We have already seen how the surrounding temperature can affect mortality (Note 3.C). It also affects our perception of comfort. People prefer a neutral temperature (i.e. neither too cool nor too warm) which depends partly on what they are currently used to. This preferred temperature is typically 21-22°C in air-conditioned buildings in the USA and Australia if the outdoor monthly mean temperature is below 10°C, but 24-25°C when the outdoor mean is above 20°C. The difference is chiefly due to clothing. However, the neutral temperature depends also on external factors such as the wind, sunshine and atmospheric humidity, and personal factors such as the amount of exercise being undertaken. The various components in the human-energy balance are shown in Figure 5.11. Clothing, wind, radiation and humidity control the loss of heat from the skin, and comfort generally arises from maintaining the skin at about 33°C. In that way, the core of the body can be kept close to the optimal 37°C, even when heavy exercise generates much heat.
The effect of the wind is generally to remove heat from the skin, cooling the body. But heat is transferred to the body by the wind when the air is hotter than the skin, worsening conditions. In that case, an energy balance can be maintained only by profuse perspiration (Section 5.5).
The air's humidity affects comfort by influencing the rate of evaporation of sweat, and hence the amount of evaporative cooling of the skin (Note 4.H). In fact, the sweating person is like a wet-bulb thermometer, so that the wet-bulb temperature has been suggested as a criterion of comfort. Unfortunately, that ignores the effects of radiation (Note 5.E), exercise (Table 6.5) and clothing (Table 6.6).
Many indices of climatic comfort have been proposed to allow for those other factors, apart from the wet-bulb temperature and the Relative Strain Index (Note 4.H). One of these involves a weather-stress index, discussed in Note 6.E, and another is a thermal sensation scale described in Note 6.F. More recently, extensive experimentation has led to the general adoption of the standard effective temperature (SET), as the basic index of comfort (Note 6.G). This is the temperature of air at 50 per cent RH (when the wind is 0.5 km/h and radiation to the body is equalled by that from it) which yields the same skin temperature and fraction of the skin which is wet as occurs in reality, in the actual temperature, humidity, radiation, activity and clothing conditions. It is assumed that the person is seated, with the corresponding metabolic rate (Table 6.5), and normal clothing (Table 6.6). This index of comfort is theoretically superior, but it depends on so many factors and is so complicated to derive that a simplified version is often used instead. This is the new Effective Temperature (ET*), the temperature of air at 50 per cent RH that creates the same heat loss from the body, assuming that standard radiation, wind, clothing and exercise conditions prevail. In other
Table 6.5 Typical metabolic rates of adults (W/m2); a rate of 60 W/m2 is considered standard but this varies widely with the individual
Relaxed standing 70
Walking on level at 3.2 km/h 115
Running at over 8 km/h 290 Driving car 60-115
House cleaning 115-200
Lifting, packing 1 20
Handling 50 kg bags 235
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