Human Engg. & Safety 2(1+1)

Lesson 12. HEAT EXCHANGE PROCESS AND PERFORMANCE, AIR POLLUTION

Evaporation

Evaporation is the method of body heat transmission to the environment by evaporation of perspiration and respiration. Evaporation can only result in a transfer of heat from the body to the environment. Sweat must be evaporated from the skin surface for the heat transfer to occur. In saturated environments (RH=100%), no heat transfer can take place through evaporation

Convection

Convection is the method of heat exchange through a fluid medium viz. air, water. Generally the convection heat transfer is made by air or fluid through movement of its molecules. The air/ fluid molecules are warmed up by touching the environment, in turn, absorb heat from the warmer air/ fluid molecules. This process is called natural  or free convection.

Conduction

Conduction is the method of heat exchange by direct contact of the body with other objects, in this method, heat is transferred from one substance to another, or among molecules within a substance, without physical movement of the substance itself or its molecules. Since clothing insulates the body, the amount of heat exchange in the body is usually minimal and is often ignored in heat balance equation.

Radiation

Generally any substance with a temperature above absolute zero emits heat radiation in the form of electromagnetic waves (similar to light waves). Thermal radiation will travel through vacuum or transparent media. The amount of heat radiated depends on the temperature and area of the radiating object, regardless of its mass.

Latent heat

Latent heat is the amount of heat that must be absorbed when a substance changes phase from solid to liquid or from liquid to gas, or the amount of heat lost (emitted) during phase changes from gas to liquid or from liquid to solid.

Radiation Heat Load

Radiant heat exchange occurs between the occupant and its surroundings. During vehicle cool down and warm-up processes, the radiation heat load has roughly the same influence as air temperature on occupant thermal comfort. The current human physiology model can simulate an arbitrary number of body segments. Each of these segments consists of four body layers (core, muscle, fat, and skin tissues) and a clothing layer. A separate series of nodes, representing the arteries and veins, provide for convective heat transfer between segments and tissue nodes and the counter current heat exchange between the arteries and the veins. Human body thermal regulation is mainly achieved by regulating blood flow, so a realistic blood flow model is important for any dynamic model of human thermal comfort. The body uses vasoconstriction and vasodilatation to regulate blood distribution in order to control skin temperature through an increase or decrease of heat loss to the environment. Veins and arteries are paired, even down to very small vessels, and veins carry heat from the arteries back to the core. The model is able to predict both core and extremity skin temperatures with reasonable accuracy under a range of environmental conditions. The validations for transient conditions are;

1. Clothing Model :

2. Contact surfaces:

3. Physiological variation:

 1. Clothing Model: Heat capacity of the clothing is important when considering transient effects.  Moisture capacitance is important to correctly model evaporative heat loss from the body through clothing. The moisture model uses the regain approach to calculate the amount of moisture that a specific fabric will absorb at a given  relative humidity.

2. Contact surfaces: In almost any environment, the body is in contact with solid surfaces and loses or gains heat via heat conduction. In the vehicle, the seat contacts a considerable fraction of the body and must be considered to accurately model the occupant. The thermal properties of the contact surface are used to simulate its surface temperature. Each body segment includes the fractions of exposed skin and clothed skin in contact with the surface.

3. Physiological variation: Human physiology varies significantly among individuals, and these differences can affect perceptions of thermal comfort; e.g., higher metabolic rate or increased body fat can cause people to feel warmer.

Sweating

Humans have about 1.6 million high-capacity (eccrine) sweat glands in their skin. This is why humans can lose about 500 grams of sweat per square meter of skin, whereas horses and camels can lose only 100 and 240 g, respectively.

Sweat which is produced by the eccrine glands in the skin consists mainly of water. It is a dilute solution of various electrolytes, principally sodium, potassium and chloride. Passive diffusion of water through the sweat glands. However in humid environments, evaporation of sweat diminishes and cooling efficiency is lost even though sweat continues to be produced.

Profuse sweating has two important disadvantages:

• Dehydration may occur if more water is lost than is replaced.

• Salt may be lost.

The adrencortical hormone, aldosterone, is involved in the conservation of sodium, causing it to be actively reabsorbed in the sweat glands and in the kidneys. As the rate of sweat production increases, less sodium ions can be reabsorbed.

Shivering

Involuntary shivering is a thermoregulatory mechanism involving active heat production. Group of motor units act out of phase with one another and muscles act against their antagonists.

