||It has been suggested that How radiation affects Perceived temperature be merged into this article. (Discuss) Proposed since April 2013.|
Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation (ANSI/ASHRAE Standard 55). Maintaining this standard of thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC (heating, ventilation, and air conditioning) design engineers.
The Predicted Mean Vote (PMV) model stands among the most recognized thermal comfort models. It was developed using principles of heat balance and experimental data collected in a controlled climate chamber under steady state conditions. Because the PMV model was derived from data collected in a controlled climate, it is not suitable for applications in naturally ventilated spaces which often have high levels of air movement. This shortcoming motivated the development of a family of empirical statistical models of thermal comfort in naturally ventilated spaces known as adaptive models. Adaptive models of thermal comfort consider occupants as dynamically interacting with their environment, and controlling their thermal comfort by means of clothing or window-opening and closing. This is in contrast to thermal comfort in sealed buildings, in which occupants experience the environment passively.
Thermal comfort is affected by heat conduction, convection, radiation, and evaporative heat loss. Thermal comfort is maintained when the heat generated by human metabolism is allowed to dissipate, thus maintaining thermal equilibrium with the surroundings. It has been long recognized that the sensation of feeling hot or cold is not just dependent on air temperature alone. Thermal comfort calculations according to ANSI/ASHRAE Standard 55  can be freely performed with the CBE Thermal Comfort Tool for ASHRAE-55.
Effects of thermal discomfort 
Thermal discomfort has been known to lead to sick building syndrome symptoms. The combination of high temperature and high relative humidity serves to reduce thermal comfort and indoor air quality. The occurrence of symptoms increased much more with raised indoor temperatures in the winter than in the summer due to the larger difference created between indoor and outdoor temperatures.
Factors determining thermal comfort 
Since there are large variations from person to person in terms of physiological and psychological satisfaction, it is hard to find an optimal temperature for everyone in a given space. Laboratory and field data have been collected to define conditions that will be found comfortable for a specified percentage of occupants.
There are six primary factors that directly affect thermal comfort that can be grouped in two categories: personal factors - because they are characteristics of the occupants - and environmental factors - which are conditions of the thermal environment. The former are metabolic rate and clothing level, the latter are air temperature, radiant temperature, air speed and humidity.
Even if all these factors may vary with time, standards usually refer to a steady state to study thermal comfort, just allowing limited temperature variations.
Metabolic rate 
The ASHRAE 55-2010 Standard defines metabolic rate as the level of transformation of chemical energy into heat and mechanical work by metabolic activities within an organism, usually expressed in terms of unit area of the total body surface. In the standard, metabolic rate is expressed in MET units, which are defined as follows:
1 MET = 58.2 W/m² (18.4 Btu/h·ft²), which is equal to the energy produced per unit surface area of an average person seated at rest. The surface area of an average person is 1.8 m² (19 ft²).
The Standard provides a table of activity levels, some given as a single value and some as ranges, since for some activities the value is not precisely defined and it depends on more factors, personal aspects and conditions under which the activity is performed. Some common values are 0.7 met for sleeping, 1.0 met for a seated and quiet position, 1.2-1.4 met for light activities standing, 2.0 met or more for activities that involve movement, walking, lifting heavy loads or operating machinery.
For intermittent activity, the Standard states that is permissible to use a time-weighted average metabolic rate if individuals are performing activities that vary over a period of one hour or less. For longer periods, different metabolic rates must be considered.
According to ASHRAE Handbook of Fundamentals, estimating metabolic rates is complex, and for levels above 2 or 3 met – especially if there are various ways of performing such activities – the accuracy is low. Therefore, the Standard is not applicable for activities with an average level higher than 2 met. Met values can also be determined more accurately than the tabulated ones, using an empirical equation that takes into account the rate of respiratory oxygen consumption and carbon dioxide production. Another physiological yet less accurate method is related to the heart rate, since there is a relationship between the latter and oxygen production.
Food and drink habits may have an influence on metabolic rates, which indirectly influences thermal preferences. These effects may change depending on food and drink intake. Body shape is another factor that affects thermal comfort. Heat dissipation depends on body surface area. A tall and skinny person has a larger surface-to-volume ratio, can dissipate heat more easily, and can tolerate higher temperatures more than a person with a rounded body shape.
