.4, with = 1.5 it is found that a conductance of 0.2 mol

limits latent heat loss, but a conductance of

does not. A conductance between these two, say 0.25 rnol ,

should therefore allow the to remain dry. This is the combined cloth- ing and boundary layer conductance. If the boundary layer conductance is 0.8 mol

then the clothing conductance would be

Survival under heat-stress conditions can be predicted using the energy budget equation, but it needs rederived without the assumption that

is combined with

as it is in Eq. (12.1 1).

most easily done

Humans and their Environment

by drawing an equivalent electrical circuit like Fig. 12.1, with thermal conductors being represented by electrical conductors, temperatures (heat concentrations) by voltages, and heat flux densities by current sources or sinks. The new diagram is like Fig. 12.1 except that the heat source in the body is M -

and an additional heat sink is added at the skin surface equal to

Writing the energy balance equation for this circuit gives

As an example of the use

we investigate the effect of clothing on the maximum operative temperature that can be tolerated by a person at various rates. We assume

(Table 12.2, vasodilated), = =

2.8 mol

see the previous example). Results of the calculations are shown

0.1 M, and

13.5. The part of the graph which shows increasing operative temperature with decreasing clothing conductance corresponds to the part of Fig. 13.4 where

is at its maximum. Adding clothing does not decrease the rate of evaporative cooling because it is already limited at the maximum sweat rate of the person. The clothing does, however, decrease the heat load on the person because the environment temperature is higher than

Heat and Vapor Conductance of Clothing

13.5. Maximum tolerable operative temperature for aperson as a function of clothing conductance. Vapor pressure is 1

F IG URE

wind speed if

The Humid Operative Temperature

the body temperature. Adding clothing therefore allows the person to tolerate a hotter environment. In a desert, with low vapor pressure and high solar loads, adding clothing (up to a point) decreases, rather than increases heat load on a person. The inflection point of the graph occurs when coat conductance becomes small enough to start controlling water loss.

Keep in mind that Fig. 13.5 is for a low vapor pressure. It does not apply at higher vapor pressure where any decrease in clothing conductance would reduce latent heat loss. If the atmospheric vapor pressure is high enough to keep the skin wet without clothing, then any addition of clothing will decrease dissipation of heat. This brings out the point that clothing must be matched to environment to be most useful in minimizing heat stress. Proper clothing for one hot environment would not necessarily be proper clothing for another.