Effects of moisture absorption in clothing on the human heat balance.

A theory of moisture absorption in clothing, with the associated effects of heat transfer, was developed and applied in a computer model. The model considers the body, underclothing, an outer layer, and the adjacent air layer. The theory was checked with an experiment involving four subjects. They wore heavy woollen clothing, which was either initially dry or humid, in both a warm and a cool environment. Model calculations and experimental results agree approximately upon the timing and magnitude of the effect of absorbing clothing on heat flows, temperatures and physiological reactions. Contrary to expectations the observed vapour resistance is lower in the heat than in the cold, probably due to differences in sweat distribution. It is pointed out that the usual way to determine the clothing characteristics by means of partitional calorimetry leads to considerable errors when the steady state has not been reached. In clothing that has high absorption properties the transient effects may be sustained for hours. Tests using the model show few beneficial effects of absorbing clothing on thermal sensation.

Effects of condensation in clothing on heat transfer.

A condensation theory is presented that enables the calculation of the rate of vapour transfer with its associated effects on temperature and total heat transfer inside a clothing ensemble consisting of underclothing, enclosed air, and outer garment. The model is experimentally tested by three experiments: (1) impermeable garments worn by subjects with and without plastic wrap around the skin, blocking sweat evaporation underneath the clothing; (2) comparison of heat loss in impermeable and semi-permeable garments and the associated discomfort and strain; (3) subjects working in impermeable garments in cool and warm environments at two work rates, until tolerance. The measured heat exchange and temperatures are calculated with satisfying accuracy by the model (mean error = 11, SD = 10 Wm-2 for heat flows and 0.3 and 0.9 degree C for temperatures, respectively). A numerical analysis shows that for total heat loss the major determinants are vapour permeability of the outer garment, skin vapour concentration and air temperature. In the cold the condensation mechanism may completely compensate for the lack of permeability of the clothing as far as heat dissipation is concerned, but in the heat impermeable clothing is more stressful.

Transfer of radiative heat through clothing ensembles.

A mathematical model was designed to calculate the temperature and dry heat transfer in the various layers of a clothing ensemble, and the total heat loss of a human who is irradiated for a certain fraction of his or her area. The clothing ensemble that is irradiated by an external heat source is considered to be composed of underclothing, trapped air, and outer fabric. The model was experimentally tested with heat balance methods, using subjects, varying the activity, wind, and radiation characteristics of the outer garment of two-layer ensembles. In two experiments the subjects could only give off dry heat because they were wrapped in plastic foil. The model appeared to be correct within about 1 degree C (rms error) and 10 Wm-2 (rms error). In a third experiment, sweat evaporation was also taken into account, showing that the resulting physiological heat load of 10 to 30% of the intercepted additional radiation is compensated by additional sweating. The resulting heat strain was rather mild. It is concluded that the mathematical model is a valid tool for the investigation of heat transfer through two-layer ensembles in radiant environments.

Vapour transfer in two-layer clothing due to diffusion and ventilation.

An experiment was carried out to measure the vapour resistance of two-layer clothing ensembles as a function of air permeability of the outer layer, open or closed apertures, wind, and walking, both for the total ensemble and for the outer garment alone. Six subjects walked on a treadmill (0.0, 2.5, and 5 km.h-1) which was placed in a wind tunnel (0.2, 0.7, and 3.0 m.s-1). They wore long underwear and an outer garment made of impermeable (imp), microporous (mpo), low air permeable (loa), or high air permeable (hia) fabric. Vapour resistances were determined by a trace gas method, calibrated against water vapour resistance. The vapour resistances of the underclothing and the outer garment were calculated from the measured data, as was the ventilation through the apertures. The vapour resistance of the underclothing was almost constant at 5 mm air equivalent. The ventilation was strongly dependent on wind and motion but still so low (54 l.min-1) that only the impermeable garment could benefit from it noticeably. The vapour resistance of the garments also varied strongly (imp 55-200 mm, mpo 12-20 mm, loa and hia 1-14 mm air equivalent). For the imp garment, this is due to leakage of air, whereas the vapour permeable garments were dominated by the diffusion and air penetration through the fabric. It is concluded that ventilation with vents cannot match the effect of vapour permeability, and that real low vapour resistances are only possible with air permeable fabrics.

Simulation of hand cooling due to touching cold materials.

A simple analytical model has been developed to simulate the cooling of the hands due to touching various types of cold material. The model consisted of a slab of tissue, covered on both sides with skin. The only active mechanism was the skin blood flow. The blood flow was controlled by body core temperature, mean skin temperature, and local hand temperature. The blood flowed along the palm before returning via the back of the hand. The control function was adapted from an earlier study, dealing with feet, but enhanced with a cold induced vasodilatation term. The palm of the hand was touching materials that were specified by conductivity and heat capacity. The hand was initially at a steady-state in a neutral environment and then suddenly grabbed the material. The resulting cooling curves have been compared to data from an experiment including six materials (foam, wood, nylon, steel, aluminium and metal at a constant temperature), three temperatures (-10, 0, and 10 degrees C), two thermal states of the body (neutral and 0.4 degrees C raised), and with and without gloves. There was a fair general agreement between the model and the experiment but the model failed to predict three specific effects: the unequal effect of equal 10 degrees C steps in cold surface temperature on the temperature of the palm of the hand, the cooling effect of nylon, and the rapid drop in back of the hand temperature. Nevertheless the overall regression was 0.88 with a standard deviation between model and experiment of about 2.5 degrees C.

The actual insulation of multilayer clothing.

The effect of geometric factors on insulation was calculated mathematically for standing humans. It was found that internal radiative heat transfer in an ensemble was significant for insulation, that intrinsic clothing insulation is a useful concept only for indoor climates, and that shape plays a minor role. The literature agrees closely on insulation and clothing surface area figures, and the latter are compatible with model predictions. Finally, it was shown that wind, body motion, the effects of posture, and the fit of garments are predictable. Sitting provides more insulation than standing for light clothing, but the reverse is true for heavy clothing. Insulation is decreased by about 20% by cycling and by about 40% by walking, and a reasonable estimate can be made of the effect of wind and wind and motion together. The effect of air motion on vapor permeability is stronger than the effect on heat transfer.