Evaporation from skin

In centrally heated houses and hospitals sometimes an open dish of water is used in a room to evaporate and give more water vapour in the air. Air extraction and circulation systems or air-conditioning plants condense out the water vapour in their purification processes and so the air feels dry . Flowers have a similar effect of adding some water vapour to the air by evaporation from their leaves and flowers. 

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When relative humidity is high, sweat does not easily evaporate from skin. Why ... 

There are many chemicals, by lowering suitable as carriers. Their bp is one of the principal criteria in selection. If bp is too low, the compound will evaporate from the dyebath at dyeing temperatures, and will be lost before it is effective in its role as a carrier. It may also steam distill (condense on the cooler parts of the equipment) and cause drips that will spot the fabric. On the other hand, if the bp is too high, the compound cannot be removed from the fabric under normal plant drying conditions and will affect lightfastness of finished goods, leave residual odor, and possibly cause skin irritation to the wearer. 

On contact with the skin, liquid ammonia produces severe burns compounded by frostbite due to the freezing effect from rapid evaporation from the skin. 

The skin receives heat from the core by passive conduction and active skin blood flow (Table 5.3). It transfers this heat to the surroundings by convection, radiation, and evaporative (perspiration and diffusion) mechanisms. All of these mechanisms are unregulated or passive except evaporation from sweating. The sweating process is actively controlled by the humarrs thermoregulatory center where the rate of sweat secretion is proportional to eleva tions in core and skin temperature from respective set point temperatures (Table 5.3). 

The value used in ISO 7933, required sweat rate, SW is based on the heat balance equation (6.1). Assuming the heat storage is equal to 0, the necessary evaporation from the skin, > to ensure a heat balance is calculated as foil... 

What is the maximum amount of heat that you can lose as one gram of water evaporates from your skin ... 

A clinical study of 21 females and 21 males used to determine the efficacy of using aspirin dissolved in chloroform which was then applied topically to patients infected with herpes zoster and post-therapeutic neuralgia with painful skin lesions has been reported. When an aspirin/chloroform combination (approximately 43.3 mg/mL) was applied, the only reported side-effect was an occasional burning sensation on the skin as the chloroform evaporated from the skin surface however, the possible impact on other major body organs (liver, kidney, etc.) was not investigated (King 1993). 

During a marathon, the rate of heat production can be more than tenfold greater than at rest and sufficient to raise the core body temperature by 1°C every eight minutes, if no cooling occurs. The core temperature is normally regulated so precisely that it does not rise more than about 1°C. The main mechanism for cooling is evaporation from the skin. Endurance runners can produce one litre of sweat per hour which removes about 2.4 MJ of heat. The energy used and therefore converted into heat in a marathon is about 12 MJ... 

DOSAGE - Skin - lOmg. Orally -2mg. Inhalation -lOmg.min/cubic meter. VX has a very low vapor pressure and therefore is very slow to evaporate. The primary danger is from skin contact or ingestion, though it is very toxic as an aerosol. 

When we sweat, water evaporates from the surface of the skin. Because it takes a lot of energy (heat) for water to change from the liquid phase to the gas phase, when sweat evaporates, it actually takes some energy (heat) away from the body. This is what scientists call an endothermic reaction. An endothermic reaction absorbs energy. 

Breathing DEHP does not appear to have serious harmful effects. Studies in rats have shown that DEHP in the air has no effect on lifespan or the ability to reproduce. As mentioned previously, almost no DEHP evaporates into air. You probably will not have any health effects from skin contact with DEHP because it cannot be taken up easily through the skin. 

In vivo experiments on 4 human volunteers, to whom 0.0026 mg/cm2 of 14C-benzene was applied to forearm skin, indicated that approximately 0.05% of the applied dose was absorbed (Franz 1984). Absorption was rapid, with more than 80% of the total excretion of the absorbed dose occurring in the first 8 hours after application. Calculations were based on urinary excretion data and no correction was made for the amount of benzene that evaporated from the applied site before absorption occurred. In addition, the percentage of absorbed dose excreted in urine that was used in the calculation was based only on data from rhesus monkeys and may not be accurate for humans. In another study, 35-43 cm2 of the forearm was exposed to approximately 0.06 g/cm2 of liquid benzene for 1.25-2 hours (Hanke et al. 1961). The absorption was estimated from the amount of phenol eliminated in the urine. The absorption rate of liquid benzene by the skin (under the conditions of complete saturation) was calculated to be low, approximately 0.4 mg/cm2/hour. The absorption due to vapors in the same experiment was negligible. The results indicate that dermal absorption of liquid benzene is of concern, while dermal absorption from vapor exposure may not be of concern because of the low concentration of benzene in vapor form at the point of contact with the skin. No signs of acute intoxication due to liquid benzene dermally absorbed were noted. These results confirm that benzene can be absorbed through skin. However, non-benzene-derived phenol in the urine was not accounted for. 

The heat of vaporization is the heat required to evaporate 1 g of a liquid. Water s large heat of vaporization (540 cal/g) requires large amounts of heat in order to vaporize it into gas. During perspiration, water evaporates from the skin and large amounts of heat are lost. Heat of fusion is the heat required to fuse or melt a substance. Water s heat of fusion is 6.02 kj/mol. 

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