**I. Index of thermal stress (ITS)**

The improved *heat balance equation* is:

_{}

where is the evaporation required to maintain heat balance, * * is the solar load, and metabolic heat production *H * is used instead of metabolic rate to account for external work. An important improvement is the recognition that not all sweat evaporates (e.g., some drips) hence required sweat rate is related to required evaporation rate by:

where *nsc* is the efficiency of sweating.

Used indoors, sensible heat transfer is calculated from:

For outdoor conditions with solar load, * * is replaced with and allowance made for solar load (R_{S}* *) by:

The equations used are fits to experimental data and are not strictly rational.

*Maximum evaporation heat loss* is:

and efficiency of sweating is given by:

but

nsc = 1, если

and

nsc = 0.29, если

The index of thermal stress *(ITS)* in g/h is given by:

where * * is the required evaporation rate , 0.37 converts into g/h and*nsc* is the efficiency of sweating (McIntyre 1980).

**II. Required sweat rate**

Similar to the other rational indices, is derived from the six basic parameters (air temperature (), radiant temperature (* *), relative humidity air velocity (*v*), clothing insulation (* *), metabolic rate (*M*) and external work (*W*)). Effective radiation area values for posture (sitting = 0.72, standing = 0.77) are also required. From this the evaporation required is calculated from:

Equations are provided for each component (see table 8 and table 9). Mean skin temperature is calculated from a multiple linear regression equation or a value of 36°C is assumed.

From the required evaporation (E_{reg}) and maximum evaporation (E_{max}) and sweating efficiency (*r*), the following are calculated:

*Required skin wettedness *

*Required sweat rate *

**III. Predicted 4-hour sweat rate (P4SR)**

Steps taken to obtain the *P4SR* index value are summarized by McIntyre (1980) as follows:

If , increase wet bulb temperature by .

If the metabolic rate *M* > 63 , increase wet bulb temperature by the amount indicated in the chart (see figure 6).

If the men are clothed, increase the wet bulb temperature by .

The modifications are additive.

The (P4SR) is determined from figure 6. The *P4SR* is then:

**IV. Heart rate**

where *M* is metabolic rate, is air temperature in °C and P_{a} is vapour pressure in Mb.

Givoni and Goldman (1973) provide equations for predicting heart rate of persons (soldiers) in hot environments. They define an index for heart rate *(IHR)* from a modification of predicted equilibrium rectal temperature,

*IHR* is then:

* *

where *M* = metabolic rate (watts), = mechanical work (watts), clo = thermal insulation of clothing, = air temperature_{}, _{ = total metabolic and environmental heat load (watts), = evaporative cooling capacity for clothing and environment (watts).}

The equilibrium heart rate (in beats per minute) is then given by:

* *

_{}* for IHR* _{}225

that is, a linear relationship (between rectal temperature and heart rate) for heart rates up to about 150 beats per minute. For *IHR* >225:

that is, an exponential relationship as heart rate approaches maximum, where:

_{= equilibrium heart rate (bpm),}

65 = assumed resting heart rate in comfortable conditions (bpm), and t = time in hours.

**V. Wet bulb globe temperature index (WBGT)**

Wet bulb globe temperature is given by:

for conditions with solar radiation, and:

for indoor conditions with no solar radiation, where T_{nwb}= temperature of a naturally ventilated wet bulb thermometer, T_{a} = air temperature, and T_{g }= temperature of a 150 mm diameter black globe thermometer.