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Review of clothing thermal comfort

Abstract: Clothing is one of the fundamental needs of the human being. People’s selection of clothing depends upon their perception and feeling about the clothing. Comfort is a basic and introductory prerequisite of the people in all situations. Comfort depends on various factors such as the environmental parameters, the metabolic heat and humidity produced by the human body and thermal properties of the clothing materials. Thermal comfort is associated with changes in many physiological and environmental variables. In this paper we have discussed the mechanism of thermal comfort and fabric properties like Insulation, Moisture vapour permeability, waterproofing and processes involved in moisture transmission through textiles.

Keywords: Heat Balance, Heat loss, Insulation, Moisture Vapor Permeability

1. Introduction

Although  in  the  recent  past,  the  textile  industry had  increasingly focused its  attention  on  higher machine productivity and lower production costs, the current  trend  within  the     industry’s  towards improved  wear  comfort . Clothing is one of the fundamental needs of the human being. It serves various and diverse purposes. Clothing selection is based on the needs and desires of the people. It may be to satisfy some aesthetic needs or to fulfill any particular demand of human being. People’s selection of clothing depends upon their perception and feeling about the clothing. In some cases it is recommended to wear certain clothing and selection is not possible, for example dress of a firefighter, military uniform, etc. However, it is very common that there is a dynamic and fundamental changes in the preferences of people with the change in the context; season, climate, age, type of activity, etc. It is highly linked with the core requirement why a person is wearing any particular clothing. Moreover, clothing requirements are rather different depending upon the type of activities of any person. However, comfort is a basic and introductory prerequisite of the people in all situations and is considered a threshold in selecting the clothing.

Comfort is difficult to explain since it is a complex and interdependent combination of physical, psychological and sensorial perceptions and highly depends of subjective evaluation of the individuals. Lower  thermal  resistance  results  in discomfort  for  the  wearer  because  excessive  heat  may be  dissipated  rapidly by vaporization  of  the  body water. A  garment  that  permits  free  access  of  liquid  (water)  can  become  uncomfortable  in  wet  weather,  when  the  reverse  movement  of  exterior water  towards  the  skin is  experienced.

And It is not possible that comfort level of every one sitting in an office could be same, even temperature, air velocity and other parameters, which they are experience are same and comparable, even then comfort sensation is quite diverse. However if more than 80% people feel comfort, then it can be said that such environment provides a comfort.

Comfort  depends on many factors such as the environmental parameters (i.e. air and radiant temperature, relative humidity, and wind velocity), the metabolic heat and humidity produced by the human body and, of course, the characteristics of the clothing materials e.g. materials’ thermal properties, which display the ability to transport heat and moisture from the human body surface into the environment. The measuring values that reflect this ability are clothing’s thermal resistance or thermal insulation, and water-vapour resistance. Moreover, numerous other factors e.g. colour, fashion, and a person’s physical and psychological state, influence the transfer of energy and the feeling of comfort. In order to obtain good thermal comfort, there must be a balance between heat production and heat dissipation.

2. Need of thermal comfort

A person produces thermal energy through the oxidation of glucose from what they have consumed. This process is named Metabolism. A great part of this energy (80%) is used for the maintenance of the body, while 20% is expended in work. Oxidation can increase with increased activity level of human body or increased body temperature. Indoors, the temperature of the human body remains constant. The normal temperature is about 37ºC (36.1∼37.2ºC). Outside these limits a person is considered to be sick, however, they can survive at a minimum temperature of 32ºC and at a maximum of 42ºC.  The body uses thermo-regulatory mechanisms to compensate the gain or loss of heat to keep body temperature in equilibrium. This system is known as the hypothalamic thermostat, which “tells” the body to increase or decrease the temperature, in accordance with thermal needs. A person is in thermal comfort when he/she is satisfied with the thermal environment and does not need to use their thermo-regulator mechanism. When temperature is more or less than this body feels discomfort, as a result human body needs some external agency to maintain this temperature and clothing is one of the most common devices to provide comfort in this regard.

