Barriers against losses

In Chapter 7, we introduced the concept of barriers against losses. Two examples were shown—one of barriers against exposure to hazardous chemicals and the other of the prevention of fires and explosions. We may use this concept to define a different set of performance indicators, i.e. that of barrier availability. This is defined as the probability that a barrier is available when it is needed and ranges from 0 to 1 (or from 0 to 100 per cent). These types of performance indicators have by their nature to be specific to the type of hazard that is to be prevented. Let us illustrate this principle by two examples. 

This is a highly efficient barrier against evaporative losses of volatile compounds, which also improves the peak width of the early eluting compounds. This system has been successfully applied to a group of pesticides, using -decane as the co-solvent and has enabled a group of volatile phosphorus pesticides to be determined (95). The experimental conditions used in this work are shown in Table 13.2. 

The anionic portions of the soil solution play a role of equal importance to the cations. The anions function in the manner outlined for cations in conductivity and concentration-cell action, and have an additional action if they react with the metal cation and form insoluble salts. Thus, if the metal is lead and the predominant anion is sulphate, a layer of insoluble lead sulphate may precipitate on the metal surface and form an effective barrier against further loss of metal. 

Usually these three biophysical mechanisms interact precisely to provide a highly efficient barrier against water loss and retain water within the tissue to maintain flexibility and catabolic activity. Nevertheless, this barrier is continually prone to perturbation by both external forces (UV, low RH, cold temperatures, and surfactants), and internal factors (cutaneous disease, psychological stress, and diabetic complications). With decreased performance of the water barrier the increased loss of water from the tissue ultimately leads to the formation of dry skin. 

The permeability barrier located in stratum corneum protects the skin against loss of water at a rate of 1.6 kg per cm2 per hour. With a body surface of 1.8 m,2 this translates into protection against a potential loss of almost 30 t water per hour 45 This remarkable and still ill-understood protective action is sometimes overlooked. Even a text book as renowned as the Molecular biology of the cell does not mention the existence of this barrier 46... 

Acne vulgaris is the result of several factors combined. The condition arises in the pilosebaceous units in the dermis, which consist of a hair follicle and associated sebaceous gland. These glands secrete sebum, a mixture of fats and waxes that protect the skin and hair by retarding water loss and forming a barrier against external agents. The hair follicle is lined with epithelial cells that become keratinised as they mature. 

Small-size carbonated soft drinks (<0.5 L) Improved barrier against CO2 loss Increased UV barrier Copolsmiers, blends... 

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