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El libro comprendido como una unidad de hojas impresas que se encuentran encuadernadas en determinado material que forman un volumen ordenado, puede dividir su producción en dos grandes períodos: La coexistencia del desarrollo de la imprenta con el comienzo del movimiento humanista y la reforma luterana impulsaron el crecimiento de la industria del libro, puesto que vieron en él un medio de difusión masivo. En medio siglo, la segunda mitad del siglo XV, el libro impreso se convirtió en un importante negocio internacional, los libreros e impresores fueron ante todo empresarios.
La superioridad de la imprenta sobre la xilografía fue incuestionable, la escritura era regular, impresión a ambas caras, rapidez de impresión y la posibilidad de volver a utilizar los caracteres para imprimir otros textos. Se puede establecer una cronología del libro antiguo dividida en siglos, tomando como base ciertas características comunes en un siglo determinado: El mismo libro, se convierte en un avance que da distinción a los lectores como progresistas en un siglo en que el progreso es una meta social ampliamente deseada y a la que pueden acceder por igual nobles y plebeyos, creando una meritocracia de nuevo cuño.
A pesar de lo anterior, la minoría que cultiva el gusto por el libro se encuentra entre los nobles y las clases altas y cultivadas de los plebeyos, pues sólo estos grupos sociales saben leer y escribir, lo que representa el factor cultural adicional para el inevitable auge del libro. Otro importante factor que fomentó el aprecio por los libros fue la Censura , que si bien solía ejercerse también en periodos anteriores a los siglos XVII y XVIII, es precisamente en esta época cuando adquiere mayor relevancia, puesto que los libros se producen por millares, multiplicando en esa proporción la posibilidad de difundir ideas que el Estado y la Iglesia no desean que se divulguen.
La draconiana medida fue complementada con un decreto que prohibía a cualquiera que no estuviera autorizado a publicar libros de tema religioso. En , otro decreto obligaba a los editores a obtener autorizaciones antes y después de publicar cada libro y en , se ordenó vigilar incluso los lugares libres de censura. En tanto la censura se volvió inefectiva e incluso los censores utilizaron dicha censura como medio para promover a astutos escritores y editores. Las signaturas se ordenan y se cosen por el lomo.
Luego este lomo es redondeado y se le pega una malla de tela para asegurar las partes. Toda esta tarea se realiza en serie, inclusive la encuadernación.
En el caso de que las hojas no sean alisadas mediante un proceso de corte, se habla de un libro intonso. Los importantes avances en desarrollo de software y las tecnologías de impresión digital han permitido la aplicación de la producción bajo demanda en inglés el acrónimo P. Esto es posible solo por estar dados de alta en los sistemas de producción de compañías internacionales como Lightning Source, Publidisa, Booksurge, Anthony Rowe, etc.
A finales de comenzó a desarrollarse lo que hoy denominamos libro digital o electrónico. En se produce un importante avance, ya que sale a la venta el primer libro electrónico: Random House's Electronic Dictionary. En el año se recogían los siguientes datos: Una investigación que tomó como base los libros de la Biblioteca del Congreso de Estados Unidos , con sede en Washington, D. Vladimir Lenin resulta el tercero, con , seguido de Abraham Lincoln , con , y de Napoleón I, con La siguiente es Juana de Arco , con Da Vinci, sin embargo, se lleva la palma en la lista de científicos e inventores, superando a Charles Darwin , Albert Einstein y Galileo Galilei.
En las bibliotecas se suele utilizar el Sistema Dewey de clasificación por materias. De Wikipedia, la enciclopedia libre. Para la localidad aragonesa, véase Libros Teruel. Ong y otros especialistas. Libros, dos mil años de historia ilustrada. Historia de la imprenta coreana. Universidad Autónoma de Barcelona. Scribes, Script and Book.
Libros y libreros en la antigüedad. Albores de la imprenta. Fondo de Cultura Económica. Consultado el 19 de enero de Consultado el 11 de julio de Espacios de nombres Artículo Discusión. Vistas Leer Editar Ver historial.
En otros proyectos Wikimedia Commons Wikiquote. Al usar este sitio, usted acepta nuestros términos de uso y nuestra política de privacidad. This ability to voluntarily adjust work rate probably depends on awareness of cardiovascular stress and fatigue. Human beings cannot consciously detect elevations in core body temperature; rather, they rely on skin temperature and skin wettedness to assess thermal discomfort.
An alternative approach to schedule modification is the adoption of prescribed work-rest cycles, where management specifies the duration of each work bout, the length of rest breaks and the number of repetitions expected.
Thermal recovery takes much longer than the period required to lower respiratory rate and work-induced heart rate: Lowering core temperature to resting levels requires 30 to 40 min in a cool, dry environment, and takes longer if the person must rest under hot conditions or while wearing protective clothing. If a constant level of production is required, then alternating teams of workers must be assigned sequentially to hot work followed by recovery, the latter involving either rest or sedentary tasks performed in a cool place.
