Clothing ventilation and human thermal response
thesisposted on 10.11.2010, 14:03 by Lisa M. Bouskill
Given the importance of heat balance being maintained between a person and their environment an appropriate clothing choice is essential. Since military personnel are required to work effectively when deployed in any of the world's climates it is important that the thermal protection afforded by their clothing is considered as well as its more obvious protective properties such as those relating to the chemical and abrasive environments. Clothing descriptions restricted to details of heat and water vapour transfer characteristics alone, as is commonly the case, are recognised as being insufficient. Of particular note, where these data are obtained under 'artificial' conditions, ie intrinsic values, they are unlikely to represent the 'resultant' values as observed when worn by human subjects engaged in actual work tasks. Where intrinsic data are used in predictive standards calculations, to estimate safe work times etc, the workforce under consideration may not always be protected. One source of change in the thermal properties of clothing, when in the workplace, occurs due to increased convective and evaporative heat transfer at the wearer's skin surface caused by air movement through the clothing. This may occur as a result of wearer body movements or increased environmental air speed. The Ventilation Index has previously been suggested as an accurate and repeatable method for quantifying clothing ventilation characteristics. Although several other measurement techniques have also been suggested, the Ventilation Index is simple (albeit laborious) to conduct, and does not require the use of expensive equipment. Work conducted towards this thesis has shown that the Ventilation Index may be suitable for use in either manikin testing or human studies assessmentso f clothing. The aim of this thesis was to investigate the suitability of the Ventilation Index as a measurementt echnique for the assessmenot f clothing ventilation characteristics, particularly to consider the relationship between clothing ventilation and wearer physiological responses and to identify the factors which can affect this. The Ventilation Index measurement systems constructed as part of this research have improved on those used previously in similar research. New materials technology has provided an improved air-tight oversuit for use during measurement of the clothing micro-environment (a constant source of fiustration, it appears, for previous authors), while extensive calibration of the whole system has proved its accuracy. Using the Ventilation Index has shown that the ingress and egress of air into and from the clothing micro-environment may induce a physiological response from the wearer of the clothing (chapter 6) such increases in air movement being reflected by a drop in insulation afforded by the clothing (chapter 7). Of particular interest to persons involved in the thermal assessment of clothing, will be the suggestion that clothing may exhibit different ventilation characteristics when tested on a thermal manikin to when worn by human subjects. This difference appearing to be related to clothing fit (investigated in chapter 9). Of interest to wearer's of protective, is the observation that air-impermeable clothing does not necessarily withstand changes in environmental air movement (chapter 10). The technique is not without criticism. The standard tracer gas technique, used to calculate clothing air exchange rate, considers only air movement occurring next to the wearer's skin. In multi-layer clothing ensembles, the movement of air in clothing layers more distant will change the clothing micro-environment and thus have consequences for the wearer. Preliminary investigation suggests that distribution of nitrogen to each clothing layer should enable assessmenot fair movement in each of these layers.