Computer models as tools for evaluating clothing risks and controls

Computer models of clothed people are now at a level of sophistication that permit them to be used as tools in clothing design and evaluation. Thermal models vary from those based upon a simple heat balance equation for the clothed body to models that include both the thermal properties of the body and a representation of human thermoregulation. Supporting models include those for modelling clothing, per se, involving dry and vapour heat transfer, and data-bases of the thermal insulation of clothing garments and ensembles. The availability of these models for micro-computers provides potential for their introduction into procedures for work design and evaluation.Computer models integrate the effects of the six basic parameters of air temperature, radiant temperature, humidity, air velocity, metabolic heat and clothing. They provide predictions of human response, usually in terms of body skin temperatures, core temperature and sweat loss. From the physiological responses, predictions can be made of likely body strain caused by wearing protective clothing, the possible risk involved and its causes and whether it is acceptable. Control measures, to maintain thermal strain within acceptable limits, in terms of environmental parameters and work breaks for example, can also be determined from model predictions.Models are easy to use, and with training and understanding can be applied usefully over a wide range of conditions. It is important, however, to be aware of and to understand their limitations. They should be used at a preliminary stage of design or assessment and will save time and other resources. The ergonomics and other organizational issues regarding the use of models requires consideration. The role of models in work design and evaluation should be determined through practical application and presented in case studies.Thermal models are defined and described. Examples of the use of a thermal model are presented for evaluating clothing risk, determining controls and investigating the ‘window of application’ for clothing.     

Relationship between clothing ventilation and thermal insulation

Air layers trapped within a clothing microenvironment contribute to the thermal insulation afforded by the ensemble. Any exchange of air between the external environment and these trapped air layers results in a change in the ensemble's thermal insulation and water vapor resistance characteristics. These effects are seldom taken into account when considering the effects of clothing on human heat balance, the thermal characteristics usually being restricted to intrinsic insulation and intrinsic evaporative resistance measurements on static manikins. Environmental assessments based on these measurements alone may therefore lead to under-(or over-) estimation of thermal stress of the worker. The aim of this study was to quantify the relationship between clothing ventilation and thermal insulation properties. A one-layer, air-impermeable ensemble and a three-layer, air-permeable ensemble were tested using an articulated, thermal manikin in a controlled climate chamber (ta = tr = 10 degrees C, PaH2O = 0.73 kPa). The manikin, which was designed for thermal insulation measurements, was also equipped with a system to determine clothing ventilation. Baseline measurements of clothing ventilation (VT) and thermal insulation (total clothing insulation: I(T)--measured, intrinsic insulation: Icl--calculated) were made of the clothing with the manikin standing stationary in still air conditions. Increased clothing ventilation was induced when the manikin "walked" (walking speeds of 0.37 m/sec and 0.77 m/sec) and by increasing the environmental air speed (Va = 1.0 m/sec). These increases in VT reduced Icl, this being ascribed to the increased heat transfer from the manikin skin surface to the cooler external environment due to the exchange of air between the clothing microenvironment and the external environment. Measured air exchanges were shown to have a potential heat exchange capacity of up to 17 and 161 W/m2 for the one- and three-layer ensembles, respectively, emphasizing the need to take clothing ventilation characteristics into consideration during thermal audits and thermal risk assessments.     

The effects of wind and human movement on the heat and vapour transfer properties of clothing.