If cost were no object, all heat stress problems could be solved by application of engineering techniques to convert hostile working environments to hospitable ones. A wide variety of techniques may be used depending on the specific conditions of the workplace and available resources. Traditionally, hot industries can be divided into two categories: In hot-dry processes, such as metal smelting and glass production, workers are exposed to very hot air combined with strong radiant heat load, but such processes add little humidity to the air.
In contrast, warm-moist industries such as textile mills, paper production and mining involve less extreme heating but create very high humidities due to wet processes and escaped steam.
The most economical techniques of environmental control usually involve reduction of heat transfer from the source to the environment. Hot air may be vented outside the work area and replaced with fresh air. Hot surfaces can be covered with insulation or given reflective coatings to reduce heat emissions, simultaneously conserving heat which is needed for the industrial process.
A second line of defence is large-scale ventilation of the work area to provide a strong flow of outside air. The most expensive option is air conditioning to cool and dry the atmosphere in the workplace. Although lowering air temperature does not affect transmission of radiant heat, it does help to reduce the temperature of the walls and other surfaces which may be secondary sources of convective and radiative heating. When overall environmental control proves impractical or uneconomical, it may be possible to ameliorate thermal conditions in local work areas.
Local or even portable reflective shielding may be interposed between the worker and a radiant heat source. Alternatively, modern engineering techniques may allow construction of remote systems to control hot processes so that workers need not suffer routine exposure to highly stressful heat environments.
For instance, a spring heat wave can precipitate an epidemic of heat illness among workers who are not yet heat acclimatized as they would be in summer. Management should therefore implement a system for predicting weather-related changes in heat stress so that timely precautions can be taken. Work in extreme thermal conditions may require personal thermal protection in the form of specialized clothing.
Passive protection is provided by insulative and reflective garments; insulation alone can buffer the skin from thermal transients. Reflective aprons may be used to protect personnel who work facing a limited radiant source.
Another form of passive protection is the ice vest, which is loaded with slush or frozen packets of ice or dry ice and is worn over an undershirt to prevent uncomfortable chilling of the skin.
The phase change of the melting ice absorbs part of the metabolic and environmental heat load from the covered area, but the ice must be replaced at regular intervals; the greater the heat load, the more frequently the ice must be replaced.
Ice vests have proven most useful in deep mines, ship engine rooms, and other very hot, humid environments where access to freezers can be arranged. Active thermal protection is provided by air- or liquid-cooled garments which cover the entire body or some portion of it, usually the torso and sometimes the head.
The simplest systems are ventilated with the surrounding, ambient air or with compressed air cooled by expansion or passage through a vortex device. Air cooling can theoretically take place through convection temperature change or evaporation of sweat phase change. However, the effectiveness of convection is limited by the low specific heat of air and the difficulty in delivering it at low temperatures in hot surroundings.
Most air-cooled garments therefore operate through evaporative cooling. The worker experiences moderate heat stress and attendant dehydration, but is able to thermoregulate through natural control of the sweat rate. Air cooling also enhances comfort through its tendency to dry the underclothing.
Disadvantages include 1 the need to connect the subject to the air source, 2 the bulk of air distribution garments and 3 the difficulty of delivering air to the limbs. These systems circulate a water-antifreeze mixture through a network of channels or small tubes and then return the warmed liquid to a heat sink which removes the heat added during passage over the body.
The heat sink may dissipate thermal energy to the environment through evaporation, melting, refrigeration or thermoelectric processes.
Liquid-cooled garments offer far greater cooling potential than air systems. A full-coverage suit linked to an adequate heat sink can remove all metabolic heat and maintain thermal comfort without the need to sweat; such a system is used by astronauts working outside their spacecraft. However, such a powerful cooling mechanism requires some type of comfort control system which usually involves manual setting of a valve which shunts part of the circulating liquid past the heat sink.
Liquid-cooled systems can be configured as a back pack to provide continuous cooling during work. Any cooling device which adds weight and bulk to the human body, of course, may interfere with the work at hand.
For instance, the weight of an ice vest significantly increases the metabolic cost of locomotion, and is therefore most useful for light physical work such as watch-standing in hot compartments. Systems which tether the worker to a heat sink are impractical for many types of work. Intermittent cooling may be useful where workers must wear heavy protective clothing such as chemical protective suits and cannot carry a heat sink or be tethered while they work.
Removing the suit for each rest break is time consuming and involves possible toxic exposure; under these conditions, it is simpler to have the workers wear a cooling garment which is attached to a heat sink only during rest, allowing thermal recovery under otherwise unacceptable conditions.