Voluntary movement also increases heat production but breaks up the insulating layers of air around the body, which increases the rate of heat loss. Both shivering and voluntary movement increase oxygen uptake and cardiac output and can lower a person’s capacity to carry out physical work.

Work in Hot Climates

Heavy physical work in the heat imposes conflicting demands on the cardiovascular system. Cardiac capacity is a limiting factor placed under considerable strain when a person in working in the heat as output rises to meet the demands of both physical work and bodily cooling.

Heat Stroke

If a worker becomes dehydrated, sweat production diminishes and the deep body temperature may increase. Rapid elevations in body temperature increase the metabolic rate. If core temperature rises above 42⁰ C, blood pressure may drop and insufficient blood is pumped to the vital organs, including the heart, kidney, and brain. rapid cooling of the body using cold sprays or wet sponges can be used to lower the temperature. Many industries take steps to prevent such dangerous situations from occurring as is described below.

Relative Humidity

Whether workers can tolerate a hot environment depends on several classes of variables. For example, dry-bulb temperature of more than 38⁰ C can be tolerated if the relative humidity is less than 20 percent because at such low humidities the cooling efficiency of sweating is high.

Heat Acclimatization

Heat acclimatization is a physiological process of adaptation rather than a psychological adjustment to life in hot environment. It involves an increase in the capacity to produce sweat and a decrease in the core temperature threshold value for the initiation of sweating. Furthermore, acclimatization reduces the skin’s blood-flow requirements, which reduces the cardiovascular load during work in the heat. Although the body can acclimatize to heat, it cannot acclimatize to dehydration. Fluid must be made available at all times even for acclimatized workers.

Physiological responses and heat exchange

When people are exposed to a hot environment, they experience first, vasodilatation (expansion of blood capillaries near the skin surface), which facilitates increased heat transfer from the core to the shell of the body to be removed by evaporation; and second , an activation of the sweat glands (in the subcutaneous layer under the skin) to facilitate evaporative heat loss (Astrand and Rodahi, 1986). At some level of heat load the limit of heat dissipation will be reached. Beyond this physiological limit the establishment of a steady state for the core temperature will become impossible, and it may increase to dangerous level.  Furthermore the physiological limit is usually reached before the body temperature rises much above 40^oC (Poulton, 1970)

To prevent the internal heat build-up the body has to dissipate some of its metabolic heat. The body attempts to achieve thermal equilibrium with its surrounding environment through the following heat exchange methods: metabolism, evaporation, convection, conduction and radiation. The heat exchange follows the second law of thermodynamics, according to which heat from a substance of a higher temperature is transferred to another object with a lower temperature. The process of heat exchange between the body and its surrounding environment can be expressed by the heat balance equation:

+ S=M+CV + CD + R-E

Where

S          =          heat storage (positive sign indicates heat gain, while negative indicates heat loss. If the heat balance is achieved, S =0);

M         =          metabolic heat (always positive)

CV      =          convective heat (positive sign indicates air temperature is higher than skin temperature and negative indicates a reversed case).

CD     =           conductive heat (positive when the contacting objects are warmer than skin and negative when the skin is warmer);

R         =          radiant heat (positive when surrounding objects are warmer than skin and negative when the skin is warmer);

E          =          evaporative heat (always negative).

Fundamentals of Human Thermoregulation

Human have a remarkably well-adapted ability to tolerate heat compared with other primates. This statement applies equally to Eskimos as to tropical rain forest dwellers despite their small differences.

Countercurrent Heat Exchange

Countercurrent heat exchange is essentially a method of conserving heat. It involves the exchange of heat between arteries and veins supplying the deep tissues. Arterial blood is precooled before it reaches the extremities and venous blood is warmed before it returns to the vital organs. An efficient heat-exchange system enables penguins for example, to spend long hours standing on ice at many degrees below freezing and to maintain a very high temperature gradient between their deep body tissues and the soles of their feet.

Effects of Heat on Performance

High internal temperatures appear to increase the speed of performance because they accelerate the body’s “internal clock.” Fox et al. (1967) showed that increase in body temperature accelerated people’s perception of the passage of time. Colquhoun and Goldman (1972) investigated the ability of subjects to detect target stimuli against a noisy background at a temperature of 38⁰ dry-bulb 33⁰ wet-bulbs. It was found that only when work in the heat was accompanied by an actual increase in body temperature was an effect on performance observed. Azer et al. (1972) provide more evidence that hot conditions have significant effects on performance only when they cause a rise in body temperature. In their experiment performance decrements occurred when subjects worked at 35⁰ C or 75relative humidity, but not at 35⁰ or 37.5⁰ and 50 percent relative humidity.