This paper integrates the research presented in the papers in this special issue of Holmér et al. and Havenith et al. [Holmér, I., Nilsson, H., Havenith, G., Parsons, K. C. (1999) Clothing convective heat exchange: proposal for improved prediction in standards and models. Annals of Occupational Hygiene, in press; Havenith, G., Holmér, I., den Hartog, E. and Parsons, K. C. (1999) Clothing evaporative heat resistance: proposal for improved representation in standards and models. Annals of Occupational Hygiene, in press] to provide a practical suggestion for improving existing clothing models so that they can account for the effects of wind and human movement. The proposed method is presented and described in the form of a BASIC computer program. Analytical methods (for example ISO 7933) for the assessment of the thermal strain caused by human exposure to hot environments require a mathematical quantification of the thermal properties of clothing. These effects are usually considered in terms of 'dry' thermal insulation and vapour resistance. This simple 'model' of clothing can account for the insulation properties of clothing which reduce heat loss (or gain) between the body and the environment and, for example, the resistance to the transfer of evaporated sweat from the skin, which is important for cooling the body in a hot environment. When a clothed person is exposed to wind, however, and when the person is active, there is a potentially significant limitation in the simple model of clothing presented above. Heat and mass transfer can take place between the microclimate (within clothing and next to the skin surface) and the external environment. The method described in this paper 'corrects' static values of clothing properties to provide dynamic values that take account of wind and human movement. It therefore allows a more complete representation of the effects of clothing on the heat strain of workers.     

Thermal characteristics of clothing ensembles for use in heat stress analysis

The Heat Stress Index was an early model for the assessment of heat stress. The International Organization for Standardization (ISO) standard for required sweat rate is the current generation of heat balance methods for occupational heat stress. The method assumes cotton clothing and works adequately for cotton/polyester blends. To extend the usefulness of the model, the thermal characteristics of a variety of commercially available and prototype protective clothing ensembles have been determined for application in the ISO method. The fundamental principle for assessing thermal characteristics of work clothing is establishing the critical environmental conditions in which test subjects were just able to maintain thermal equilibrium. Critical conditions were found for warm, humid conditions; hot, dry conditions; intermediate conditions of temperature and humidity; and/or moderate conditions in which metabolic rate was increased to a limiting thermal load. Typically, five subjects at each condition for each ensemble were used. Metabolic rate, average skin temperature, and the environmental conditions (air temperature and vapor pressure) were noted at the critical conditions, and the total insulation was estimated for each ensemble. From these values, the total evaporative resistance, the clothing factor for dry heat exchange (CFcl), and the clothing factor for evaporative cooling (CFpcl) were determined. When compared with reports of others on thermal characteristics the results agreed when pumping factors and clothing wetness were considered. The result was higher than expected values for CFcl and lower values for CFpcl.     

Theoretical analysis of three methods for calculating thermal insulation of clothing from thermal manikin

There are three methods for calculating thermal insulation of clothing measured with a thermal manikin, i.e. the global method, the serial method, and the parallel method. Under the condition of homogeneous clothing insulation, these three methods yield the same insulation values. If the local heat flux is uniform over the manikin body, the global and serial methods provide the same insulation value. In most cases, the serial method gives a higher insulation value than the global method. There is a possibility that the insulation value from the serial method is lower than the value from the global method. The serial method always gives higher insulation value than the parallel method. The insulation value from the parallel method is higher or lower than the value from the global method, depending on the relationship between the heat loss distribution and the surface temperatures. Under the circumstance of uniform surface temperature distribution over the manikin body, the global and parallel methods give the same insulation value. If the constant surface temperature mode is used in the manikin test, the parallel method can be used to calculate the thermal insulation of clothing. If the constant heat flux mode is used in the manikin test, the serial method can be used to calculate the thermal insulation of clothing. The global method should be used for calculating thermal insulation of clothing for all manikin control modes, especially for thermal comfort regulation mode. The global method should be chosen by clothing manufacturers for labelling their products. The serial and parallel methods provide more information with respect to the different parts of clothing.     

Readdressing personal cooling with ice

An ice-based system of personal, non-restrictive cooling of workers exposed to high temperature work environments in nuclear power plants was evaluated. The garments were designed to be worn under the protective clothing donned for penetration into radiation areas. The cooling system consisted of direct body contact with small packets of frozen water enclosed in the pockets of a shirt in high ambient temperatures (55 degrees C) and moderate metabolic heat production (200-300 kcal/hr). Mean exposure time without cooling (control) was 52 min for workloads demanding 200 kcal/hr energy expenditure. A long garment with 7.2 kg of frozen water (LFWG) increased mean exposure time over the control by 242% (163% for the same garment with 6.2 kg of frozen water). A short version garment with 3.8 kg of frozen water (SFWG) increased the stay time by 115%. In field observations, the LFWG with 6.2 kg of frozen water improved stay time by 125%. The leveling off of the body temperatures and heart rates during the work both in the laboratory and in the field confirmed a reduction of heat strain associated with the use of these garments. Calculated heat balance equations of heat uptake by the ice predicted these results very closely. It was concluded that direct body contact with frozen water provided predictable adequate body cooling for the work and ambient conditions investigated in this study.     

