biophysical explanation

ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 5 CHAPTER II DISCUSSION 2.1 Definition of Biophysics In the Anonymous (2007) suggested that biophysics is the study of biological phenomena by using the methods and concepts of physics, whereas in Anonymous (2005) suggested that biophysics is the study interdisciplinary about the phenomenon and biological problems using the principles and techniques of physics. Biophysics was originally developed after World War II, the study of nuclear physics applications in systems biological, which include research on radiation effects on living things, and start biophysics at the time regarded as a new scientific field. Biophysics depend on techniques derived from physics but is focused on problems biological. Referring to the definitions that have been raised about the biophysical, then in context of this paper in particular a worker who perform activities in the open, the biophysics can be viewed as the study of biological phenomena on a workers who interact with the local physical environment when you're doing work activities of the principles, concepts, and methods of physics. In this case Campbell (1977) called the study of physics in this context as the biophysical environment. According to Campbell (1977) developments in the field of biophysics in particular environments focused on two areas, namely: 1) Use of mathematical models to quantify the rate of heat transfer and mass, and 2) Use the continuity equation which has led to the energy balance analysis. Therefore it can be argued that the biophysical environment studied about how the application of physics concepts on the interactions between living things with their physical environment, so in the context of this paper to learn about ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 6 application of physics concepts on the interaction between workers and their physical environment when conducting activities in the open. 2.2 Identification of Practical Aspects of Biophysics in Field Activities To be able to identify the biophysical aspects of work activity field, referring to the understanding of biophysics as described in section 2.1, it can be done through the study of theory as well dibaringi with direct observation. Someone who conduct activities in the open, any form of activity done will be influenced by the surrounding physical environment. In this context it microclimate which consists of: solar radiation, air temperature, air humidity, and wind speed which is the element of the physical environment, become very important as influential factor. Someone who do outdoor activities such as the activity field work, of course, will use equipment and tools in To achieve the purpose for which activity. The most important equipment usually is the suit of clothes, and other equipment such as protective clothing (PPD), such as hats, umbrellas, jackets / coats, and included here is a shoe. Equipment used will be tailored to tujan in doing activity. For field work activity, to be sure the equipment used is the equipment associated with the activity practicum units planned. It can be argued that all the factors that are outside the human self (As a principal activity) including micro-climates as well as equipment and tools used will give effect to the body of actor activity. The influence can vary, for example on: the heat energy balance or thermoregulation, health, comfort, and others. In the context of this paper is interesting is how the physical processes duration and the occurrence of such influence. The process of ongoing physical and the influence of microclimate, fixtures and equipment used to body of the perpetrator is an activity of biophysical processes. Figure 2.1 shows the position ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 7 biophysical process when there is interaction between the actors activities with micro-climate, equipment and tools used. Factors aktitvitas actors presented in Figure 2.1, together with the three factors involved in the interaction, in which biophysical processes take place, namely micro-climate, equipment and tools used are important factors in a work system (Corlett and Clark, 1995). In the context of this paper biophysical processes yang discussed in a working system is a process related to the thermal energy balance body or body thermoregulation. 2.3 Thermal Energy Balance Concept As can be seen in Figure 2.1 that man as the perpetrator activity would interact with the surrounding environment, as well as with equipment and equipment used or worn at the time doing activities. Process Biophysics occurs when the interaction takes place. In this man as a worker can Micro Climate (Environment Physical) Equipment which Used Performer Activity Equipment reply Used Figure 2.1. Process At Biophysics Happens Interaction Between Actors Activities with Micro Climate, Used Equipment and Equipment Used. ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 8 viewed as a system. Biophysical concepts that are important in the process biophysics in this context is the law of conservation of energy. According to Campbell (1977) concept of conservation of energy, which is also commonly written in the form of the continuity equation, the application of advanced biophysical environment eventually comes down to energy balance analysis. Analysis of energy balance can be done using pendakatan system. By looking at human body as a system, Havenith (1999, 2002), Blazejczyk (2000) and Epstein and Moran (2006) write the heat balance equation to the human body as the following equation, Heat stored (Store) Heat production = heat loss = (Metabolic rate - employment external) (Conduction + radiation + convection + evaporation + respiration) (1) When these factors in the equation (1) given a mathematical symbols, then equation (1) can be rewritten as, S = (M W) (C + R + H + E + eres) (2) In this case: S, stored heat; M, the body's metabolic rate, W, external work; C, conduction heat loss; R, radiation heat loss, H, convection heat loss; E, evaporative heat loss; and eres, respiration heat loss. Brake and Bates (2002) write the equation (1) in the form like equation (3), by adding a factor F, ie heat loss due to fluid intake, (M W) = Qsk + Qres + F + S (3) where Qsk is heat loss through the skin and Qres is heat loss through respiration. What was written by Brake and Bates (2002) as the heat balance equation, exactly the same as equation (2) presented by Havenith (1999, ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 9 2002) by considering that Qsk + Qres = C + R + H + E + eres + S, and with ignore the factor F. The factors that states lose body heat as it has been expressed in equation (2) or equation (3) for line conduction, convection, and radiation, following the general equation of transfer or the transfer of heat (Havenith, 2004; Campbell, 1977; Monteith and Unsworth, 1990) which forms generally can be written like equation (4), The heat loss = gradient. surface area detainee (4) From equation (4) can be stated that for each path; conduction (C), convection (H) and radiation (R), the amount of heat transferred depends on the driving force (driving force), the gradient of temperature and vapor pressure (T / z and Px / z), body surface area involved (A) and resistance where the heat flow, which can be either insulating clothing. Z in the gradient magnitude stated distance of two points where the heat or steam trasnfer progress. According to Havenith (1999, 2001, 2002, and 2004) heat release process and the process heat production in directed energy balance to maintain normal body temperature about 37 0C. This value is achieved by balancing the amount of heat generated in the body with the amount of heat loss. Figure 2.2 shows the representation schematic route forms of energy that occurs when workers perform activities in open nature. Heat production is determined by metabolic activity. At the time resting, heat produced by the body for basic body functions such as respiration and function heart by giving the body cells of oxygen and food (nutrients) that required in performing these basic functions. ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 10 During work activities, the need for active muscles against oxygen and food increased, and as a result of metabolic activity also increased. When cells Active muscle burns food for mechanical activity, part of the energy released into outside the body as external work, but most are released into the muscle as heat. The ratio between external work and energy consumed is called the efficiency by which the body does work. This process is similar to processes that occur in engine of a car. A small percentage of fuel energy used to drive car body, and most were released as waste heat. Body, such as machine a car, need to release this summer. When the heat is not released heat body heats up to deadly levels. Figure 2.2. Schematic representation Forms Line Energy As A Worker Interacting With Physical Environment in the Conduct of Activities. Adapted from Havenith (1999, 2002) and Blazejczyk, 2000) with using the actual image in the field work activity. M ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 11 Fox, Bowers and Foss (1988) depicts a more detailed transfer paths heat from the heat production starts in activity until the release of heat into outside the body in interaction with physical environment (microclimate) around the body. This is shown in Figure 2.3. Furthermore Havenith (1999, 2002) suggests, for most tasks, such as walking on a level, the value for efficiency (in the definition of physics) is close to zero. Only the heat released by friction of shoes and other things that released to outside the body, whereas most other energy used by otototot which eventually becomes heat in the body. For the heat loss from the body, there are several paths. Pathway that plays little is conduction. Conduction only becomes an important factor for people who work in the water, or people who work for handling cold products or work in a supine position in which the body bersentuan with heat transfer medium. The pathway that is more important for the release of heat is convection, when the air cooler flow along the surface of the skin. Therefore, heat is transferred from the skin into the surrounding air. Heat will also be transferred in the form of electromagnetic radiation or also called long-wave radiation. Third there is a difference between body surface temperature and the surface temperature of the object or objects around him it will happen heat transfer through radiation. Finally, the body also has other channels to release heat to the outside of the body, namely heat is lost through evaporation. Because the body's ability to sweat, moisture that appear on the skin surface through the pores of the skin can berevaporasi, by which amount of heat released to the outside of the body. Apart from convective and evaporative heat loss from the skin, the type of heat loss occurs from the lungs through respiration. Because the air coming out of the lungs generally colder and drier than on the surface of the lungs. Through respiration processes of the body loses an amount of heat that can reach 10% of total heat produced by the body. ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 12 Figure 2.3. Heat transfer pathways. Internally, the heat transfer occurs from the core to the shell to the skin and from muscle to the shell to the skin. Externally, there is radiation, convection, and conduction. In the painting back from Fox, Bowers and Foss (1988). T Pv Shell (tissue subsurface skin). ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 13 For your body stable, heat loss must be balanced with the heat produced. Otherwise, the heat content of the body will change, which causes body temperature rises or falls. This balance can be written as in equation (1) or equation (2). So if the rate of metabolic heat production through higher than the sum of all heat loss, heat stored will be marked positive (Surplus), which means increased body heat content and body temperature will rise. If the stored heat is negative (deficit), the heat loss is greater than the heat produced. The body becomes cold, and body temperature will fall. 2.4 Metabolism According Soegiardo (1993) understanding of metabolism is the "change" or change / replacement. Preparation of cell substance, and preparation of materials and oxidized called anabolisme, while solving the food substances that have been absorbed and backfilled called catabolism. Anabolisme and catabolism is called metabolism. Anabolisme is a process of synthesis for storage, while catabolism is the process demolition of nutrients to be converted into other forms to obtain energy. Pulat (1992) suggested that the metabolism of energy supply necessary for the musculoskeletal system. Metabolism is a chemical process convert food into mechanical work and heat. Meanwhile, according to Kroemer and Granjean (2000) metabolism is a fundamental biological process to absorb nutrients in the form of food and drinks and change the chemical energy contained into mechanical energy and heat. Ganong (1983) suggested that the amount of energy released by catabolism of food substances in the body equals the number who were released when the substance the food is burned outside the body. The energy generated by the process of catabolism in the body of work shaped the outside, heat and stored energy (another representation of equation 1): Energy Output Working out + = Deposit energy + Heat (5) ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 14 The amount of energy released per unit time, called the rate of metabolism. Contraction isotonis muscle showed working with efficiencies of approximately 50%. Equality work efficiency is, The work done Efficiency = The amount of energy used (6) Virtually all the energy in isometric muscle contraction will be in the form of heat, because of outside work done very little or even totally absent. Working outside formulated as follows: outside of work = force x distance produced by the force (7) Outside of work (mechanical work) by Soegiardo (1993) is a human effort to weight transfer to another place or lift the goods to another place. Furthermore, even further Ganong (1983) describes the energy stored in the form high-energy compounds. Total energy savings can be fickle, and on people who are fasting to achieve zero or even negative. Therefore in people who do not exercise, or do not eat, essentially the entire energy output will be in the form of heat. According Pulat (1992) the basic energy source for muscle contraction is glycogen or glucose in the blood. But this is not the source of initial energy. At the beginning muscle activity (3 to 5 seconds), Adenosine tripospat (ATP), a compound (Compound) contained high-energy phosphate in muscle tissue, were deployed (Mobilized) as described briefly in persamaman (8). ATP ADP + P + free energy (Energy Generating) (8a) Creatine phosphate + ADP creatine + ATP (Set Back ATP) (8b) Phosphate Glucose + 2 + 2 ADP 2 lactate + 2 ATP (Working aerobic) (8c) ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 15 Phosphate Glucose + 38 + 38 ADP + 6O2 6CO2 +44 H2O +38 ATP (Anaerobic work) (8d) ATP is split into adenosine dipospat (ADP) (Armstrong, 2006) and release some energy for the continuation of muscle activity. ATP must be generated again. The first energy source for the re-establishment of ATP is creatine phosphate (Creatine phosphate), a compound other high-energy phosphate, has existed in the network muscle in small amounts. The reaction of creatine phosphate with ADP release free (Free) creatine (creatine) and ATP. After depletion (depletion), creatine phosphate (15 seconds), if the activity continues, blood glucose or glycogen mobilized. Glucose, a blood sugar is important, circulated to the muscle tissue through the permeable capillary structure. Glucose is first converted through various phases into piruvik acid (pyruvic acid). Furthermore, the solution can be via two routes / paths are possible, namely anaerobic work and aerobics, or according to Armstrong (2006), anaerobic metabolism and aerobic metabolism (Figure 2.4). Further explained by Pulat (1992): 1) Work anaerobic (Anaerobic work). If sufficient oxygen is supplied to in muscle tissue, piruvik acid converted to lactic acid (lactic acid) while ATP is regenerated (resurrected). Accumulation of lactic acid between muscle fibers causes muscle fatigue and the development of taste ill. 2) Aerobic Work (Aerobic work). With an adequate supply of oxygen, acid piruvik broken down into water and CO2, while removing a large amount of ATP. This is reaction is more efficient than the reactions in anaerobic work. Oxygen is the key to working efficiently. Supply oxygen to muscle fibers require that more blood pumped per unit time into the muscle, as well as air breathing more to be able to oxygenate more blood through the system breathing. Body thermoregulation 2.5 ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 16 As mentioned in section 2.3, more specifically that can be seen in equation (1), (2) or equation (3), application of heat energy balance principle in system of human body shows that the balance between heat produced heat released and the body will determine the increase