Note: More details on indoor climate assessment are given in REHVA Guidebook 14.
The thermal indoor environment is a composition of many and diverse aspects. Hence, the perspective on thermal comfort may change its evaluation by occupants.
3.1Human Responses to the Thermal Environment
Responses to our thermal indoor environment have a considerable effect on health, comfort, and performance. There has been considerable scientific investigation of these responses and formal methods have been developed to design and develop the interior environment. Existing methods for the evaluation of the general thermal state of the body, both in comfort and in heat- or cold-stress considerations, are based on an analysis of the heat balance for the human body. Under cool to thermoneutral conditions, heat gain is balanced by heat loss, no heat is stored, and body temperature equilibrates; that is:
S = M - W - C - R - Esk - Cres - Eres - K W/m² (1)
S = heat storage in the human body;
M = metabolic heat production;
W = external work;
C = heat loss by convection;
R = heat loss by radiation;
Esk = evaporative heat loss from skin;
Cres = convective heat loss from respiration;
Eres = evaporative heat loss from respiration;
K = heat loss by conduction.
The four environmental factors influencing this heat balance are: air and mean radiant temperature (°C), air speed (m/s), and partial water vapor pressure (Pa). The three personal variables are: metabolic heat production due to the activity level (W/m² or met), the thermal resistance of clothing (clo or m²K/W), and the evaporative resistance of clothing (m²Pa/W). These parameters must be in balance so that the combined influence will result in a thermal storage equal to zero. A negative thermal storage indicates that the environment is too cool and vice versa. To provide thermal comfort, the mean skin temperature also has to be within certain limits and the evaporative heat loss must be low.
Human responses to the thermal environment and to internal heat production serve to maintain a narrow range of internal body temperatures of 36 to 38 °C. The human body has a very effective thermoregulation system, which uses the blood flow for heat transport (high blood flow: enhanced heat dissipation – low blood flow: reduction of heat losses) with the hypothalamus acting as the main "thermostat".
There are two categories of responses of a human to the thermal environment: voluntary or behavioral responses, and involuntary or physiological autonomic responses. Voluntary or behavioral responses consist generally of avoidance or reduction of thermal stress by modification of the body's immediate environment or by modification of clothing insulation. Physiological responses consist of peripheral vasoconstriction to reduce the body's thermal conductance and increased heat production by involuntary shivering in the cold, and peripheral vasodilation to increase thermal conductance and secretion of sweat for evaporative cooling in hot environments. Autonomic responses are proportional to changes in internal and mean skin temperatures. Physiological responses also depend on the point in a diurnal cycle, on physical fitness, and on the sex of the individual. Behavioral responses rely on thermal sensations and thermal discomfort. Thermal discomfort appears to be closely related to the level of autonomic responses so that warm discomfort is closely correlated with skin wetness, and cold discomfort similarly relates to cold extremities and shivering activity.
However, there is no physiological acclimatization to cold environments; the most common way to compensate for cold environments is behavioral adaptation by clothing adjustment. In warm environments, sweating is a very efficient way of losing heat. However, the sweat rate is limited, as well as, how much a person can sweat during a day. Clothing, posture, and reduced activity are all behavioral ways of adapting to hot environments. Studies have also shown that peoples' expectations may change and influence their acceptability of the thermal environment. Besides the general thermal state of the body, a person may find the thermal environment unacceptable or intolerable if the body experiences local influences from asymmetric radiation (opposite surfaces with high temperature difference, solar radiation on single parts of the body, air velocities, vertical air temperature differences or contact with hot or cold surfaces (floors, machinery, tools, etc.).
In existing standards, guidelines or handbooks, different methods are used to evaluate the general thermal state of the body in moderate environments, cold environments and hot environments; but all are based on the heat balance and listed factors [ISO EN 7730:2005], [ISO EN 11079:2007], and [ISO EN 7996:1985].
Due to individual differences, it is impossible to specify a thermal environment that will satisfy everybody. There will always be a percentage of dissatisfied occupants. However, it is possible to specify environments predicted to be acceptable for a certain percentage of the occupants. If the occupants have some kind of personal control (change of clothing, setting of room temperature in a single office, increase of air velocity, change of activity level and posture), the overall satisfaction with the environment will increase significantly and every occupant may be satisfied. Due to local or national priorities, technical developments and climatic regions, in some cases a higher thermal quality (fewer dissatisfied) or lower quality (more dissatisfied) may be sufficient.