Assessment of heat stress exposure limits

Although heat acclimatization improves the capacity of the person to work under hot thermal conditions, the dangers and problems associated with exposure to heat stress cannot be totally relieved. Even for an acclimatized person, the maximum tolerable levels of heat exposure depend on the combination of ambient temperature, radiernt heat, humidity, air movement, metabolic heat during work, and the duration of exposure.

Factors Affecting Heat Stress And Strain

The following three general factors affect the level of heat stress of individuals who work in hot environments:

• Environmental thermal conditions: that is ambient temperature radiant heat, humidity and air movement.

• Physical workload, which positively affects the body’s heat generated internally, through metabolism.

• Clothing: some clothes are permeable and allow heat exchange between the body and the surrounding environment. Others are impermeable, especially those used in personal protective garments for use in chemically hazardous environments, which constrict the heat exchange process. Holzl et al. (1983) found that at 79⁰F (26.1⁰C) ET*, individuals with a clothing value of 0.54 clo, with no difference due to gender.

The threshold limit of heat stress should be modified to account for clothing. Clothing can be as unrestrictive as shorts and a T-shirt or as restrictive as a suit of armor. Restrictive clothing can interference with the body’s evaporation capability. As Ramsey (1978) stated, for unrestrictive clothing, a positive 2⁰C (3.6⁰F)is added to the threshold limit. As much as 10⁰C (18⁰F) may be subtract from the heat stress TLVs to account for a suit of armor.

The level of heat strain experienced by an individual exposed to heat stress is influenced by the above three heat stress factors and the personal characteristics, such as gender, age, race, physical fitness, status of nutrition, medical history, skill, body build and heat acclimatization. These personal characteristics are described in the following section.

Perception of air and thermal quality

In the previous sections, individual differences in heat tolerance were noted. Individual differences in temperature preference also exist. Investigations have shown that some people find temperatures as low as 18⁰C comfortable, whereas others prefer temperatures higher than 23⁰C. Sundstrom (1986) suggests on the basis of findings such as these that individuals be given a certain amount of control of the temperature of their workplaces.

The threshold at which air is perceived as stuffy begins at a relative humidity of 60 percent at 24⁰C and 80 percent at 18⁰C. Dehumidifiers can be used to lower the relative humidity in a building to acceptable levels.            Low relative humidity causes bodily secretions to dry up. Under these conditions, the occupants may complain of dry, blocked noses and eye irritation. Contact lens wearers may experience eye discomfort since proper adhesion of the lens to the eye depends on a continuous supply of lachrymal fluid to maintain a thin, moist film over the cornea. Somewhat counterintuitively, low relative humidity inside houses and offices can be a problem in cold, wet countries if artificially heated buildings are not ventilated adequately and if humidifiers are not provided.

Applications and Discussion

Temperature regulation in the human is normally maintained in a very close range, around 37⁰C. As a worker is exposed to warm or cool environments, physiological changes take place to maintain the required body temperature. Through a process of acclimatization, the body adapts somewhat to the thermal load of the environment. However as the thermal load exceeds the body’s ability to adapt, engineering and administrative controls or persona protective equipment must be incorporated to protect the worker.

There are many occupations in which workers are routinely exposed to hot and cold environments. If the worker is not properly protected the consequences could become fatal. This chapter has provided tools for understanding the thermal environment and physiological responses to change in temperature. The ergonomist should be able to assess a thermal environment, understand the exposure risks for workers and to develop guidelines for employees exposed to such environment.

 

References

Astrand and Rodahi (1986). Textbook of Work Physiology: Physiological Bases of Exercise, 3^rd edn. McGraw-Hill, New York.

Poulton, (1970).          Environment and Human Efficiency. Thomas Springfield, IL

Fox et al. (1967). Time judgement body temperature. Journal ofExperimental Psychology, 75:88-98.

Azer et al. (1972).  Effects of heat stress on performance. Ergonomics, 15:681-691.

Colquhoun and Goldman (1972). Vigilance under induced hyperthermia. Ergonomics, 15:621-632.

Holzl et al. (1983). A validation study of the ASHRAE summer comfort envelope. ASHRAE transaction 89: 126-138.

Ramsey (1978). Abbreviated guidelines for heat stress exposure. American Industrial Hygiene Association Journal 39: 491-495.

Sundstrom (1986). Workplaces. Cambridge University Press, Cambridge, England.