In this blog we have already talked about thermophysiology in articles entitled “Thermophysiology and its applications in CBRN” and “Thermophysiological tolerance of CBRN underwear”.
This article by Uttam et al, Thermophysiological clothing comfort, J. Textile Eng. Fashion Technol, 2021,7,98-103 details the different characteristics of the thermophysiological properties of textiles that make up clothing. The authors focus on the comfort aspect of this clothing, but we will go further by extending it to the field of PPE, which is also very dependent on thermophysiological themes.
What is it about?
While clothing is worn to protect the human body, it must also maintain body temperature under unfavorable environmental conditions. In a hot environment, the heat generated by the body must be dissipated into the environment, while conversely, the garment must protect the body from the aggression of cold and humidity.
A good garment helps to maintain body temperature at different levels of physical activity and in various environmental conditions. This is called thermophysiological comfort. The heat and moisture transport properties of textiles play a very important role here.
General perception of comfort
Comfort can be defined as a pleasant state resulting from the psychological, physiological and physical states between a person and his environment.
The garment protects the body against climatic influences by managing the heat and humidity between the body and the textile in such a way as to create a “microclimate” near the skin. This physiological effect is fundamental, especially in the case of functional fabrics such as that used in PPE (personal protective equipment) or sportswear. No clothing system is suitable for all situations: a fabric that is suitable for a given climate is generally unsuitable for another environment.
Clothing comfort depends on a number of factors:
- thermophysiological comfort: it is the result of the balance between temperature and humidity (liquid and vapor) allowing a good thermoregulation;
- Sensory or tactile comfort: it results from the sensation that a fabric provides when it is worn next to the skin. It is a function of its softness, its flexibility, the possible noise during the movement. A good garment does not irritate, does not sting and does not stick to the skin when wet;
- The comfort of body movement is the ability of a fabric to allow freedom of movement;
- Aesthetic comfort is a subjective perception of the garment for the eyes, and provides an overall feeling of comfort to the wearer. Parameters that contribute to aesthetic comfort are color, fabric appearance, the way it is cut, pilling, folding, etc.
Concept of thermophysiological comfort
In generally, the body maintains its own constant temperature at 37 ± 0.5˚C in different climates. Thermophysiological comfort is achieved when the body’s core temperature is maintained around 37°C. A 5˚C increase or decrease in body temperature can be fatal.
Physical activity in excess of what is needed to maintain body heat at 37°C results in excess heat production that must be transferred to the environment or the body temperature will rise. Although some heat dissipation occurs through respiration, most of this heat is removed via the body surface, usually through dry heat transfer. When the removal is insufficient, the human body begins to sweat, the objective being to cool the body by evaporating sweat from the skin.
Under these conditions, clothing must always guarantee a high level of moisture transmission.
Several mechanisms contribute to the removal of dry heat:
. conduction, by direct contact between 2 substances ;
. convection, by fluid transmission (liquid/gas);
. radiation by electromagnetic waves.
As for the evaporation of sweat on the skin, it requires a large amount of thermal energy which is taken from the skin allowing its cooling.
The body continuously produces heat through its metabolic processes, which must be transferred to the environment. Of all the energy absorbed in the form of food, only 15 to 30% is converted into useful work, the remaining 70 to 85% is dissipated as heat. In the resting state, it already reaches 100 W, it rises to about 300 W during physical work of medium difficulty, and reaches values of more than 1000 W during a short period of maximum physical performance in the case of an athlete for example.
An air temperature of 28-29˚C is necessary for an individual to sit comfortably without wearing clothing. At lower temperatures, the body decreases its temperature. Convective losses occur because the body transfers heat to the air that is in contact with it. This warm air is then immediately replaced by cold air by natural convection or by air currents generated by body movement or air circulation.
The garment must insulate the body so that the body does not get cold.
During a heat balance in equilibrium, the thermal energy produced by the body is compensated by the transfer of heat from the body to the outside by respiration, conduction, convection, radiation and evaporation.
The factors affecting thermal comfort are both environmental (air temperature, wind speed, relative humidity) and individual factors: activity level, clothing insulation.
Clothing isolation is an important factor. Most isolation systems applied to reduce body heat loss consist of textile structures that are mixtures of textile fibers and air. The textile fibers impede the flow of radiant heat and stabilize the air within the textile structure to reduce convective heat loss because the air has a lower conductivity than the fibers.
Thermoregulation through the clothing system
The thermoregulation of the human body is well known: in a steady state, the heat produced by the body is balanced by the heat loss to the environment (convection, radiation, evaporation and respiration). When body heat increases beyond the set value, vasodilation of the blood vessels is activated to increase blood flow to the skin in order to accelerate heat loss through the skin. Above this value, the sweating mechanism is activated to accelerate heat loss by evaporation of liquid sweat. Conversely, when the body senses that its temperature has dropped below the set value, vasoconstriction of the blood vessels is activated to decrease blood flow to the skin in order to reduce heat loss. In addition, the muscles are activated, resulting in the production of heat-generating shivers.
Thermoregulation by the clothing system depends mainly on the transmission of heat through the clothing, the process of moisture transfer, the thermal barrier between the human body and the environment formed by the clothing.