The human body exchanges heat with its environment by various pathways: Conduction is the transmission of heat between two solids in contact. Such exchanges are observed between the skin and clothing, footwear, pressure points seat, handles , tools and so on. In practice, in the mathematical calculation of thermal balance, this heat flow by conduction is approximated indirectly as a quantity equal to the heat flow by convection and radiation which would take place if these surfaces were not in contact with other materials.
Convection is the transfer of heat between the skin and the air surrounding it. Air circulation, known as natural convection, is thus established at the surface of the body. This exchange becomes greater if the ambient air passes over the skin at a certain speed: Moreover, it receives the radiation emitted by neighbouring surfaces. F clR is the factor by which clothing reduces radiation heat exchange. This expression may be replaced by a simplified equation of the same type as that for exchanges by convection:.
Every wet surface has on it a layer of air saturated with water vapour. If the atmosphere itself is not saturated, the vapour diffuses from this layer towards the atmosphere. The layer then tends to be regenerated by drawing on the heat of evaporation 0. P sk,s is the saturated pressure of water vapour at the temperature of the skin expressed in kPa. P a is the ambient partial pressure of water vapour expressed in kPa. F pcl is the factor of reduction of exchanges by evaporation due to clothing.
A correction factor operates in the calculation of heat flow by convection, radiation and evaporation so as to take account of clothing. In the case of cotton clothing, the two reduction factors F clC and F clR may be determined by:. As regards the reduction of heat transfer by evaporation, the correction factor F pcl is given by the following expression:. An insulation of 1 clo corresponds to 0. ISO standard gives the thermal insulation provided by different combinations of clothing.
In the case of special protective clothing that reflects heat or limits permeability to vapour under conditions of heat exposure, or absorbs and insulates under conditions of cold stress, individual correction factors must be used.
To date, however, the problem remains poorly understood and the mathematical predictions remain very approximate. The appliances and methods for measuring these physical parameters of the environment are the subject of ISO standard , which describes the different types of sensor to use, specifies their range of measurement and their accuracy, and recommends certain measurement procedures. This section summarizes part of the data of that standard, with particular reference to the conditions of use of the most common appliances and apparatus.
There are numerous types of thermometers on the market. Mercury thermometers are the most common. Their advantage is accuracy, provided that they have been correctly calibrated originally. Their main disadvantages are their lengthy response time and lack of automatic recording ability. Electronic thermometers, on the other hand, generally have a very short response time 5 s to 1 min but may have calibration problems. Whatever the type of thermometer, the sensor must be protected against radiation.
This is generally ensured by a hollow cylinder of shiny aluminium surrounding the sensor. Such protection is ensured by the psychrometer, which will be mentioned in the next section. The saturated water vapour pressure P S,t at any temperature t is given by:. The psychrometric diagram figure The parameters of humidity most often used in practice are:. The range of measurement and the accuracy recommended are 0.
The mean radiant temperature t r has been defined previously; it can be determined in three different ways:. The black sphere thermometer consists of a thermal probe, the sensitive element of which is placed at the centre of a completely closed sphere, made of a metal that is a good conductor of heat copper and painted matt black so as to have a coefficient of absorption in the infrared zone close to 1.
The sphere is positioned in the workplace and subjected to exchanges by convection and radiation. The temperature of the globe t g then depends on the mean radiant temperature, the air temperature and the air velocity. For a standard black globe 15 cm in diameter, the mean temperature of radiation can be calculated from the temperature of the globe on the basis of the following expression:. In practice, the need must be stressed to maintain the emissivity of the globe close to 1.
The main limitation of this type of globe is its long response time of the order of 20 to 30 min, depending on the type of globe used and the ambient conditions. The measurement is valid only if the conditions of radiation are constant during this period of time, and this is not always the case in an industrial setting; the measurement is then inaccurate. These response times apply to globes 15 cm in diameter, using ordinary mercury thermometers. They are shorter if sensors of smaller thermal capacity are used or if the diameter of the globe is reduced.
The equation above must therefore be modified to take account of this difference in diameter. The WBGT index makes direct use of the temperature of the black globe. It is then essential to use a globe 15 cm in diameter. On the other hand, other indices make use of the mean radiant temperature. A smaller globe can then be selected to reduce the response time, provided that the equation above is modified to take account of it.
The air velocity must be measured disregarding the direction of air flow. Otherwise, the measurement must be undertaken in three perpendicular axes x, y and z and the global velocity calculated by vectorial summation:. The range of measurements recommended by ISO standard extends from 0. It should be measured as a 1- or 3-min average value. There are two categories of appliances for measuring air velocity: The measurement is carried out by counting the number of turns made by the vanes during a certain period of time.
In this way the mean velocity during that period of time is obtained in a discontinuous manner. These anemometers have two main disadvantages:. They are very directional and have to be oriented strictly in the direction of the air flow. When this is vague or unknown, measurements have to be taken in three directions at right angles. The range of measurement extends from about 0. This limitation to low velocities is important when, for instance, it is a question of analysing a thermal comfort situation where it is generally recommended that a velocity of 0.