Design and Evaluation of a Ventilated Garment for Use in Temperatures up to 200°C

The protection of personnel against high air and radiant temperatures is a problem that has been confronting industry for many years now, and for many industrial situations it still has not been solved. The experiments reported here were intended to determine the most suitable form of insulation for a hot entry suit for use primarily in furnace wrecking where mean radiant temperatures of 200°C. are met and where heat-reflecting garments are unsuitable due to the rapid deterioration of the reflecting surface.

From a preliminary consideration of the problem it was concluded that a ventilated garment was required and that conventional ventilated garments in which air is induced to flow parallel to the body surfaces (axial ventilation) are basically unsound in design as the air is not utilized for the transfer of heat in the most efficient manner. A new form of ventilation was therefore developed in which air flows out through a permeable suit (radial ventilation). This form of ventilation produces what is called dynamic insulation, and this method of insulation, when compared with two alternative methods on a physical model, was found to be very effective.

The model experiments were confirmed by comparative trials of three ventilated suits each using one of three different forms of insulation thought to be suitable for use in heat-protective clothing.

Physiological measurements made on the subjects and physical measurement made on the suits confirmed that dynamic insulation is the most suitable insulation for a hot entry suit for furnace wrecking.

With the air flows used in these experiments, dynamic insulation had a thermal conductance one-fifth that of conventional static insulation, and sweat losses and oral temperature rises were reduced by one-third and one-half respectively.

     

Limiting metabolic rate (thermal work limit) as an index of thermal stress

The development of a rational heat stress index called thermal work limit (TWL) is presented. TWL is defined as the limiting (or maximum) sustainable metabolic rate that euhydrated, acclimatized individuals can maintain in a specific thermal environment, within a safe deep body core temperature (< 38.20 degrees C) and sweat rate (< 1.2 kg/hr(-1)). The index has been developed using published experimental studies of human heat transfer, and established heat and moisture transfer equations through clothing. Clothing parameters can be varied and the protocol can be extended to unacclimatized workers. The index is designed specifically for self-paced workers and does not rely on estimation of actual metabolic rates, a process that is difficult and subject to considerable error. The index has been introduced into several large industrial operations located well inside the tropics, resulting in a substantial and sustained fall in the incidence of heat illness. Guidelines for TWL are proposed along with recommended interventions. TWL has application to professionals from both the human and engineering sciences, as it allows not only thermal strain to be evaluated,. but also the productivity decrement due to heat (seen as a reduced sustainable metabolic rate) and the impact of various strategies such as improved local ventilation or refrigeration to be quantitatively assessed.     

Protective clothing in hot environments

The high level of protection required by personal protective clothing (PPC) severely impedes heat exchange by sweat evaporation. As a result work associated with wearing PPC, particularly in hot environments, implies considerable physiological strain and may render workers exhausted in a short time. Recent development of algorithms for describing the heat transfer, accounting for pumping and wind effects, comprises improvement of the prediction of thermal stress. Realistic corrections can then be made to the available measures of thermal insulation and evaporative resistance of a given clothing ensemble. Currently this information is incorporated in international standards for assessment of thermal environments. Factors, such as directional radiation and wetting of layers, were studied in a recently completed EU research project. The development of advanced thermal manikins and measurement procedures should provide better measures for predictive models. As with all methods and models, the results need validation in realistic wear trials in order to prove their relevance and accuracy.     

