Ingvar Holmér

Workplace heat stress, health and productivity an increasing challenge for low and middle-income countries during climate change

Background: Global climate change is already increasing the average temperature and direct heat exposure in many places around the world. Objectives: To assess the potential impact on occupational health and work capacity for people exposed at work to increasing heat due to climate change. Design: A brief review of basic thermal physiology mechanisms, occupational heat exposure guidelines and heat exposure changes in selected cities. Results: In countries with very hot seasons, workers are already affected by working environments hotter than that with which human physiological mechanisms can cope. To protect workers from excessive heat, a number of heat exposure indices have been developed. One that is commonly used in occupational health is the Wet Bulb Globe Temperature (WBGT). We use WBGT to illustrate assessing the proportion of a working hour during which a worker can sustain work and the proportion of that same working hour that (s)he needs to rest to cool the body down and maintain core body temperature below 38°C. Using this proportion a ‘work capacity’ estimate was calculated for selected heat exposure levels and work intensity levels. The work capacity rapidly reduces as the WBGT exceeds 26–30°C and this can be used to estimate the impact of increasing heat exposure as a result of climate change in tropical countries. Conclusions: One result of climate change is a reduced work capacity in heat-exposed jobs and greater difficulty in achieving economic and social development in the countries affected by this somewhat neglected impact of climate change.

Personal factors in thermal comfort assessment: clothing properties and metabolic heat production

In the assessment of thermal comfort in buildings, the use of the Predicted Mean Vote (PMV) model is very popular. For this model, data on the climate, on clothing and on metabolic heat production are required. This paper discusses the representation and measurement of clothing parameters and metabolic rate in the PMV context. Several problems are identified and for some of these solutions are provided. For clothing insulation it was shown that effects of body motion and air movement are so big that they must be accounted for in comfort prediction models to be physically accurate. However, effects on dry heat exchange are small for stationary, light work at low air movement. Also algorithms for convective heat exchange in prediction models should be reconsidered. For evaporative heat resistance of the clothing worn, which is currently not an input factor in the PMV model, it was shown that in cases where special clothing with high vapour resistance is worn (e.g. clean-room clothing), comfort may be limited by the clothing as it will induce a high skin wettedness. Thus, for such cases clothing vapour resistance should not be neglected in the calculation of comfort using the PMV model, or the induced skin wettedness should be calculated separately. The effects on thermal comfort of reductions in vapour resistance due to air and body movements are also shown to have a substantial impact on the comfort limits in terms of skin wettedness and cannot be neglected either. For metabolic heat production it was concluded that for precise comfort assessment a precise measure of metabolic rate is needed. In order to improve metabolic rate estimation based on ISO 8996, more data and detail is needed for activities with a metabolic rate below 2 MET. Finally, it was shown that the methods for determining metabolic rate provided in ISO 8996 (typically used in comfort assessment and evaluations) do not provide sufficient accuracy to allow determination of comfort (expressed as PMV) in sufficient precision to classify buildings to within 0.3 PMV units as proposed in the upcoming revision of ISO 7730.

The UTCI-clothing model

The Universal Thermal Climate Index (UTCI) was conceived as a thermal index covering the whole climate range from heat to cold. This would be impossible without considering clothing as the interface between the person (here, the physiological model of thermoregulation) and the environment. It was decided to develop a clothing model for this application in which the following three factors were considered: (1) typical dressing behaviour in different temperatures, as observed in the field, resulting in a model of the distribution of clothing over the different body segments in relation to the ambient temperature, (2) the changes in clothing insulation and vapour resistance caused by wind and body movement, and (3) the change in wind speed in relation to the height above ground. The outcome was a clothing model that defines in detail the effective clothing insulation and vapour resistance for each of the thermo-physiological model’s body segments over a wide range of climatic conditions. This paper details this model’s conception and documents its definitions.

Protective clothing and heat stress

The high level of protection required by 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. Current methods of describing evaporative heat exchange with PPC are insufficient, will overestimate evaporative heat loss and should not be recommended. More reliable measures of the resistance to evaporative heat transfer by PPC should be developed and standardized. Direct measurements of evaporative resistance of PPC may be carried. However, a more promising method appears to be the definition of evaporative resistance on the basis of the icl-index for the fabric layers. The icl-index is a permeation efficiency ratio, which in combination with clothing insulation determines the evaporative heat transfer. Current methods should be further developed to account for effects of moisture condensation and microclimate ventilation.

Cooling vests with phase change material packs: the effects of temperature gradient, mass and covering area.

Phase change material (PCM) absorbs or releases latent heat when it changes phases, making thermal-regulated clothing possible. The objective of this study was to quantify the relationships between PCM cooling rate and temperature gradient, mass and covering area on a thermal manikin in a climatic chamber. Three melting temperatures (24, 28, 32°C) of the PCMs, different mass, covering areas and two manikin temperatures (34 and 38°C) were used. The results showed that the cooling rate of the PCM vests tested is positively correlated with the temperature gradient between the thermal manikin and the melting temperature of the PCMs. The required temperature gradient is suggested to be greater than 6°C when PCM vests are used in hot climates. With the same temperature gradient, the cooling rate is mainly determined by the covering area. The duration of the cooling effect is dependent on PCM mass and the latent heat. Statement of Relevance: The study of factors affecting the cooling rate of personal cooling equipment incorporated with PCM helps to understand cooling mechanisms. The results suggest climatic conditions, the required temperature gradient, PCM mass and covering area should be taken into account when choosing personal PCM cooling equipment.