3. What is comfort?

The word ‘Comfort’ has a variety of meanings as it relates to clothing and to the wearer. The word ‘Comfort’ refers to how an individual feels. There are three main viewpoints for analyzing comfort of any fabric.

The first notion of comfort is always thermal comfort. It is comfort or discomfort related to how a hot or cold a person feels. Thermal comfort is associated with changes in many physiological and environmental variables like the activity level of the individuals and clothing properties, such as the fabric insulation values and water vapor permeability. Thermal comfort is mostly quantified using physiological parameters though it is a psychological concept. Second way to interpret comfort is the tactile sensations that result from the fabrics in contact with the skin. The buzz words in tactile comfort are stiffness, thickness, and fuzziness etc.

A third component of comfort arises from the fit of the garment. A poorly fitting garment, especially too small or too large can impede mobility and performance, although impact on comfort may not be as great, but it influences the psychological perceptions of the wearer through personal or cultural preferences regarding fit and fashion size trends. Whether we are considering thermal comfort, sensory skin-feel comfort, comfort due to fit, or the psychological comfort of clothing, each of these can have considerable impact on the individual physical and cognitive performance. For this reason ‘Comfort’ must be seen as an essential element in all areas.
However there has been no one clear definition of comfort, since this subjective feeling differs from person to person, but a lot of researchers have investigated comfort over the past years. For example, Slater defined comfort as a pleasant state of physiological, psychological and physical harmony between a human being and the environment [1]. Li defined comfort as a holistic concept, which is a state of multiple interactions of physical, physiological, and psychological factors [2].Some other researchers termed comfort as:

“The lack of unpleasantness”
“Condition or feeling of pleasurable ease, well-being, and contentment”
“A neutral state compared to the more active state of pleasure” [3]

4. Types of comfort

The thermal comfort of clothing can be distinguished in following way (4):
1. Thermo physiological wear comfort which concerns the heat and moisture transport properties of clothing and the way that clothing helps to maintain the heat balance of the body during various levels of activity.

2. Skin sensational wear comfort which concerns the mechanical contact of the fabric with the skin, its softness and pliability in movement and its lack of prickle, irritation and cling when damp

4.1 Thermal comfort

Thermal comfort is very subjective issue. It is that state when an individual prefers neither warmer nor cooler condition and that condition are comfortable when largest percentage people in any particular grouping are comfortable. It can be defined as “the absence of any unpleasant sensations of being too cool or too warm, or of having too much perspiration on the skin [5].” The thermal comfort of clothing is associated with the thermal balance of the human body and its thermal responses to the dynamic interactions with the clothing and environment systems [6].

Man, being a homeotherm, strives to keep his body core at a constant temperature i.e. 37oC and a rise or fall of ±50C can be fatal [7]. In the cold conditions the blood supply to the extremities is reduced and shivering occurs. In hot days or during high activity level, blood comes to the skin surface to reduce the body temperature [8]. Clothing has a vital part to play in maintaining this heat balance as it modifies the heat loss from the skin surface under the same time has the secondary effect of altering the moisture loss from the skin. The heat balance also varies with climatic conditions. It should be the main property of textiles to conserve the heat that body divers away, and dissipate heat from body surrounding when body generates it. Because of above two different actions, it is impossible to design a single clothing system which acts comfortable to body for all the seasons and reasons. A clothing system which is suitable for one climate may not be suitable for another climate. Good thermal insulation properties are required for clothing and textiles used specially in cold climates. In warm climate, or when the wearer performs hard work, it is important that the clothing transmits the moisture secreted by the body. So it is necessary to understand the mechanism of thermal comfort.

4.1.1 Mechanism of thermal comfort

Heat Balance

The human body tries to maintain a constant core temperature of about 370C.The actual value varies slightly from person to person but the temperature of any one person is maintained within narrow limits. In most climates, body temperature is above that of the external environment so that there has to be an internal source of heat in order to maintain the temperature difference. The required heat comes from the body’s metabolism that is necessary burning of calories to provide power to the muscles and other internal functions. However, the body must be kept in thermal balance. The metabolic heat generated together with the heat received from external sources must be matched by the loss from the body of an equivalent amount of heat. If the heat gain and the heat loss are not in balance then the body temperature will either rise or fall. The heat balance is mathematically expressed as below (15).