Prediction of clothing thermal insulation and moisture vapour resistance of the clothed body walking in wind

Clothing thermal insulation and moisture vapour resistance are the two most important parameters in thermal environmental engineering, functional clothing design and end use of clothing ensembles. In this study, clothing thermal insulation and moisture vapour resistance of various types of clothing ensembles were measured using the walking-able sweating manikin, Walter, under various environmental conditions and walking speeds. Based on an extensive experimental investigation and an improved understanding of the effects of body activities and environmental conditions, a simple but effective direct regression model has been established, for predicting the clothing thermal insulation and moisture vapour resistance under wind and walking motion, from those when the manikin was standing in still air. The model has been validated by using experimental data reported in the previous literature. It has shown that the new models have advantages and provide very accurate prediction.     

Effect of cold conditions on manual performance while wearing petroleum industry protective clothing

The purpose of this study was to investigate manual performance and thermal responses during low work intensity in persons wearing standard protective clothing in the petroleum industry when they were exposed to a range of temperatures (5, -5, -15 and -25℃) that are relevant to environmental conditions for petroleum industry personnel in northern regions. Twelve men participated in the study. Protective clothing was adjusted for the given cold exposure according to current practices. The subjects performed manual tests five times under each environmental condition. The manual performance test battery consisted of four different tests: tactile sensation (Semmes-Weinstein monofilaments), finger dexterity (Purdue Pegboard), hand dexterity (Complete Minnesota dexterity test) and grip strength (grip dynamometer). We found that exposure to -5℃ or colder lowered skin and body temperatures and reduced manual performance during low work intensity. In conclusion the current protective clothing at a given cold exposure is not adequate to maintain manual performance and thermal balance for petroleum workers in the high north.     

热流法测定服装隔热值的研究与应用

本文报道了采用热流法取代传统的代谢法测定服装的隔热值。实验结果证明，热流法符合传热学规律，其突出优点是同时可以测定整体服装的隔热值和身体各个部位的隔热值。比代谢法简便易行，节省人力，物力和时间，因而在工业卫生，劳动保护，服装设计和研究部门具有广泛使用价值。     

Development of a draft British standard: the assessment of heat strain for workers wearing personal protective equipment.

Existing methods for estimating heat stress, enshrined in British/International Standards (the Wet Bulb Globe Temperature (WBGT) index [BS EN 27243] and the Required Sweat Rate equation [BS EN 12515; ISO 7933 modified]), assume that the clothing worn by the individual is water vapour permeable; the WBGT index also assumes that the clothing is relatively light. Because most forms of personal protective equipment (PPE) either have a higher insulative value than that assumed or are water vapour impermeable, the Standards cannot be accurately applied to workers wearing PPE. There was, therefore, a need to develop a British Standard which would allow interpretation of these existing Standards for workers wearing PPE. Relevant information was obtained through reviewing the literature and consulting experts. Two questionnaire surveys of potential users of the Standards were conducted, and physiological data collected both experimentally and in work situations were considered. The information collected was used to develop the draft British Standard. It provides information and data on: The general effect of PPE on heat balance of the body (the ability of the body to maintain its 'core' temperature within an acceptable range). The effect of specific forms of PPE on metabolic heat production rate. The thermal insulation and evaporative resistance of types of PPE. The effect of the closure of the garments to the body on heat transfer. The effect of the PPE on the proportion of the body covered. The effect of an air supply (for example, Breathing Apparatus [BA]) to the wearer. Guidance is given on conducting an analysis of the work situation, taking account of the impact of PPE. Detailed methods of interpreting both BS EN 27243 and BS EN 12515 for workers wearing PPE are given, taking account of the factors listed above. Three worked examples using BS EN 27243 and BS EN 12515 are given in the Annex of the draft Standard.     