They are appliances giving an instantaneous estimate of speed at one point of space: These appliances are also often very directional, and the remarks above also apply. Finally, the measurement is correct only from the moment when the temperature of the appliance has reached that of the environment to be evaluated. This response can be powerful and effective, but it can also produce a strain on the body which leads to discomfort and eventually to heat illness and even death.
It is important therefore to assess hot environments to ensure the health and safety of workers. Heat stress indices provide tools for assessing hot environments and predicting likely thermal strain on the body. Limit values based upon heat stress indices will indicate when that strain is likely to become unacceptable. The mechanisms of heat stress are generally understood, and work practices for hot environments are well established.
These include knowledge of the warning signs of heat stress, acclimatization programmes and water replacement. There are still many casualties, however, and these lessons seem to have to be relearned. In , Leithead and Lind described an extensive survey and concluded that heat disorders occur for one or more of the following three reasons:.
They concluded that many deaths can be attributed to neglect and lack of consideration and that even when disorders do occur, much can be done if all the requirements for the correct and prompt remedial treatment are available. A heat stress index is a single number which integrates the effects of the six basic parameters in any human thermal environment such that its value will vary with the thermal strain experienced by the person exposed to a hot environment.
The index value measured or calculated can be used in design or in work practice to establish safe limits. Much research has gone into determining the definitive heat stress index, and there is discussion about which is best. For example, Goldman presents 32 heat stress indices, and there are probably at least double that number used throughout the world. Many indices do not consider all six basic parameters, although all have to take them into conside ration in application.
The use of indices will depend upon individual contexts, hence the production of so many. Some indices are inadequate theoretically but can be justified for specific applications based on experience in a particular industry. The recent surge in standardization e.
It will be necessary, however, to gain experience with the use of any new index. Most heat stress indices consider, directly or indirectly, that the main strain on the body is due to sweating. For example, the more sweating required to maintain heat balance and internal body temperature, the greater the strain on the body. For an index of heat stress to represent the human thermal environment and predict heat strain, a mechanism is required to estimate the capacity of a sweating person to lose heat in the hot environment.
An index related to evaporation of sweat to the environment is useful where persons maintain internal body temperature essentially by sweating. These conditions are generally said to be in the prescriptive zone WHO Hence deep body temperature remains relatively constant while heart rate and sweat rate rise with heat stress.
At the upper limit of the prescriptive zone ULPZ , thermoregulation is insufficient to maintain heat balance, and body temperature rises. This is termed the environmentally driven zone WHO In this zone heat storage is related to internal body temperature rise and can be used as an index to determine allowable exposure times e.
Heat stress indices can be conveniently categorized as rational, empirical or direct. Rational indices are based upon calculations involving the heat balance equation; empirical indices are based on establishing equations from the physiological responses of human subjects e.
The most influential and widely used heat stress indices are described below. The Heat Stress Index is the ratio of evaporation required to maintain heat balance Ereq to the maximum evaporation that could be achieved in the environment Emax , expressed as a percentage Belding and Hatch Equations are provided in table The HSI as an index therefore is related to strain, essentially in terms of body sweating, for values between 0 and Mild to moderate heat strain.
Little effect on physical work but possible effect on skilled work. Severe heat strain, involving threat to health unless physically fit. Very severe heat strain. Personnel should be selected by medical examination. Ensure adequate water and salt intake. An important improvement is the recognition that not all sweat evaporates.
An important improvement is the recognition that not all sweat evaporates e. For outdoor conditions with solar load, T g is replaced with T a and allowance made for solar load R s by:. Similar to the other rational indices, SW req is derived from the six basic parameters air temperature T a , radiant temperature T r , relative humidity air velocity v , clothing insulation I cl , metabolic rate M and external work W.
From this the evaporation required is calculated from:. Equations are provided for each component see table From the required evaporation E req and maximum evaporation E max and sweating efficiency r , the following are calculated:. If the men are clothed, increase the wet bulb temperature by 1.
The P4SR is determined from figure The P4SR is then:. Givoni and Goldman provide equations for predicting heart rate of persons soldiers in hot environments. They define an index for heart rate IHR from a modification of predicted equilibrium rectal temperature,. The equilibrium heart rate HR f in beats per minute is then given by:. This index calculated sweating required for heat balance from an improved heat balance equation but, most importantly, also provided a practical method of interpretation of calculations by comparing what is required with what is physiologically possible and acceptable in humans.
Extensive discussions and laboratory and industrial evaluations CEC of this index led to it being accepted as International Standard ISO b.