or decrease in body temperature. In scrutinizing the application of the principles of biophysics environment for workers outdoor activities, such as field work activity, changes in temperature body is a very important parameter for evaluating the job performed. Changes in body temperature, in addition determined by environmental factors when a worker interacts with its physical environment, but is physiologically there is a specific mechanism for the regulation of body temperature. The mechanism is called thermoregulation of the body. According to Fox, Bowers and Foss (1988), the main function of the system is termoregulatori to maintain body temperature (core temperature = temperature of the core) to stay on set point (37 0C). To carry out the functions of the components used termoregulatori basis: 3) receptor or thermal sensors: that is the organs that are sensitive to thermal stimulation (Cold, heat), Metabolism Anaerobic Metabolism Aerobic Food: Carbohydrates Fat Protein Respiration: O2 ATP Lactic Acid Hot ATP CO2 H2O Nitrogen Hot Acetyl CoA Figure 2.4. Sources of Energy and Metabolism Line (Armstrong, 2006) ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 17 4) thermal effectors: the organs that respond to stimuli delivered by the receptor and which produce regulatory or arrangements for repair, and 5) termoregulatori center located in the central nervous system that regulates (Mengkoordiner) information coming from the receptor with the regulatory action organs effector. 2.5.1 Thermal Receptors The human body has at least two main areas of thermal receptors; one located in the hypothalamus of the brain (reseptror center), the other in the skin (receptor side) (Fox, Bowers and Foss, 1988). Both regions contain two types of sensor receptor, one sensitive to heat and the other sensitive to cold. Receptors in hypothalamus is sensitive to fluctuations in a narrow temperature (between 0.2 to 0.5 0C). Receptors in the skin, both are sensitive to hot or cold consisted mainly free nerve endings. Receptors are located throughout permukaanm body, and usually cold receptors more sensitive than heat sensitive. Central receptors and edges connect with the nerves to the cortex and also to regulatory centers in the hypothalamus. Cortical relations, where the human relationships are consciously accept or hot sensations cold, encourage people to voluntarily setting (voluntary regulation), such as: search for shaded areas or sun, starting or avoid physical activity, wear or remove clothing, and stretch themselves (Stretching out) in a hot environment or indent itself (curling up) in cold environment. Regulations that starts from the hypothalamus is reflex and by because it accidentally (involuntary). 2.5.2 thermal effector Organ-orgam thermal effectors were: muscle-skeletal muscle (skeletal muscles), muscles encircling fine arterioles (small branches of arteries) that supply blood to skin, sweat glands, and endocrine glands (Fox, Bowers and Foss, 1988). In ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 18 cold environment, the muscles affecting shivering (shivering), which raises metabolic heat production, at the same time, arterioles that supply blood to the skin shrink (constrict, cutaneous vasoconstriction). Conversely, in a warm environment or heat, there was dilation arteioles (cutaneous vasodilation), and sweating occurs. Importance of vasomotor control (with the contraction and dilation) by the arterioles that supply blood to the skin comes from the fact that the heat from the central body must was first transported by convection and conduction to the surface circulatory before heat can be released into the environment through conduction, convection, radiation, and evaporation. For example, with shrinkage arterioles, skin blood flow decreased and by therefore consequently heat transfer from the center of the body also decreases. Conversely, with dilation of arterioles, increased blood flow allows the dissipation (release) core body heat to the environment faster. Secretion of sweat, so important in prevent excessive heating in the human body due to sweat secretion in eventually will dievaporasikan, and secretion is derived from approximately 2.5 million sweat glands. These sweat glands are widely dispersed throughout surface of the body, but mostly concentrated on the palms, soles of the feet, neck and body. Endocrine glands involved in the regulation of the thyroid and the temperature is adrenal medulla. More than a few weeks of cold exposure, metabolic heat production increase due to increased output kelencar thyroxin and thyroid. Also during the exposure cold, increased levels of epinephrine from the adrenal medulla and nonrepinephrine causes increased heat production along the increased heat conservation via cutaneous vasoconstriction. 2.5.3 termoregulatori Center Various responses have been described coordinated by the central arrangement thermal contained in the hypothalamus (Fox, Bowers and Foss, 1988; Christopherson, 2005). This central role somewhat similar to the role of a thermostat in the house. Temperature room (can be equated with the body's internal temperature) was measured by a thermometer (Can be equated with organs of the receptor) and compared with the set point (temperature ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 19 reference), thermostat (can be equated with hypothalamic centers) automatically convey information to the systems of heating or cooling (equated with effectors), which improved in accordance with the temperature set point value via mekanismemekanisme as has been described. Return to the set point and then effector system automatically stops. Thermal receptors initiate appropriate action after the effector body temperature internally compared with the temperature set point which is usually 37 0C or 98.6 oF, however, set point can be changed and it is understood as the primary role of the receptors edge in temperature regulation. For example, when the skin is heated, set point down or reduced. Consequently, this led to the body sweating and cutaneous vasodilation occurs and therefore cooling the body more quickly. Conversely when the skin is exposed cold, ie, set point increases or increased and more rapid improve the conservation and heat production. 