Effect of wearing gloves on the thermal balance of Korean women wet-suit divers in cold water

Effect of wearing neoprene gloves on the thermal exchanges of wet-suited divers was studied in 8 Korean diving women. Subjects, clad with 5-6-mm-thick neoprene wet suits (jacket, pants, and boots) either with or without wearing 3-mm-thick neoprene gloves, were immersed for 3 h in water of critical temperature (17.3 degrees +/- 0.8 degree C) while the rectal and skin (chest, leg, arm, and hand) temperatures and oxygen consumption were measured. Overall thermal insulation of the subject plus suit was calculated from the rectal-to-water temperature difference divided by the estimated rate of skin heat loss. The skin heat loss was assumed to equal metabolic heat production minus respiratory heat loss, corrected for changes in heat storage when mean body temperature changed. All measurements were carried out in a resting condition. During the 3rd h of immersion, the rectal temperature was lower with gloves (delta Tre = 0.30 degree +/- 0.04 degree C; P less than 0.05) whereas metabolic heat production was not significantly different. Consequently, the total thermal insulation was nearly 16% lower with gloves than without gloves. In both the hands and forearms, the regional heat flux determined directly using a heat flux transducer was higher and the thermal insulation index was lower with gloves than without gloves. These results indicate that in wet-suited subjects resting in cold (17 degrees C) water gloves do not provide additional protection against heat loss, but rather decrease the efficiency of thermoregulatory mechanisms. We suggest that sensory input from cold receptors in the distal extremities is particularly important in thermoregulation during immersion in cold water.     

Methods of evaluating protective clothing relative to heat and cold stress: thermal manikin, biomedical modeling, and human testing

Personal protective equipment (PPE) refers to clothing and equipment designed to protect individuals from chemical, biological, radiological, nuclear, and explosive hazards. The materials used to provide this protection may exacerbate thermal strain by limiting heat and water vapor transfer. Any new PPE must therefore be evaluated to ensure that it poses no greater thermal strain than the current standard for the same level of hazard protection. This review describes how such evaluations are typically conducted. Comprehensive evaluation of PPE begins with a biophysical assessment of materials using a guarded hot plate to determine the thermal characteristics (thermal resistance and water vapor permeability). These characteristics are then evaluated on a thermal manikin wearing the PPE, since thermal properties may change once the materials have been constructed into a garment. These data may be used in biomedical models to predict thermal strain under a variety of environmental and work conditions. When the biophysical data indicate that the evaporative resistance (ratio of permeability to insulation) is significantly better than the current standard, the PPE is evaluated through human testing in controlled laboratory conditions appropriate for the conditions under which the PPE would be used if fielded. Data from each phase of PPE evaluation are used in predictive models to determine user guidelines, such as maximal work time, work/rest cycles, and fluid intake requirements. By considering thermal stress early in the development process, health hazards related to temperature extremes can be mitigated while maintaining or improving the effectiveness of the PPE for protection from external hazards.     

Heated manikins as a tool for evaluating clothing

More than 50 manikins are in use world-wide. They represent various levels of development in terms of technique and performance. The more sophisticated are articulated manikins able to assume different postures and body movements. They are electrically heated and divided into several (up to 36) individually controlled segments. A few may even allow for controlled sweating and water immersion.The continuing and growing interest in manikins is based on the fact that they: represent a realistic and objective method for assessment of clothing thermal function; comprise a quick, accurate and reproducible method for measurement of thermal insulation; are cost-effective instruments for comparative measurements and for produce development; and provide input values for thermal modelling and prediction of safe and comfortable working conditions.It must be borne in mind, however, that manikin data at best only represent the performance of clothing under specified test conditions. Size, fit, posture, type and intensity of work movements, wind, wetting and other factors influence clothing heat transfer in such a way that the resulting insulation provided by an ensemble during real conditions may be much lower than the measured standard value. Walking movements alone may reduce insulation by 20–30%. The manikin value does not account for individual variation in terms of requirement and preferences. Therefore, manikin measurements rely extensively on experience and knowledge derived from human experiments and should be regarded as complementary to, rather than a replacement for, practical testing.     