Differences between observed and predicted responses of workers led to the inclusion of cautionary notes concerning methods of assessing dehydration and evaporative heat transfer through clothing in its adoption as a proposed European Standard prEN Essentially, if what is calculated as required can be achieved, then these are predicted values e. If they cannot be achieved, the maximum values can be taken e. More detail is given in a decision flow chart see figure If required sweat rate can be achieved by persons and it will not cause unacceptable water loss, then there is no limit due to heat exposure over an 8-hour shift.
If not, the duration-limited exposures DLE are calculated from the following:. If the above are not satisfied, then:. Fuller details are given in ISO b. More developments with this approach can be made; however, an alternative approach is to use a thermal model. Givoni and Goldman , also provide empirical prediction models for the assessment of heat stress. The Effective Temperature index Houghton and Yaglou was originally established to provide a method for determining the relative effects of air temperature and humidity on comfort.
Three subjects judged which of two climatic chambers was warmer by walking between the two. Using different combinations of air temperature and humidity and later other parameters , lines of equal comfort were determined. Immediate impressions were made so the transient response was recorded. This had the effect of over-emphasizing the effect of humidity at low temperatures and underestimating it at high temperatures when compared with steady-state responses.
Although originally a comfort index, the use of the black globe temperature to replace dry bulb temperature in the ET nomograms provided the Corrected Effective Temperature CET Bedford Research reported by Macpherson suggested that the CET predicted physiological effects of increasing mean radiant temperature. ET and CET are now rarely used as comfort indices but have been used as heat stress indices. It is the amount of sweat secreted by fit, acclimatized young men exposed to the environment for 4 hours while loading guns with ammunition during a naval engagement.
The single number index value which summarizes the effects of the six basic parameters is an amount of sweat from the specific population, but it should be used as an index value and not as an indication of an amount of sweat in an individual group of interest. It was acknowledged that outside of the prescriptive zone e. The P4SR nomograms figure The P4SR appears to have been useful under the conditions for which it was derived; however, the effects of clothing are over-simplified and it is most useful as a heat storage index.
Heart rate prediction as an index Fuller and Brouha proposed a simple index based on the prediction of heart rate HR in beats per minute. Body temperature and pulse rates are measured during recovery following a work cycle or at specified times during the working day.
At the end of a work cycle the worker sits on a stool, oral temperature is taken and the following three pulse rates are recorded:. The ultimate criterion in terms of heat strain is an oral temperature of The component of thermal strain possible heat stress index can be calculated from:. It was developed in a US Navy investigation into heat casualties during training Yaglou and Minard as an approximation to the more cumbersome Corrected Effective Temperature CET , modified to account for the solar absorptivity of green military clothing.
WBGT limit values were used to indicate when military recruits could train. It was found that heat casualties and time lost due to cessation of training in the heat were both reduced by using the WBGT index instead of air temperature alone. The specification of the measuring instruments is provided in the standard, as are WBGT limit values for acclimatized or non- acclimatized persons see table For example, for a resting acclimatized person in 0.
The simplicity of the index and its use by influential bodies has led to its widespread acceptance. Like all direct indices it has limitations when used to simulate human response, and should be used with caution in practical applications. It is possible to buy portable instruments which determine the WBGT index e. Dasler , provides WBGT limit values based on a prediction of exceeding any two physiological limits from experimental data of impermissible strain. The limits are given by:.
This index therefore uses the WBGT direct index in the environmentally driven zone see figure The temperature of a wet black globe of appropriate size can be used as an index of heat stress. The principle is that it is affected by both dry and evaporative heat transfer, as is a sweating man, and the temperature can then be used, with experience, as a heat stress index.
Olesen describes WGT as the temperature of a 2. The temperature is read when equilibrium is reached after about 10 to 15 minutes of exposure. It is a 3-inch The sensing element of a thermometer is located at the centre of the sphere, and the temperature is read on a colour coded dial. Lind proposed a simple, direct index used for storage- limited heat exposure and based on a weighted summation of aspirated wet bulb temperature T wb and dry bulb temperature T db:. Allowable exposure times for mine rescue teams were based on this index.
It is widely applicable but is not appropriate where there is significant thermal radiation. NIOSH provides a comprehensive description of working practices for hot environments, including preventive medical practices. A proposal for medical supervision of individuals exposed to hot or cold environments is provided in ISO CD It should always be remembered that it is a basic human right, which was affirmed by the Declaration of Helsinki, that, when possible, persons can withdraw from any extreme environment without need of explanation.
Where exposure does take place, defined working practices will greatly improve safety. It is a reasonable principle in environmental ergonomics and in industrial hygiene that, where possible, the environmental stressor should be reduced at the source. NIOSH divides control methods into five types. These are presented in table Reduce surface temperatures or place reflective shield between radiant source and workers. Change emissivity of surface.
Use doors which open only when access required. Increase air movement, decrease water vapour pressure. Use fans or air conditioning. Wet clothing and blow air across person. Perform jobs at cooler times of day and year. Provide cool areas for rest and recovery. Extra personnel, worker freedom to interrupt work, increase water intake.