Heat Exchange and Thermal Insulation Compared in Woolen and Nylon Garments During Wear Trials

Heat exchange and thermal insulation were determined in four subjects during 120 minutes exposure (60 minutes exercise and 60 minutes rest) at +8°C in a climatic chamber. Two types of garment assemblies were investigated, each comprising a jacket, trousers, socks, mittens, and a hat of identical construction made of wool and nylon. Mean skin and rectal temperatures were significantly higher at rest in wet woolen garments compared to wet nylon garments. No significant difference was obtained in heat exchange and thermal insulation of garments when wool was compared to nylon during walking, running, and resting in wet and dry clothing, respectively. Overall thermal insulation (Rc ) was significantly lower by 15% in wet clothing compared to dry clothing at rest. Rc was significantly reduced by 30-40% during walking and by 50-60% during running in comparison with resting in dry clothing. Reduction was partly explained by reduced insulation of surface air layer, but predominantly by ventilation of clothing due to the pumping effect.

Personal cooling with phase change materials to improve thermal comfort from a heat wave perspective

UNLABELLED The impact of heat waves arising from climate change on human health is predicted to be profound. It is important to be prepared with various preventive measures for such impacts on society. The objective of this study was to investigate whether personal cooling with phase change materials (PCM) could improve thermal comfort in simulated office work at 34°C. Cooling vests with PCM were measured on a thermal manikin before studies on human subjects. Eight male subjects participated in the study in a climatic chamber (T(a) = 34°C, RH = 60%, and ν(a) = 0.4 m/s). Results showed that the cooling effect on the manikin torso was 29.1 W/m(2) in the isothermal condition. The results on the manikin using a constant heating power mode reflect directly the local cooling effect on subjects. The results on the subjects showed that the torso skin temperature decreased by about 2-3°C and remained at 33.3°C. Both whole body and torso thermal sensations were improved. The findings indicate that the personal cooling with PCM can be used as an option to improve thermal comfort for office workers without air conditioning and may be used for vulnerable groups, such as elderly people, when confronted with heat waves. PRACTICAL IMPLICATIONS Wearable personal cooling integrated with phase change materials has the advantage of cooling human bodys micro-environment in contrast to stationary personalized cooling and entire room or building cooling, thus providing greater mobility and helping to save energy. In places where air conditioning is not usually used, this personal cooling method can be used as a preventive measure when confronted with heat waves for office workers, vulnerable populations such as the elderly and disabled people, people with chronic diseases, and for use at home.

Physiological evaluation of the resistance to evaporative heat transfer by clothing

A new method has been developed to determine the ‘effective’ evaporative resistance of clothing in vivo. It is based on direct measurements of the water vapour pressure gradient between skin and ambient air and of the steady state rate of evaporative heat loss. Air is sampled by a system of tubes terminating at six different loci on the skin surface underneath clothing and pumped to an oxygen analyser via a mixing chamber. Water vapour pressure is derived from measurements of oxygen partial pressure in the atmospheric air using the general gas law. Evaporative heat loss is obtained from continuous weighing of the subject on an electronic balance, after correction for respiratory heat loss and metabolic weight loss. The technique was used to evaluate the heat transfer properties of two types of rainwear and an overall. A rainwear made of a new fabric (Gore-Tex) produced a significantly lower evaporative resistance than a rainwear made of traditional material (nylon). It is concluded that the present method...

Heated manikins as a tool for evaluating clothing

Abstract 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.

Evaporative cooling: effective latent heat of evaporation in relation to evaporation distance from the skin

Calculation of evaporative heat loss is essential to heat balance calculations. Despite recognition that the value for latent heat of evaporation, used in these calculations, may not always reflect the real cooling benefit to the body, only limited quantitative data on this is available, which has found little use in recent literature. In this experiment a thermal manikin, (MTNW, Seattle, WA) was used to determine the effective cooling power of moisture evaporation. The manikin measures both heat loss and mass loss independently, allowing a direct calculation of an effective latent heat of evaporation (λeff). The location of the evaporation was varied: from the skin or from the underwear or from the outerwear. Outerwear of different permeabilities was used, and different numbers of layers were used. Tests took place in 20°C, 0.5 m/s at different humidities and were performed both dry and with a wet layer, allowing the breakdown of heat loss in dry and evaporative components. For evaporation from the skin, λeff is close to the theoretical value (2,430 J/g) but starts to drop when more clothing is worn, e.g., by 11% for underwear and permeable coverall. When evaporation is from the underwear, λeff reduction is 28% wearing a permeable outer. When evaporation is from the outermost layer only, the reduction exceeds 62% (no base layer), increasing toward 80% with more layers between skin and wet outerwear. In semi- and impermeable outerwear, the added effect of condensation in the clothing opposes this effect. A general formula for the calculation of λeff was developed.