Clothing insulation 
The amount of thermal insulation worn by a person has a substantial impact on thermal comfort, because it influences the heat loss and consequently the thermal balance. Layers of insulating clothing can either help keep a person warm or, at the same time, in case of high physical activity, prevent heat loss and possibly lead to overheating. Generally, the thicker the garment is, the greater insulating ability it has. Depending on the type of material the clothing is made out of, air movement and relative humidity can decrease the insulating ability of the material.
The standard amount of insulation required to keep a resting person warm in a windless room at 70 °F (21.1 °C) is equal to 1 clo.
Clo units can be converted to R-value in SI units (m²·K/W) or RSI) by multiplying clo by 0.155 (1 clo = 0.155 RSI).
In IP units, 1 clo corresponds to an R-value of 0.88 °F·ft²·h/Btu.
ASHRAE 55-2010 contains tables with more information about clothing levels for common ensembles or single garnments.
Air temperature 
The air temperature is the average temperature of the air surrounding the occupant, with respect to location and time. According to ASHRAE 55 standard, the spatial average takes into account the ankle, waist and head levels, which vary for seated or standing occupants. The temporal average is based on three-minutes intervals with at least 18 equally spaced points in time. Air temperature is measured with a dry-bulb thermometer and for this reason it is also known as dry-bulb temperature.
Radiant temperature 
The radiant temperature is related to the amount of radiant heat transferred from a surface, and it depends on the emissivity of the material - i.e. the ability to absorb or emit heat. The mean radiant temperature, defined as the uniform temperature of an imaginary enclosure in which the radiant heat transfer from the human body is equal to the radiant heat transfer in the actual non-uniform enclosure, is a key variable for thermal comfort calculations for the human body.
Air speed 
Air speed is defined as the rate of air movement at a point, without regard to direction. According to ASHRAE 55 standard, it is the average speed of the air to which the body is exposed, with respect to location and time. The temporal average is the same as the air temperature, while the spatial average is based on the assumption that the body is exposed to a uniform air speed, according to the SET thermo-physiological model. However, some spaces might provide strongly nonuniform air velocity fields and consequent skin heat losses that cannot be considered uniform. Therefore, the designer shall decide the proper averaging, especially including air speeds incident on unclothed body parts, that have greater cooling effect and potential for local discomfort.
Relative humidity 
While the human body has sensors within the skin that are fairly efficient at feeling heat and cold, relative humidity (RH) is harder to detect. The influence of humidity on the perception of an indoor environment can play a part in the perceived temperature and their thermal comfort. As a matter of fact, relative humidity affects the evaporation from the skin, which is the prevailing way of heat loss at high temperatures, normally from 26°C (80°F). At lower RH more sweat is allowed to evaporate from the body, while at higher values it is harder for this process to happen, because the air's moisture content is already elevated. Therefore, very humid environments (RH > 70-80%) are usually uncomfortable because the air is close to the saturation level, thus strongly reducing the possibility of heat loss through evaporation. On the other hand, very dry environments (RH < 20-30%) are also uncomfortable because of their effect on the mucous membranes. The recommended level of indoor humidity is in the range of 30-60%, but new methods allow lower and higher humidities, depending on the other factors involved in thermal comfort.
A way to measure the amount of relative humidity in the air is to use a system of dry-bulb and wet-bulb thermometers. While the former measures the temperature with no regard to moisture - such in weather reports - the latter has a small wet cloth wrapped around the bulb at its base, so the measurement takes into account water evaporation in the air. The wet-bulb reading will thus always be at least slightly lower than the dry bulb one. The difference between these two temperatures can be used to calculate the relative humidity: the larger the temperature difference between the two thermometers, the lower the level of relative humidity.
The wetness of skin in different areas also affects perceived thermal comfort. Humidity can increase wetness on different areas of the body, leading to a perception of discomfort. This is usually localized in different parts of the body, and local thermal comfort limits for skin wettedness differ by locations of the body. The extremities are much more sensitive to thermal discomfort from wetness than the trunk of the body. Although local thermal discomfort can be caused from wetness, the thermal comfort of the whole body will not be affected by the wetness of certain parts.