Keep workers physically fit. Ensure water loss is replaced and maintain electrolyte balance if necessary. Supervisors trained in recognizing signs of heat illness and in first aid. Basic instruction to all personnel on personal precautions, use of protective equipment and effects of non-occupational factors e.
Contingency plans for treatment should be in place. Memos to supervisors to make checks of drinking fountains, etc. Check facilities, practices, readiness, etc. Increase workers, increase rest. Remind workers to drink. Use if it is not possible to modify worker, work or environment and heat stress is still beyond limits.
Individuals should be fully heat acclimatized and well trained in use and practice of wearing the protective clothing. Examples are water-cooled garments, air-cooled garments, ice-packet vests and wetted overgarments. It must be remembered that wearing protective clothing that is providing protection from toxic agents will increase heat stress.
All clothing will interfere with activities and may reduce performance e. There has been a great deal of military research into so-called NBC nuclear, biological, chemical protective clothing. In hot environments it is not possible to remove the clothing, and working practices are very important. A similar problem occurs for workers in nuclear power stations. Methods of cooling workers quickly so that they are able to perform again include sponging the outer surface of the clothing with water and blowing dry air over it.
Other techniques include active cooling devices and methods for cooling local areas of the body. The transfer of military clothing technology to industrial situations is a new innovation, but much is known, and appropriate working practices can greatly reduce risk. The following hypothetical example demonstrates how ISO standards can be used in the assessment of hot environments Parsons Workers in a steel mill perform work in four phases.
They don clothing and perform light work for 1 hour in a hot radiant environment. They rest for 1 hour, then perform the same light work for an hour shielded from the radiant heat. They then perform work involving a moderate level of physical activity in a hot radiant environment for 30 minutes. If the levels exceed the reference values table This can be achieved by engineering controls and working practices.
A complementary or alternative action is to conduct an analytical assessment according to ISO The WBGT values for the work are presented in table The environmental and personal factors relating to the four phases of the work are presented in table It can be seen that for part of the work the WBGT values exceed those of the reference values. It is concluded that a more detailed analysis is required.
The analytical assessment method presented in ISO was performed using the data presented in table The results for acclimatized workers in terms of alarm level are presented in table If greater accuracy is required, or individual workers are to be assessed, then ISO and ISO will provide detailed information concerning metabolic heat production and clothing insulation.
ISO describes methods for measuring physiological strain on workers and can be used to design and assess environments for specific workforces. Mean skin temperature, internal body temperature, heart rate and mass loss will be of interest in this example.
ISO CD provides guidance on medical supervision of an investigation. In order to survive and work under colder or hotter conditions, a warm climate at the skin surface must be provided by means of clothing as well as artificial heating or cooling.
An understanding of the mechanisms of heat exchange through clothing is necessary to design the most effective clothing ensembles for work at extreme temperatures. Heat transfer through clothing, or conversely the insulation of clothing, depends largely on the air that is trapped in and on the clothing. Clothing consists, as a first approximation, of any sort of material that offers a grip to air layers.
This statement is approximate because some material properties are still relevant. These relate to the mechanical construction of the fabrics for instance wind resistance and the ability of fibres to support thick fabrics , and to intrinsic properties of fibres for instance, absorption and reflection of heat radiation, absorption of water vapour, wicking of sweat.
For not too extreme environmental conditions the merits of various fibre types are often overrated. The notion that it is air, and in particular still air, that provides insulation, suggests that thick air layers are beneficial for insulation. This is true, but the thickness of air layers is physically limited.
Air layers are formed by adhesion of gas molecules to any surface, by cohesion of a second layer of molecules to the first, and so on. However, the binding forces between subsequent layers are less and less, with the consequence that the outer molecules are moved by even tiny external motions of air. In quiet air, air layers may have a thickness up to 12 mm, but with vigorous air motion, as in a storm, the thickness decreases to less than 1 mm.
The exact function depends on the size and shape of the surface. Heat conduction of still and moving air Still air acts as an insulating layer with a conductivity that is constant, regardless of the shape of the material. Natural convection adds to this effect. For a graph showing the effect of air velocity on the insulating ability of a layer of air, see figure Heat transfer by radiation Radiation is another important mechanism for heat transfer.
Every surface radiates heat, and absorbs heat that is radiated from other surfaces. Radiant heat flow is approximately proportional to the temperature difference between the two exchanging surfaces. A clothing layer between the surfaces will interfere with radiative heat transfer by intercepting the energy flow; the clothing will reach a temperature that is about the average of the temperatures of the two surfaces, cutting the temperature difference between them in two, and therefore the radiant flow is decreased by a factor of two.
