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Objectives: To revise the criteria used in the present 'Required Sweat Rate' standard ISO 7933 (1989) for the prediction of the maximum duration of work in hot environments. Methods: Review of the literature and in particular, of the bases for the present criteria. Results: A new method is proposed, to take into account the increase in core temperature associated with activity in neutral environments. The prediction of maximum wetness and maximum sweat rates are revised, as well as the limits for maximum water loss and core temperature. Conclusions: An improved set of maximum values and limits is described, to be used in the revised version of the ISO 7933 standard. Due to the major modifications to the 'Required Sweat Rate' index and in order to avoid any confusion, it is suggested that the revised model be renamed the 'Predicted Heat Strain' (PHS) model.

Background: Helicopter pilots are often exposed to periods of high heat strain, especially when wearing survival suits. Therefore, a prototype of a ventilated vest was evaluated on its capability to reduce the heat strain of helicopter pilots during a 2-h simulated flight. Hypothesis: It was hypothesized that the ventilated vest would reduce pilot heat stress. Methods: Five male and one female helicopter pilots flew for 2 h in a simulator in three different conditions; 15°C wet bulb globe temperature (WBGT) without ventilation, 32°C WBGT without ventilation, and 32°C WBGT with a ventilated vest. Results: Wearing the ventilated vest significantly reduced the increase in rectal temperature and increased thermal comfort. This made it possible for all subjects to complete the 2-h session. Conclusion: With the ventilated vest the subjects experienced less heat stress, thereby allowing all subjects to successfully complete the experiment, though two of the six pilots could not complete the 2-h flying task in the hot condition without cooling due to heat-related problems. Copyright © by Aerospace Medical Association. Chemicals / CAS: politef, 9002-84-0, 9039-02-5

Clothing heat and vapour resistances are important inputs for standards and models dealing with thermal comfort, heat- and cold-stress. A vast database of static clothing heat resistance values is available, and this was recently expanded with correction equations to account for effects of movement and wind on the static value of heat resistance in order to obtain the dynamic heat resistance of clothing ensembles. For clothing vapour resistance, few data were available so far. Indices for vapour permeability (i(m)) and reduction factors for vapour transfer (F(pcl)) of clothing were used instead, using a relation between heat and vapour resistance to derive the clothing vapour resistance from the value for clothing heat resistance. This paper reviews the two commonly used approaches (i(m) and F(pcl)), as well as five alternative approaches to the problem. The different approaches were evaluated for their accuracy and their usability.The present paper shows that the currently used relations are not adequate when the wearer of the clothing starts moving, or is exposed to wind. Alternative approaches are shown to improve the determination of dynamic clothing vapour resistance, though some are thought to be too complex. An empirical description of the relation between the clothing permeability index (i(m)) and the changes in clothing heat resistance due to wind and movement was selected as the most promising method for deriving clothing vapour resistance. For this method the user needs to know the static heat resistance, the static i(m) value of the clothing and the wind- and movement-speed of the wearer. This method results in a predicted maximal decrease in clothing vapour resistance by 78%, when clothing heat resistance is reduced by 50%, which is consistent with theoretical expectations and available data. Copyright (C) 1999 British Occupational Hygiene Society. A review is carried out on the two commonly used approaches as well as five alternative approaches to the issues of clothing heat and vapor resistances. The different approaches are evaluated for their accuracy and usability. It is shown that the currently used relations are not adequate when the wearer of the clothing starts moving, or is exposed to wind. Alternative approaches are shown to improve the determination of dynamic clothing vapor resistance.

This paper integrates the research presented in the papers in this special issue of Holmer et al. and Havenith et al. [Holmer, 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., Holmer, 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. Copyright (C) 1999 British Occupational Hygiene Society. A method is described that corrects static values of clothing properties to provide dynamic values that take into account wind and human movement. The method allows a more complete representation of the effects of clothing on the heat strain of workers.