Recently, the effects of low relative humidity and high air velocity were tested on humans after bathing. Researchers found that low relative humidity engendered thermal discomfort as well as the sensation of dryness and itching. It is recommended to keep relative humidity levels higher in a bathroom than other rooms in the house for optimal conditions.
Local thermal discomfort 
Even though the comfort models based on the predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) usually describe compliance to thermal comfort for the body as a whole, thermal dissatisfaction may also occur just for a particular part of the body, due to local sources of unwanted heating, cooling or air movement. According to the ASHRAE 55-2010 standard, there are four main causes of thermal discomfort to be considered. A section of the standard specifies the requirements for these factors, that apply to a lightly clothed person engaged in near sedentary physical activity. This is because people with higher metabolic rates and/or more clothing insulation are less thermally sensitive, and consequently have less risk of thermal discomfort.
Radiant temperature asymmetry 
The thermal radiation field about the body may be nonuniform due to hot and cold surfaces and direct sunlight. This asymmetry may cause local discomfort and reduce the thermal acceptability of the space. In general, people are more sensitive to asymmetric radiation caused by a warm ceiling than that caused by hot and cold vertical surfaces. ASHRAE standard gives the predicted percentage of dissatisfied occupants (PPD) as a function of the radiant temperature asymmetry and specifies the acceptable limits .
Draft is unwanted local cooling of the body caused by air movement, most prevalent when the thermal sensation of the whole body is cool (below neutral). Draft sensation depends on the air speed, air temperature, activity, and clothing. Sensitivity to draft is greatest where the skin is not covered by clothing, especially the head, neck, shoulders, ankles, feet, and legs.
Vertical air temperature difference 
Thermal stratification that results in the air temperature at the head level being higher than at the ankle level may cause thermal discomfort. ASHRAE standard 55 gives the predicted percentage of dissatisfied occupants as a function of the air temperature difference between the head level and ankle level. Thermal stratification in the opposite direction is rare and perceived more favorably by occupants.
Floor surface temperature 
Occupants may feel uncomfortable due to contact with floor surfaces that are too warm or too cool. The temperature of the floor, rather than the material of the floor covering, is the most important factor for foot thermal comfort for people wearing shoes. ASHRAE standard 55 specifies the allowable range of surface temperatures of the floor for people wearing lightweight shoes.
Thermal stress 
The concept of thermal comfort is closely related to thermal stress. This attempts to predict the impact of solar radiation, air movement, and humidity for military personnel undergoing training exercises or athletes during competitive events. Values are expressed as the Wet Bulb Globe Temperature or Discomfort Index. Generally, humans do not perform well under thermal stress. People’s performances under thermal stress is about 11% lower than their performance at normal thermal conditions. Also, human performance in relation to thermal stress varies greatly by the type of task you are completing. Some of the physiological effects of thermal heat stress include increased blood flow to the skin, sweating, and increased ventilation.
Adjustment mechanisms 
The body has several thermal adjustment mechanisms to survive in drastic temperature environments. In a cold environment the body utilizes vasoconstriction; which reduces blood flow to the skin, skin temperature and heat dissipation. In a warm environment, vasodilation will increase blood flow to the skin, heat transport, and skin temperature and heat dissipation. If there is an imbalance despite the vasomotor adjustments listed above, in a warm environment sweat production will start and an evaporative cooling mechanism will be provided. If this is insufficient, hyperthermia will set in, body temperature may reach 40 C and heat stroke may occur. In a cold environment shivering will start, involuntarily forcing the muscles to work and increasing the heat production by up to a factor of 10. If equilibrium is not restored, hypothermia will set in which, can be fatal. Long term adjustments to extreme temperatures of a few days to six months may result in cardiovascular and endocrine adjustments. A hot climate may create increased blood volume, improving the effectiveness of vasodilation, enhanced performance of the sweat mechanism, and the readjustment of thermal preferences. In cold or underheated conditions, vasoconstriction can become permanent resulting in decreased blood volume, and increased body metabolic rate.