As the number of intercepting layers is increased, the rate of heat transfer is decreased. Multiple layers are thus effective in reducing radiant heat transfer. In battings and fibre fleeces radiation is intercepted by distributed fibres, rather than a fabric layer. The density of the fibre material or rather the total surface of fibre material per volume of fabric is a critical parameter for radiation transfer inside such fibre fleeces.
Fine fibres provide more surface for a given weight than coarse fibres. As a result of the conductivities of enclosed air and radiation transfer, fabric conductivity is effectively a constant for fabrics of various thicknesses and bindings. The heat insulation is therefore proportional to the thickness.
Air layers also create a resistance to the diffusion of evaporated sweat from humid skin to the environment. This resistance is roughly proportional to the thickness of the clothing ensemble.
For fabrics, the vapour resistance is dependent on the enclosed air and the density of the construction. In real fabrics, high density and great thickness never go together. Due to this limitation it is possible to estimate the air equivalent of fabrics that do not contain films or coatings see figure Coated fabrics or fabrics laminated to films may have unpredictable vapour resistance, which should be determined by measurement.
From Fabric and Air Layers to Clothing Multiple layers of fabric Some important conclusions from the heat transfer mechanisms are that highly insulating clothing is necessarily thick, that high insulation may be obtained by clothing ensembles with multiple thin layers, that a loose fit provides more insulation than a tight fit, and that insulation has a lower limit, set by the air layer that adheres to the skin.
In cold-weather clothing it is often hard to obtain thickness by using thin fabrics only. A solution is to create thick fabrics, by mounting two thin shell fabrics to a batting.
The purpose of the batting is to create the air layer and keep the air inside as still as possible. There is also a drawback to thick fabrics: The insulation of a clothing ensemble depends to a large extent on the design of the clothing. Design parameters which affect insulation are number of layers, apertures, fit, distribution of insulation over the body and exposed skin. Some material properties such as air permeability, reflectivity and coatings are important as well.
Furthermore, wind and activity change the insulation. Is it possible to give an adequate description of clothing for the purpose of prediction of comfort and tolerance of the wearer? Various attempts have been made, based on different techniques. Most estimates of complete ensemble insulation have been made for static conditions no motion, no wind on indoor ensembles, because the available data were obtained from thermal mannequins McCullough, Jones and Huck Measurements on human subjects are laborious, and results vary widely.
Since the mids reliable moving mannequins have been developed and used Olesen et al. Also, improved measurement techniques allowed for more accurate human experiments. A problem that still has not been overcome completely is proper inclusion of sweat evaporation in the evaluation.
Sweating mannequins are rare, and none of them has a realistic distribution of sweat rate over the body. Humans sweat realistically, but inconsistently. Often I is expressed in the unit clo not a standard international unit. One clo equals 0. The use of the unit clo implicitly means that it relates to the whole body and thus includes heat transfer by exposed body parts.
I is modified by motion and wind, as explained earlier, and after correction the result is called resultant insulation. This is a frequently used but not generally accepted term.
Total heat transfer from the body includes heat that is transferred by exposed skin usually head and hands and heat passing through the clothing. Intrinsic insulation see box is calculated over the total skin area, not only the covered part. Exposed skin transfers more heat than covered skin and thus has a profound influence on the intrinsic insulation.
This effect is enhanced by increasing wind speed. For thick ensembles the reduction in insulation is dramatic. Typical ensemble thickness and coverage Apparently both the insulation thickness and the skin coverage are important determinants of heat loss. In real life the two are correlated in the sense that winter clothing is not only thicker, but also covers a larger proportion of the body than summer wear. The lower limit is set by the insulation of the adjacent air and the upper limit by usability of the clothing.
Uniform distribution may provide the best insulation in the cold, but it is impractical to have much weight and bulk on the limbs. Therefore the emphasis is often on the trunk, and the sensitivity of local skin to cold is adapted to this practice.
Limbs play an important role in controlling human heat balance, and high insulation of the limbs limits the effectiveness of this regulation. Ventilation of clothing Trapped air layers in the clothing ensemble are subject to motion and wind, but to a different degree than the adjacent air layer.
Wind creates ventilation in the clothing, both as air penetrating the fabric and by passing through apertures, while motion increases internal circulation. Havenith, Heus and Lotens found that inside clothing, motion is a stronger factor than in the adjacent air layer. This conclusion is dependent on the air permeability of the fabric, however.
For highly air-permeable fabrics, ventilation by wind is considerable. Lotens showed that ventilation can be expressed as a function of effective wind speed and air permeability. Thickness of a clothing ensemble provides a first estimate of insulation.
Typical conductivity of an ensemble is 0. At an average thickness of 20 mm, that results in an I cl of 0. However, loose-fitting parts, such as trousers or sleeves, have a much higher conductivity, more on the order of 0. Other methods use table values for clothing items. These items have been measured previously on a mannequin. An ensemble under investigation has to be separated into its components, and these have to be looked up in the table. Making an incorrect choice of the most similar tabulated clothing item may cause errors.