Heat and moisture transport properties
The transfer of heat and moisture from a garment is of critical importance to human survival and plays a significant role in maintaining thermophysiological comfort. Heat transfer through clothing can occur in three ways: transmission of dry heat by conduction and radiation, diffusion of perspiration as water vapor, and diffusion of liquid perspiration.
This creates a characteristic microclimate between the skin and the garment, which determines the feeling of comfort. Heat and humidity are dependent on both the environment and human factors as well as factors related to the garment such as the nature of the fabric and the openings of the garment.
There are a large number of thermal measurements characterizing the fabrics which we will not detail here. However, let us mention the air permeability which measures the passage of air from one side to the other of the textile under a pressure difference on both sides. It is expressed in L/s.m2 (volume of air per unit area of fabric flowing in a unit of time). The more important it is, the more the textile is permeable to the air. The thermal evaporative resistance measures the energy required to make water vapor pass through a membrane of a fabric. It is expressed in m2.Pa/W. The lower it is, the less resistant the fabric is to evaporation.
The transmission of moisture from the fabric significantly affects the thermophysiological comfort of the human body. Moisture is maintained by perspiration in liquid or vapor form.
The two types of perspiration raise different issues: one concerns the ability of water vapor to pass through the fibrous material, especially the outer layer; the other concerns the ability of the fibrous material in contact with the skin to absorb or process liquid sweat: a wet fabric that sticks to the skin is not considered comfortable.
Diffusion of perspiration in the form of water vapor
The water vapor permeability of the fabric is a very important property of the fibrous material that helps maintain the wearer’s thermal balance. During exercise or in very hot climates, the human body maintains thermal balance through perspiration and evaporation. The garment must be able to remove this moisture. A fabric with low vapor permeability is unable to allow sufficient perspiration to pass through, resulting in sweat accumulation in the garment and thus discomfort (thermal evaporative resistance).
Diffusion of liquid perspiration
The wicking effect (liquid moisture transfer) plays an important role in moisture transfer when the moisture content of clothing is very high and the body produces large quantities of liquid perspiration. The water produced on the surface of the body in the form of perspiration must be removed as quickly and efficiently as possible. The wicking effect of clothing has a significant influence on the wearer’s perception of the comfort of moisture. Fabrics that are intended to be worn, such as workwear in tropical climates, or as sportswear, must have very high wicking properties or they will prevent the evaporation of sweat and the associated skin cooling effect.
Water absorption and drying properties
During intense physical activities, a garment worn next to the skin must have a good capacity to absorb sweat and release it into the atmosphere, as well as a quick-drying property to obtain a better tactile comfort.
The air permeability of a fabric can influence the comfort behavior of clothing in several ways.
A fibrous material that is permeable to air is also, in general, likely to be permeable to water, either in the vapor or liquid phase. Air permeability can be important in both hot and cold climates. High permeability allows for freer access of air to the skin surface, which promotes moisture removal in hot weather and reduces discomfort. Conversely, this easy access can destroy comfort in cold weather, as any cold air allowed to reach the body surface will remove heat at a high rate.
Special case of PPE
Beyond the basic comfort of an outfit, thermoregulation phenomena must be rigorously respected in the context of PPE.
This is particularly true for military protective clothing or CBRN-type first aid. The suits must be resistant to chemical warfare agents in liquid, vapor or aerosol form, as well as to TICs.
It is easy to understand from what has just been said that the use of suits that are completely air and watertight will not allow thermoregulation phenomena to take place normally: the heat produced by the body accumulates since no heat transfer mechanism to the outside is possible. The production of sweat humidifies the microclimate between the skin and the clothing and the liquid sweat that cannot evaporate accumulates and does not play the role of cooling the skin. Hyperthermia then occurs, this is the “heat stroke”, endangering the life of the person wearing the equipment.
It can be deduced from this that PPE must allow the passage of air and moisture in the form of vapor or liquid while preventing the entry of toxic products in gas or liquid form.
Ouvry® has developed filtering suits that allow the passage of air (transmission of dry heat), the passage of moisture in vapor form and the evaporation of sweat. An outer hydrophobic layer prevents the entry of toxic liquids and aerosols, while a layer of activated carbon under the surface traps gaseous toxic products before they reach the individual’s skin.
In addition, this type of combination spreads a possible pumping effect over the entire internal surface of the equipment, thus reducing the concentration of the toxic. Moreover, the latter is trapped by the internal active carbon.
You may usefully refer to the following articles: CBRN protective clothing, Polycombi®, Polyindus®, PolyAgry®.
Thermophysiological comfort is the most important attribute of clothing and includes heat and moisture transport properties. The feeling of comfort depends on the combined effect of physical activity, climate and clothing. Heat is transmitted through the fabric by four mechanisms: conduction, convection, radiation and evaporation. Thermophysiological comfort of clothing can be achieved through dry heat transmission by conduction and radiation, moisture transmission (water vapor permeability, liquid or wicking water transmission, water absorption and drying properties) and air permeability. These properties of the garment can be improved by using different fiber materials, modifying the structure of the fiber, yarn, fabric and garment, and applying an appropriate finish to the fabric that can improve the heat and moisture transport properties.