Q = M ± R ± C∞v ± C∞d – E
Where,
Q     = Heat gain or loss
C∞v = Convective gain or loss
E   = Evaporative loss
M   = Metabolism
R   = Radiant gain or loss

Table 1:Typical metabolic heat generation for various activities (16)

Activity Heat generation, W/m2
Resting
   Sleeping 40
   Reclining 45
   Seated, quiet 60
   Standing, relaxed 70
Walking (on the level)
   0,89 m/s 115
   1,34 m/s 150
   1,79 m/s 220
Office activities
  Reading 55
  Writing 60
  Typing 65
  Filing, seated 70
  Filing, standing 80
  Walking about 100
  Lifting/packing 120
Miscellaneous occupational activities
  Cooking 90-115
  House cleaning 115-120
  Seated, heavy limb movement 130
  Machine Work
     Sawing (table saw) 105
     Light (electrical industry) 115-140
     Heavy 235
  Handling 50 kg bags 235
  Pick and shovel work 235-280
Miscellaneous leisure activities
  Dancing, social 140-225
  Tennis, singles 210-270
  Basketball 290-440

The efficiency of the human organism is such that of the energy taken in as food only 15-30% is converted into useful work with the remaining 70-85% of the energy being wasted as heat. Any level of physical activity above the needed to maintain body temperature will result in an excess of heat energy, which must be dissipated, otherwise the body temperature will rise. A lower level of physical activity will lead to a fall in body temperature if the available heat is not conserved by increased insulation.

The approximate energy costs, which are associated with human activity, are shown in Table 1 and range from a minimum value of about 40W when sleeping to an absolute maximum of about 440W (corresponding to hard physical work) can be kept up for a number of hours. If a person is comfortable (i.e. in heat balance) at rest then a burst of hard exercise will mean that there is a large amount of excess heat and also perspiration to be dissipated. On the other hand if the person is in heat balance during strenuous exercise then he or she will feel cold when resting owing to the large reduction of heat generation.

 Heat loss

There are five mechanisms like conduction, convection, radiation, evaporation and respiration that allow the body to lose heat to the environment in order to maintain its thermal balance. Thermal conductivity measures of how rapidly heat flows through a material that is exposed to a difference in temperatures. Convection gently removes the warmed air from your skin and replaces it with cooler air. Heat can also exchange by radiation. In radiation skin emits electromagnetic waves toward human body surroundings and they emit electromagnetic waves toward skin. The amount of heat transferred by these waves depends on the temperature of each surface and on how well they absorb and emit light. However, while conduction and convection transfer heat in proportion to the temperature difference between objects, radiation transfers heat in proportion to the difference between the fourth powersof their temperatures. That is why radiative heat transfer towards or from the skin is most noticeable when body exposed to an unusually hot or cold object. The rate of heat loss by evaporation is the removal of heat from the body by the evaporation of perspiration from the skin. The heat loss by evapo­ration is made up of two, the insensible heat loss by skin diffusion, and the heat loss by regulatory sweating (17).  The way the heat loss is divided between the mechanisms depends on the external environment.

The requirements for heat balance vary with the climate; in hot climates the problem is one of heat dissipation whereas in cold climates it is one of heat conservation. Clothing has a large part to play in the maintenance of heat balance as it modifies the heat loss from the skin surface and at the same time has the secondary effect of altering the moisture loss from the skin surface. However, no one clothing system is suitable for all occasions a clothing system that is suitable for one climate is usually completely unsuitable for another. The main fabric properties that are of importance for maintaining thermal comfort are Insulation, Moisture vapour permeability and waterproofing.