2.6 Influential Factors on thermoregulation In general, the factors that influence the body's thermoregulation, clearly depicted by Blazejczyk (2000) as shown in Figure 2.5. According to Havenith (1999, 2002), the body's capacity to detain or release heat to the environment depends on a number of external parameters, which mainly are: temperature, humidity, wind speed, and clothing insulation. 2.6.1 Temperature The higher the temperature, the less body heat can be released through convection, conduction and radiation. If the ambient temperature rises above skin temperature, body will gain heat from the environment than releasing heat to the environment (Havenith, 1999, 2002, 2004). There are three relevant temperatures. 2.6.1.1 Air Temperature The temperature determines the level of convective heat loss from the skin to environment, or vice versa if the air temperature exceeds skin temperature. The fact has been ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 20 described by the equation of heat transfer, as stated in equation (4), where the heat always flows from high temperature point to point at a lower temperature Low (Christopherson, 2005; Havenith, 1999; Campbell, 1977; Fox, Bowers and Foss, 1988). The amount of heat that flows depends on the temperature difference or gradient T temperature (T / z) between the air with the body surface as the driving force (driving force). So when the body's surface temperature is higher than the air temperature then the heat convection will flow from the surface of the body into the air. If the opposite occurs, the body surface will acquire additional convective heat from the air flowing direction vice versa, but this only occurs on the surface of the body that can cause physiological responses, such as through sweat because thermoregulation processes as described by Fox, Bowers and Foss (1988) and Christopherson (2005). But the important principle that must always be remembered by Christopherson (2005) is that except on inanimate objects, on the human body heat always flows from in the body to the outside and never vice versa, because if the opposite occurs it means death. Therefore man can not live in the area or a room with temperature exceeds the body's core temperature (370C), except in a very short time. 2.6.1.2 The temperature of radiant (radiant temperature) Radiant temperature is the temperature of any object or objects around the body who is doing an activity. This temperature will determine the level of heat trasfer or radiation heat exchange between the body with the surrounding environment. In tempattempat with hot objects such as in steel mills or when working in the field in a state of blazing sun, radiant temperature can exceed the skin temperature and consequently occur trasnfer radiant heat from the environment to the skin (Havenith, 1999, 2002). The principle of heat transfer described in section 2.6.1.1 also fixed applicable in radiation heat transfer. In the context of activities carried out in the wild open as the field work activity according to Campbell (1977) and Monteith and Unsworth (1990), air and all the objects in the vicinity, including vegetation and surface and land can be a source of long wave radiation (thermal radiation). Therefore Air temperature is also a temperature that determines the beam. ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 21 Sweaty Skin Temperature Peripheral blood flow Solar radiation absorbed Weight & High Core Body Temperature Working Acquisition of the total heat (Gain) Deposit Hot Neto Total heat release (Loss) Clothing Pertukatan heat on body surface: - Convection - Radiation - Evaporation Hot Pertukatan respiratory Solar radiation Physical environment parameters: - Air Temperature - The temperature of the surface (ground) - Air humidity - Wind speed Factors change: Circulation atmosphere Season climate Climatic factors regional Climatic factors local Figure 2.5. Physiological parameters and Physical Environment Influential and Associated with Body Heat exchange Human Environment (Blazejczyk, 2000). ASPECT biophysics field work activity DEPARTMENT OF PHYSICS STUDENTS FMIPA UNIMA. DR.Rolles Palilingan Nixon, MS 10/21/2008. 22 2.6.1.3 Surface Temperature Surface temperature in addition to causing risks such as skin burns and pain on the skin (surface temperature above 450C), or in cold risk frostbite (Frostbite) on the nose, fingers and toes and other ill due to cold temperatures surfaces that come into contact with the body determine the conductive heat exchange. In addition temperature, surface properties, such as conductivity, specific heat, heat capacity also important and determine the conductive heat exchange (Havenith, 1999, 2002). According to Havenith (1999) and also Fox, Bowers and Foss (1988) heat exchange conduction is only important in jobs associated with objects heat. However, in the context of field work activity continues to be the air temperature important factor because, according to Holman (1986) although the dominant transfer paths in this context is the radiation and convection, but in the air layer is thin on the body surface, heat transfer initially occurs by conduction between molecules of the body's skin or clothing with air molecules, which then proceed through the process of convection.