Effects of natural ventilation on thermal comfort 
Many buildings use a HVAC (Heating Ventilation Air Conditioning) unit to control their thermal environment. Recently, with the current energy and financial situation, more environmentally friendly methods for indoor temperature control are being used. One of these is natural ventilation. Buildings designed with natural ventilation in mind, according to the climate of the site, drastically reduce the need for mechanical heating or cooling. Badly designed, this process can make the controlled indoor air temperature more susceptible to the outdoor weather, and during the seasonal months the temperatures inside can become too extreme. During the summer months, the temperature inside can rise too high and cause the need for open windows and fans to be used. In contrast, the winter months could call for more insulation and layered clothing to deal with the less than ideal temperatures.
Operative temperature 
The ideal standard for thermal comfort can be defined by the operative temperature. This is the average of the air dry-bulb temperature and of the mean radiant temperature at the given place in a room. In addition, there should be low air velocities and no 'drafts,' little variation in the radiant temperatures from different directions in the room, and humidity within a comfortable range.
The operative temperature intervals vary by the type of indoor location. They also vary by the time of year. ASHRAE has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per square meter of 0.1. In the summer the suggested temperature is between 23.5 °C (74.3 °F) and 25.5 °C (77.9 °F), and airflow velocity of 0.18 m/s. In the winter, the recommended temperature is between 21.0 and 23.0 °C with an airflow velocity of 0.15 m/s.
Thermal sensitivity of individuals 
The thermal sensitivity of an individual is quantified by the descriptor FS, which takes on higher values for individuals with lower tolerance to non-ideal thermal conditions. This group includes pregnant women, the disabled, as well as individuals whose age is below fourteen or above sixty, which is considered the adult range. Existing literature provides consistent evidence that sensitivity to hot and cold surfaces declines with age. There is also some evidence of a gradual reduction in the effectiveness of the body in thermoregulation after the age of sixty. This is mainly due to a more sluggish response of the counteraction mechanisms in the body that are used to maintain the core temperature of the body at ideal values.
Situational factors include the health, psychological, sociological and vocational activities of the persons.
Sex differences 
While thermal comfort preferences between genders seems to be small, there are some differences. Studies have found men report discomfort due to rises in temperature much earlier than women. Men also estimate higher levels of their sensation of discomfort than women. One recent study tested men and women in the same cotton clothing, performing mental jobs while using a dial vote to report their thermal comfort to the changing temperature. Many times, females will prefer higher temperatures. But while females were more sensitive to temperatures, males tend to be more sensitive to relative humidity levels.
An extensive field study was carried out in naturally ventilated residential buildings in Kota Kinabalu, Sabah, Malaysia. This investigation explored the gender thermal sensitivity to the indoor environment in non air-conditioned residential buildings. Multiple hierarchical regression for categorical moderator was selected for data analysis; the result showed that females were slightly more sensitive than males to the indoor air temperatures. Whereas, under thermal neutrality; it was found that males and females have similar thermal sensation.
Thermal comfort models 
When discussing thermal comfort, there are two main different models that can be used: the static model (PMV/PPD) and the adaptive model.
Static comfort model: PMV/PPD 
The static model is based on the physiological approach, according to which the comfort zone can be the same for all occupants, disregarding location and adaptation to the thermal environment. It basically states that the indoor temperature should not change as the seasons do. Rather, there should be one set temperature year-round. This is taking a more passive stand that humans do not have to adapt to different temperatures since it will always be constant.
This model is based on the PMV/PPD model, that uses the Predicted Mean Vote formula by P. O. Fanger. The PMV is the average comfort vote, using a seven-point thermal sensation scale from cold (-3) to hot (+3), predicted by a theoretical index for a large group of subjects when exposed to particular environmental conditions. Zero is the ideal value, representing thermal neutrality. This model was originally developed by collecting data from a large number of surveys on people subjected to different conditions within a climate chamber. These data were then used to derive a mathematical model of the relationship between all the environmental and physiological factors involved. The comfort zone is defined by the combinations of the six key factors for thermal comfort for which the PMV is within the recommended limits (-0.5<PMV<+0.5). The PMV model is calculated with the air temperature and mean radiant temperature in question along with the applicable metabolic rate, clothing insulation, air speed, and humidity. If the resulting PMV value generated by the model is within the recommended range, the conditions are within the comfort zone.