In order to obtain the intrinsic insulation of the ensemble, the single insulation values have to be put in a summation equation McCullough, Jones and Huck In order to calculate total insulation, f cl has to be estimated see box. A practical experimental estimate is to measure the clothing surface area, make corrections for overlapping parts, and divide by total skin area DuBois and DuBois Other estimates from various studies show that f cl increases linearly with intrinsic insulation.
For a clothing ensemble, vapour resistance is the sum of resistance of air layers and clothing layers. Usually the number of layers varies over the body, and the best estimate is the area-weighted average, including exposed skin.
Evaporative resistance is less frequently used than I, because few measurements of C cl or P cl are available. Woodcock avoided this problem by defining the water vapour permeability index i m as the ratio of I and R, related to the same ratio for a single air layer this latter ratio is nearly a constant and known as the psychrometric constant S, 0. Typical values for i m for non-coated clothing, determined on mannequins, are 0. Values for i m for fabric composites and their adjacent air can be measured relatively simply on a wet hotplate apparatus, but the value is actually dependent on air flow over the apparatus and the reflectivity of the cabinet in which it is mounted.
Extrapolation of the ratio of R and I for clothed humans from measurements on fabrics to clothing ensembles DIN is sometimes attempted. This is a technically complicated matter. One reason is that R is proportional only to the convective part of I, so that careful corrections have to be made for radiative heat transfer.
Another reason is that trapped air between fabric composites and clothing ensembles may be different. In fact, vapour diffusion and heat transfer can be better treated separately. More sophisticated models are available to calculate insulation and water vapour resistance than the above-explained methods. These models calculate local insulation on the basis of physical laws for a number of body parts and integrate these to intrinsic insulation for the whole human shape.
For this purpose the human shape is approximated by cylinders figure The model by McCullough, Jones and Tamura requires clothing data for all layers in the ensemble, specified per body segment. These models have similar accuracy, which is better than any of the other methods mentioned, with the exception of experimental determination. Unfortunately and inevitably the models are more complex than would be desirable in a widely accepted standard. Effect of activity and wind Lotens and Havenith also provide modifications, based on literature data, of the insulation and vapour resistance due to activity and wind.
Insulation is lower while sitting than standing, and this effect is larger for highly insulating clothing. However, motion decreases insulation more than posture does, depending on the vigour of the movements. During walking both arms and legs move, and the reduction is larger than during cycling, when only the legs move.
Also in this case, the reduction is larger for thick clothing ensembles. Wind decreases insulation the most for light clothing and less for heavy clothing. This effect might relate to the air permeability of the shell fabric, which is usually less for cold-weather gear.
There is no definite agreement in the literature about the magnitude of motion or wind effects. The importance of this subject is stressed by the fact that some standards, such as ISO , require resultant insulation as an input when applied for active persons, or persons exposed to significant air motion. This requirement is often overlooked.
Moisture Management Effects of moisture absorption When fabrics can absorb water vapour, as most natural fibres do, clothing works as a buffer for vapour. This changes the heat transfer during transients from one environment to another. As a person in non-absorbing clothing steps from a dry to a humid environment, the evaporation of sweat decreases abruptly.
In hygroscopic clothing the fabric absorbs vapour, and the change in evaporation is only gradual. At the same time the absorption process liberates heat in the fabric, increasing its temperature. This reduces the dry heat transfer from the skin. In first approximation, both effects cancel each other, leaving the total heat transfer unchanged.
The difference with non-hygroscopic clothing is the more gradual change in evaporation from the skin, with less risk of sweat accumulation. Absorption capacity of fabric depends on the fibre type and the fabric mass. Fabrics can be classified as follows:. Water retention in fabrics, often confused with vapour absorption, obeys different rules. Free water is loosely bound to fabric and spreads well sideways along capillaries. This is known as wicking. Transfer of liquid from one layer to another takes place only for wet fabrics and under pressure.
Clothing may be wetted by non-evaporated superfluous sweat that is taken up from the skin. The liquid content of fabric may be high and its evaporation at a later moment a threat to the heat balance. This typically happens during rest after hard work and is known as after-chill. The ability of fabrics to hold liquid is more related to fabric construction than to fibre absorption capacity, and for practical purposes is usually sufficient to take up all the superfluous sweat.
Clothing may get wet by condensation of evaporated sweat at a particular layer. Condensation occurs if the humidity is higher than the local temperature allows. In cold weather that will often be the case at the inside of the outer fabric, in extreme cold even in deeper layers. Where condensation takes place, moisture accumulates, but the temperature increases, as it does during absorption. The difference between condensation and absorption, however, is that absorption is a temporary process, whereas condensation may continue for extended times.
Latent heat transfer during condensation may contribute very significantly to heat loss, which may or may not be desirable.