Physiological strain in work with gas protective clothing at low ambient temperature

Wearing an impermeable gas protective suit (Dr?ger 500 or 600) and a self-contained breathing apparatus (Dr?ger PA 80; total weight 27 kg (59.5 lb], seven experienced firemen and one mechanic performed simulated repair and rescue tasks in a chemical plant. The subjects' mean (+/- SD) age, height, weight and estimated maximal oxygen consumption were: 36 +/- 4 years; 181 +/- 6 cm; 83 +/- 8 kg; and 42 +/- 5 mL/min/kg, respectively. The operations took place outdoors (ambient temperature 2.0 degrees C (35.6 degrees F), wind velocity 0-4 m/s). The total work time averaged 37 minutes. During tasks of search, handling vents, and sawing and replacing bolts, the mean (+/- SE) heart rates measured by a Depex recording device were 146 +/- 2, 148 +/- 2, and 147 +/- 5 beats/min, respectively. The mean rectal temperature increased 0.8 degrees C during the whole work period. Weight loss due to sweat averaged 300 g. In conclusion, typical tasks with gas protective clothing caused marked physiological strain among subjects in average physical condition even though the thermal strain was relatively low because the weather was cool. The results emphasized the need to evaluate physical fitness during the periodic check-ups of workers who may have to use gas protective clothing.     

Heat exposure in the Canadian workplace

Exposure to excessive heat is a physical hazard that threatens Canadian workers. As patterns of global climate change suggest an increased frequency of heat waves, the potential impact of these extreme climate events on the health and well‐being of the Canadian workforce is a new and growing challenge. Increasingly, industries rely on available technology and information to ensure the safety of their workers. Current Canadian labor codes in all provinces employ the guidelines recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) that are Threshold Limit Values (TLVs) based upon Wet Bulb Globe Temperature (WBGT). The TLVs are set so that core body temperature of the workers supposedly does not exceed 38.0°C. Legislation in most Canadian provinces also requires employers to install engineering and administrative controls to reduce the heat stress risk of their working environment should it exceed the levels permissible under the WBGT system. There are however severe limitations using the WGBT system because it only directly evaluates the environmental parameters and merely incorporates personal factors such as clothing insulation and metabolic heat production through simple correction factors for broadly generalized groups. An improved awareness of the strengths and limitations of TLVs and the WGBT index can minimize preventable measurement errors and improve their utilization in workplaces. Work is on‐going, particularly in the European Union to develop an improved individualized heat stress risk assessment tool. More work is required to improve the predictive capacity of these indices. Am. J. Ind. Med. 53:842–853, 2010. © 2010 Wiley‐Liss, Inc.     

Heat balance when wearing protective clothing.

This issue of the Annals of Occupational Hygiene is dedicated to the topic of heat stress evaluation. For this evaluation, several evaluation programs and international standards are available. In order to understand the reasoning and underlying theory behind these programs and standards, a basic knowledge of heat exchange processes between workers and their environment is needed. This paper provides an overview of the relevant heat exchange processes, and defines the relevant parameters (air and radiant temperature, humidity, wind speed, metabolic heat production and clothing insulation). Further it presents in more detail the relation between clothing material properties and properties of clothing ensembles made from those materials. The effects of clothing design, clothing fit, and clothing air permeability are discussed, and finally an overview of methods for the determination of clothing heat and vapour resistance is given.     

Comparisons of air and liquid personal cooling for intermittent heavy work in moderate temperatures.

Personal microenvironmental cooling has been used to enhance safety and extend the work capacity of laborers wearing protective clothing. Previous studies of air and liquid cooling have used either very low work rates or high environmental temperatures. Emergency work tasks frequently require high work rates and occur in moderate ambient temperatures. The purpose of this research was to examine the efficacy of intermittent personal cooling during rest and to compare liquid and air cooling systems in subjects engaged in hard work. Fourteen volunteers wearing chemical protective clothing performed treadmill walking at a metabolic rate of 430 W for 45 min followed by a 15-min rest at a wet-bulb globe temperature of 25 degrees C. During rest, volunteers received either no cooling, air cooling, or liquid cooling. Both cooling systems partially alleviated heat strain and increased work time with the air system offering slightly more effective cooling.