The Predicted Percentage of Dissatisfied (PPD) is related to the PMV as is defined as an index that establishes a quantitative prediction of the thermally dissatisfied people assuming that who votes -2, -3, +2 or +3 on the thermal sensation scale is dissatisfied. The model is also based on the simplification that PPD is symmetric around a neutral PMV.
ASHRAE Standard 55-2010 sets an acceptable range of conditions that must be complied in order to apply this method and draw the comfort zone: occupants’ metabolic rates between 1.0 and 1.3 met, clothing between 0.5 and 1.0 clo, air speeds under 0.2 m/s.
Elevated air speed method 
According to the standard, a different model is to be used to allow a higher air speed, in order to increase the maximum operative temperature for acceptability under certain conditions. The Elevated Air Speed Method is based on the fact that different combinations of air movement and temperatures may result in equal levels of heat loss from the skin. The model applies to a lightly clothed person who is engaged in near sedentary activity. As a matter of fact, any benefits gained by increasing air speed depend mainly on clothing and metabolic activity. Elevated air speed is more effective at increasing heat loss with lower levels of clothing and if the occupant is engaged in higher activities, so in this case the method would be conservative. Clothing insulation higher than 0.7 clo would lead to a wrong estimation of the effects of increased air movement.
Adaptive comfort model 
The adaptive model is based on the concept that there is a strong relationship between indoor comfort and outdoor climate, taking into account that humans can adapt to and tolerate different temperatures during different times of the year. The adaptive hypothesis predicts that contextual factors and past thermal history modify building occupants' thermal expectations and preferences. Field studies are performed in these areas to see what the majority of people would prefer as their set-point temperature indoors at different times of the year.
The ASHRAE-55 2010 Standard has recently introduced the prevailing mean outdoor temperature as input variable for the adaptive model. It is based on the arithmetic average of the mean daily outdoor temperatures (DBT) over no fewer than 7 and no more than 30 sequential days prior to the day in question. It can also be calculated by weighting the temperatures with different coefficients, assigning increasing importance to the most recent temperatures. In case this weighting is used, there is no need to respect the upper limit for the subsequent days. In order to apply the adaptive model the prevailing mean temperature calculated must be greater than 10°C (50°F) and less than 33.5°C (92.3°F) and some other criteria must be met according to the standard.
This model applies especially to occupant-controlled, natural conditioned spaces, where the outdoor climate can actually affect the indoor conditions and so the comfort zone. In fact, studies by de Dear and Brager  showed that occupants in naturally ventilated buildings were tolerant of a wider range of temperatures. This is due to both behavioral and physiological adjustments, since there are different types of adaptive processes. ASHRAE Standard 55-2010 states that differences in recent thermal experiences, changes in clothing, availability of control options and shifts in occupant expectations can change people thermal responses. There are basically three categories of thermal adaptation, namely Behavioral Adjustment, Physiological and Psychological. The latter, that refers to an altered thermal perception and reaction due to past experiences and expectations, is the key factor to explain the difference between field observations and PMV predictions (based on the static model) in naturally ventilated buildings. In these buildings the relationship with the outdoor temperatures is twice as strong as predicted.
Thermal comfort in different regions 
In different areas of the world, thermal comfort needs may vary based on climate. In China there are hot humid summers and cold winters causing a need for efficient thermal comfort. Energy conservation in relation to thermal comfort has become a large issue in China in the last several decades due to rapid economic and population growth. Researchers are now looking into ways to heat and cool buildings in China for lower costs and also with less harm to the environment.
In tropical areas of Brazil, urbanization is causing a phenomenon called urban heat islands (UHI). These are urban areas, which have risen over the thermal comfort limits due to a large influx of people and only drop within the comfortable range during the rainy season. Urban Heat Islands can occur over any urban city or built up area with the correct conditions. Urban Heat Islands are caused by urban areas with few trees and vegetation to block solar radiation or carry out evapotranspiration, many structures with a large proportion of roofs and sidewalks with low reflectivity that absorb heat, high amounts of ground-level carbon dioxide pollution that retains heat released by surfaces, great amounts of heat generated by air conditioning systems of densely packed buildings and large amount of automobile traffic generating heat from engines and exhaust.
