Thermal comfort theory in uniform, steady state environments is nowadays widely accepted and measurement methodologies exist, which have been defined in standards like ISO 7730 or ASHRAE Standard 55 [28], [3]. Most of thermal comfort knowledge available today has been gained in the in the past 30 years. However, this knowledge is mainly based on research conducted under the assumption of homogeneous and steady-state laboratory conditions. In practice, there is hardly any steady-state or homogeneous hypothetical environment. For example, local discomfort may arise due to local convective cooling by draught, cooling and heating by radiation, as well as cold feet and warm head caused by vertical temperature differences [5], [75]. Assessment of thermal non-uniform environments is difficult due to a lack of general knowledge about the superposition and influence of multiple thermal sources [26]. Today thermal comfort in transient and non-uniform environments is far from fully understood and researchers are still arguing about the exact interrelationships [122]. Therefore a general theory cannot be presented in this thesis, however some findings of the latest research will be summarized.
In uniform and steady state environments, there is a linear relationship between whole body thermal sensation, thermal acceptability and thermal comfort. On the ASHRAE thermal sensation scale (Chapter 2.4.1.1), overall thermal comfort is assumed to occur between -1
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(slightly cool) and +1 (slightly warm) [2], [28]. These linear correlations are not applicable to non-uniform and transient environments. According to [136], subjects report more discomfort, the more the non-uniformity of thermal sensation they perceive, even if they are in overall thermal neutrality. Overall thermal comfort is strongly influenced by single body parts. Thermal sensation across the body depends on [134]:
The body‟s local thermo regularity mechanism,
Asymmetry in clothing insulation,
Thermal sensitivity of the individual body parts,
Rate of change in body‟s skin and core temperatures.
Feet adapt very slowly to the environment. Long adaption times up to 5 hours may occur due to poor blood circulation [36]. It is well known that cold feet are less comfortable than warm feet. This may be justified due to low core temperatures at feet level. It could be possible that the body therefore reacts more sensitive to further decrease in temperature.
The opposite applies to the head which prefers cooler ambient temperatures. Thermal comfort standards therefore point out that increasing temperature from the foot to the head may be less acceptable than increasing temperature level from head to feet area [1], [2], [28]. The head is crucial for thermal comfort sensation and less importance is accounted for hands and feet [36]. According to [134], the back, chest and pelvis are the most dominant body parts and the brain seems to be more sensitive to cooling changes in parts near the body core than to changes in extremities. These parts were found to show preference for warm sensation [133].
If skin temperature is in a middle range, the correlation between skin temperature and thermal sensation will be close to linear. Small decreases in skin temperature of chest, back, neck and head induce a large cooling sensation whilst other body parts like face, hands and arms have less sensitive cooling [132]. The higher the face skin temperature, the more the sensitivity. The lower the face skin temperature, the less the thermal sensitivity [77].
However, local sensation does not only depend on skin temperatures, but also on the overall thermal state of the body. This means that for a given overall thermal state, different output sensations may exist for the same input stimuli. Local thermal sensation is much warmer during cold tests when the whole body is cold and much colder during warm tests when the whole body is warm [132].
According to [134], overall body thermal sensation is a complaint-driven process. The strongest local sensations tend to dominate overall sensation and whole body sensation tends to follow the cooler local body sensations [41]. A study investigated human responses to local cooling with air jets in warm conditions and found that the air jet velocity preferred by the subjects was not
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the one corresponding to thermal neutrality, but the one that decreased the sensation of warmth without causing too much discomfort due to draft [136].
Similar observations have been made for local thermal comfort. Local thermal comfort assessment is affected when the whole body thermal sensation is different from the local thermal state. In general, any action that leads to a decrease in whole body‟s heat stress is felt as pleasant. In hypo-thermic states, warm stimuli are felt as pleasant and unpleasant in hyper- thermic states. If the subject is in neutral state, neither warm nor cold stimuli are felt as pleasant. However, there are also exceptions to these findings. Independent from the body‟s thermal state a warm pelvis is found to be comfortable when the body is cold, but a preference for a cold pelvis in overall warm body state can‟t be observed [133].
Analogous, cool breathing air is felt pleasant if the whole body is warm, but warm breathing air is still felt unpleasant when the whole body is cold. Additionally, extreme local cold-warm sensations are felt to be uncomfortable independent of the body‟s thermal state.
Overall thermal comfort is better specified with local comfort votes than local sensation votes. Under stable conditions overall thermal comfort is, similar to overall thermal sensation, a complaint driven process. When two body parts are strongly uncomfortable, the whole body comfort sensation will be close to the most uncomfortable part, independent of the comfort level of the other parts. However, in transient conditions, overall thermal comfort is better than predicted by the two most uncomfortable body parts. This may be due to physiological reasons or because comfort levels are varying all the time, a decisive judgment might therefore be less definite [134].
According to [137], thermal sensation change with time has significant effect on thermal comfort. According to [138], persons were seated in a Room with =+25˚C, were asked to enter a room with >+30˚C and to report their thermal state. It was noticed that skin temperatures and thermal state vote (TSV) increased gradually. When the subjects returned to the cooler room, the TSV changed immediately whilst skin temperatures decreased gradually. It is therefore concluded that the body reacts more sensitive to sudden cold changes than to sudden exposure to hot environments and skin temperatures can only be of limited use. This is confirmed by [122], which states that thermal comfort in transient environments can be predicted more precisely from air temperature than from skin temperature.