Insulation

An air temperature of 28-290C would be required for a person to be able to sit in comfort without wearing any clothes. At air temperatures lower than this, therefore, the body will lose heat without the added insulation given by clothing. It losses by convection can be prevented, the air itself offers a very high resistance to heat conduction having a value of thermal resistance which is only slightly less than that of a vacuum. Convection losses arise because the body loses heat to the air in contact with it. This heated air is then immediately replaced with cooler air either through natural convection or through air currents. The air currents can be caused by either body movement or by external airflow such as in windy conditions. Convection losses can there be reduced by keeping the air surrounding the body at rest. Air tends to ‘cling’ to solid surfaces so that material with a large exposed surface area, such as a mass of fine fibres, acts as a good resister of air movement. In clothing the majority of the bulk is composed of air, for example a worsted suiting fabric is made up of 25% fibre and 75% air whereas knitted underwear and quilted fabrics filled with fibre battings or down and feathers may contain 10% or less actual fibre, with the rest consisting of air.(9)

Moisture Vapor Permeability

Perspiration is an important mechanism, which the body uses to lose heat when its temperature starts to rise. Heat is taken from the body in order to supply the latent heat needed to evaporate the moisture from the skin. There are two forms of perspiration.

Insensible in this form the perspiration is transported as a vapour and it passes through the air gaps between yarns in a fabric,

Liquid this form occurs at higher sweating rates and it wets the clothing, which is in contact with the skin.

 Moisture transport

In order to keep the wearer dry ad hence comfortable, clothing that is worn during vigorous activity, such as sports clothing, has to be able to deal with the perspiration produced by such activity. There are two main properties of clothing that affect the handling of moisture. Firstly there is the ease with which clothing allows the perspiration to be evaporated from the skin surface during the activity. Secondly, after the activity had ceased, there is a need for the moisture that is contained in the clothing layer next to the unnecessarily through having a wet skin. Some workers also consider that the extent to which the wet fabric clings to the skin is also important to the comfort of a garment.

5. Processes involved in moisture transmission through textiles

Water vapour can pass through textile layers by the following mechanisms:

• Diffusion: – In the diffusion process, the vapour pressure gradient acts as a driving force in the transmission of moisture from one side of a textile layer to the other. Water vapour can diffuse through a textile structure in two ways, simple diffusion through the air spaces between the fibres and yarns and along the fibre itself [10]. In the case of diffusion along the fibre, water vapour diffuses from the inner surface of the fabric to the fibres’ surface and then travels along the interior of the fibres and its surface, reaching the outer fabric surface. At a specific concentration gradient the diffusion rate along the textile material depends on the porosity of the material and also on the water vapour diffusivity of the fibre

• Absorption, transmission and desorption of the water vapour by the fibres. Sorption-desorption is an important process to maintain the microclimate during transient conditions. A hygroscopic fabric absorbs water vapour from the humid air close to the sweating skin and releases it in dry air. This enhances the flow of water vapour from the skin to the environment comparatively to a fabric which does not absorb and reduces the moisture built up in the microclimate [11-14]. In the absorption-desorption process an absorbing fabric works as a moisture source to the atmosphere. It also works as a buffer by maintaining a constant vapour concentration in the air immediately surrounding it, i.e. a constant humidity is maintained in the adjoining air, though temperature changes due to the heat of sorption.

• Adsorption and migration of the water vapour along the fibre surface.
• Transmission of water vapour by forced convection. Convection is a mode of moisture transfer that takes place while air is flowing over a moisture layer.This is known as the forced convection method. The mass transfer in this process is controlled by the difference in moisture concentration between the surrounding atmosphere and the moisture source

Conclusion

The human body continuously produces heat by its metabolic processes. The heat is lost from the surface of the body by convection, radiation, evaporation and respiration. In a steady-state situation, the heat produced by the body is balanced by the heat lost to the environment. If not, the body temperature will increase and the human feels uncomfortable. Human thermal comfort depends on the metabolic rate (internal heat production), the heat losses from the body and the climatic conditions. As is well-known, the clothing properties like Insulation, Moisture vapour permeability, waterproofing is affected by these parameters. Clothing reduces the body’s heat loss. Therefore, clothing properties are very important for human thermal comfort.

6. References

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