In the hot humid region of Saudi Arabia, the issue of thermal comfort has been important in mosques where Muslims go to pray. They are very large open buildings which are used only intermittently (very busy for the noon prayer on Fridays) making it hard to ventilate them properly. The large size requires a large amount of ventilation but this requires a lot of energy since the buildings are used only for short periods of time. Some mosques have the issue of being too cold from their HVAC systems running for too long and others remain too hot. The stack effect also comes into play due to their large size and creates a large layer of hot air above the people in the mosque. New designs have placed the ventilation systems lower in the buildings to provide more temperature control at ground level. Also new monitoring steps are being taken to improve the efficiency.
Thermal comfort of livestock 
Although thermal comfort of humans is the main focus of thermal comfort studies, the needs of livestock must be met as well for better living and production. The Department of Animal Production in Italy produced a study on ewes, which tested rumen function and diet digestibility of ewes chronically exposed to a hot environment. These two bodily functions were reduced by the hot temperatures offering insight that thermal comfort levels are important to livestock productivity.
These factors were explored experimentally in the 1970s. Many of these studies led to the development and refinement of ASHRAE Standard 55 and were performed at Kansas State University by Ole Fanger and others. Perceived comfort was found to be a complex interaction of these variables. It was found that the majority of individuals would be satisfied by an ideal set of values. As the range of values deviated progressively from the ideal, fewer and fewer people were satisfied. This observation could be expressed statistically as the % of individual who expressed satisfaction by comfort conditions and the predicted mean vote (PMV)
This research is applied to create Building Energy Simulation (BES) programs for residential buildings. Residential buildings can vary much more in thermal comfort than public and commercial buildings. This is due to their smaller size, the variations in clothing worn, and different uses of each room. The main rooms of concern are bathrooms and bedrooms. Bathrooms need to be at a temperature comfortable for a human with or without clothing. Bedrooms are of importance because they need to accommodate different levels of clothing and also different metabolic rates of people asleep or awake.
Thermal comfort research in clothing is currently being done by the military. New air-ventilated garments are being researched to improve evaporative cooling in military settings. Some models are being created and tested based on the amount of cooling they provide.
Thermal comfort for patients and hospital staff 
Whenever the studies referenced tried to discuss the thermal conditions for different groups of occupants in one room, the studies ended up simply presenting comparisons of thermal comfort satisfaction based on the subjective studies. No study tried to reconcile the different thermal comfort requirements of different types of occupants who compulsorily must stay in one room. Therefore, it looks to be necessary to investigate the different thermal conditions required by different groups of occupants in hospitals to reconcile their different requirements in this concept. To reconcile the differences in the required thermal comfort conditions it is recommended to test the possibility of using different ranges of local radiant temperature in one room via a suitable mechanical system.
Although different researches are undertaken on thermal comfort for patients in hospitals, it is also necessary to study the effects of thermal comfort conditions on the quality and the quantity of healing for patients in hospitals. There are also original researches that show the link between thermal comfort for staff and their levels of productivity, but no studies have been produced individually in hospitals in this field. Therefore, researches for coverage and methods individually for this subject are recommended. Also research in terms of cooling and heating delivery systems for patients with low levels of immune system protection such as HIV patients, burned patients, etc are recommended. However, there is research that shows the link between thermal comfort for staff and their levels of productivity, but no study have been produced in hospitals in this field. There are important areas, which still need to be focused on including thermal comfort for staff and its relation with their productivity, using different heating system to prevent hypothermia in the patient and to improve the thermal comfort for hospital staff simultaneously.
Finally, the interaction between people, systems and architectural design in hospitals is a field in which require further work needed to improve the knowledge of how to design buildings and systems to reconcile many conflicting factors for the people occupying these buildings.
See also 
- Air conditioning
- Building insulation
- Mahoney tables
- P. Ole Fanger
- Room air distribution
- ANSI/ASHRAE Standard 55-2010, Thermal Environmental Conditions for Human Occupancy
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