Besides influencing peoples' comfort the thermal environment may also have an affect on peoples' health and performance.
Extreme cold or hot environments are of high risk for the human body (heat stroke, frostbites, etc.). But even more moderate thermal conditions can affect the health significantly as is demonstrated by the rates of mortality in nursing homes [Marmor 1978] and ordinary households [Rogot et al. 1992] during spells of hot weather. However, when thermal indoor conditions are less extreme, elevated room temperatures have been associated with increased prevalence of symptoms typical of Sick Building Syndrome (SBS), non-specific building-related symptoms of headache, chest tightness, difficulty in breathing, fatigue, irritation of eyes and mucous membranes which are alleviated when the individual leaves the building [WHO 1983]. Research studies revealed an intensity of headaches, difficulty to concentrate or think clearly, when thermal indoor conditions were increased from 20 °C/40 %RH to 26 °C/60 %RH. Similarly, it has been found that a temperature of 26 °C tended to elevate neurobehavioral symptoms among subjects in tropical regions compared to 20 °C. These studies indicate a benefit to keep temperatures in buildings at the lower end of thermal comfort range [Fang et al. 2004], [Willem 2006].
Thermal conditions can affect productivity and the performance of work by several mechanisms [Wyon and Wargocki, 2006, 2007a,b]:
thermal discomfort distracts attention and generates complaints that increase maintenance costs,
warmth lowers arousal (the state of activation of an individual), exacerbates SBS symptoms and has a negative effect on mental work,
cold conditions lower finger temperatures and thus have a negative effect on manual dexterity,
rapid temperature swings have the same effects on office work as slightly raised room temperatures, while slow temperature swings cause discomfort that can distract concentration and increase complaints; and (v) vertical thermal gradients reduce perceived air quality or lead to a reduction in room temperature that then causes complaints of cold at floor level.
The hypothesis that thermal conditions within the thermal comfort zone do not necessarily lead to optimum work performance is supported by the results of several studies [Tham et al. 2003], [Pepler and Warner 1968]. They showed that subjects performed best at a temperature lower than thermal neutrality. Jensen et al. (2008) introduced a dose-response relationship between thermal sensation and relative office work performance, based on a statistical analysis of data from laboratory and field measurements, which also supports this hypothesis (Figure 2 ).
Figure 2 Relationship between subjective thermal sensation vote and relative performance (addition task) obtained using data from laboratory and field measurements; reproduced from Jensen et al. (2008).
Figure 2 Relationship between hourly measured room temperature [°C] and perception of an influence of indoor climate on the productivity.
In order to establish quantitative relations between indoor environmental quality and work performance, Seppänen and Fisk (2005) re-analyzed data including 150 assessments of performance from 26 studies. The data were obtained in office environments, factories, field laboratories, and school classrooms. A meta-analysis was used to derive a model that integrates the economic outcome of improved health and performance into building cost-benefit calculations, together with initial, energy and maintenance costs. The model considers the effects of ventilation rate, perceived air quality, indoor temperature, and both the intensity and prevalence of SBS symptoms on performance. Considering the effect of temperature, the model calculates the percentage change in performance per degree temperature increase. Since the analyzed studies varied greatly in sample size and outcome, the data were weighted by both sample size and relevance of the outcome (i.e., objectively reported work performance was assigned a higher weighting factor than simple visual tasks). The analysis showed an increased performance at thermal conditions up to 20 to 23 ºC and a decreased performance at indoor temperatures above 23 to 24 ºC. Maximum performance was predicted to occur at a temperature of 21.6 ºC. Figure 2 shows the predicted relationship between relative performance and temperature that was derived using the results of this analysis.
In a recent series of field experiments by Wargocki and Wyon (2006, 2007a, b) the performance of schoolwork by 10 to 12 year-old children was measured during weeklong experimental periods of improved classroom air quality. Outdoor air supply rates were raised from 3 to 9.5 liter per second and person. Additionally, cooling was applied during weeks with moderately elevated classroom temperatures to reduce temperatures from 23 to about 20 °C. Results show that doubling the ventilation rate would improve school performance by 8 to 14 %, while reducing the room temperature by 1 K would improve it by 2 to 4 % only, depending on the nature of the task.
Figure 2 Relative performance as a function of temperature [Seppänen et al. 2006], adapted from [REHVA Guidebook 6, 2006].