Skin surface resistance

The majority of SSTIs are caused by gram-positive organisms and, less commonly, gram-negative bacteria present on the skin surface. Staphylococcus aureus and Streptococcus pyogenes account for the majority of SSTIs. Community-associated methicillin-resistant S. aureus (CA-MRSA) has recently emerged and it is often isolated in otherwise healthy patients. 

Resistance of the host to infection is determined by an intact skin surface and mucous membranes, as well as the innate and acquired immunity of the host. 

The skin represents the largest organ in the human body, and one of its primary functions can be seen as a physical barrier to absorption of toxicants. The other major routes of toxicant entry into the body are through the respiratory and gastrointestinal tract, which can be seen to offer less resistance to toxicant absorption than the skin. In general, the respiratory tract offers the most rapid route of entry, and the dermal the least rapid. One reason for this major difference is primarily because membrane thickness, which is really the physical distance between the external environment (skin surface, air in the lung, or lumen of the gut) and the blood capillaries, varies across these portals of entry. The overall entry depends on both the amount present and the saturability of the transport processes involved. 

Equation (5.4) is valid as long as the skin depth is large in comparison to the mean free path of the electrons in the metal. This holds true in the microwave range at room temperature, for cryogenic temperature the surface resistance lies above the values predicted by Equation (5.4) and exhibits a f2 3 rather than a f1 2 frequency dependence (anomalous skin effect [7]). 

As an example, for copper with a room temperature conductivity of 5.8 x 107(O m) 1 the surface resistance at 10 GHz is 26 mil, the skin depth is 0.66 pm. Therefore, the Q of a cavity resonator with a geometric factor of several hundred is in the 104 range. However, for planar resonators like the ones shown in Figure 5.8 the G values are only a few Ohms leading to Q values of only a few hundred. This is too small for many filter and oscillator applications. 

The skin depth Ss is inversely proportional to the square root of the frequency, and at 1 MHz is 0.064mm for silver, 0.066mm for copper and 0.19mm for a typical solder. A surface resistivity ps can be defined by... 

By observing that the skin s electrical resistance substantially decreased as layers were sequentially removed by tape stripping, Yamamoto and Yamamoto [5] demonstrated that the stratum comeum layer is the primary contributor to the skin s electrical resistance. However, Resell et al. [6] have shown that even the slightest abrasion to the skin markedly decreases the skin s resistance. In addition, in Leveque and De Rigal s review [7], it is mentioned that stripping can produce a cracked skin surface, resulting in a defective contact between... 

A surface temperature of 46 C causes pain on the skin. Therefore, direct contact with a metal block at this temperature or above is painful. I/owever, a person can stay in a room at 100 C for up to 30 min without any damage or pain on the skin because of the convective resistance a,t the. skin surface and evaporative cooling. We can even put our hands into an oven at 200 C lor a siiort time without getting burned. 

Heat loss by evaporation is maximum when the skin is completely wetted. Also, clothing offers resistance to evaporation, and the rate of evaporation in clothed bodies depends on the moisture permeability of the clothes. Tlie maximum evaporation rale for an average man is about 1 L/h (0.3 g/s), which represents an upper limit of 730 W for the evaporative cooling rate. A person cM lose as much as 2 kg of water per hour during a workout on a hot day, biit any excess sweat slides off the skin surface without evaporating (Fig. 13-44). 

Root knot and other pest nematodes cause poorly colored, deformed potatoes with rotted areas under the skin, surface blemishes, and surface cracks. Control pest nematodes by applying a chitin or parasitic nematodes to the soil. Prevent problems by planting nematode-resistant cultivars such as Heart , Jasper , Jeweir. Kandee, Nemagold , and Nugget . 

While the use of water alone can soften hair, the addition of a fatty acid salt (i.e., soap) improves the performance of water as a shaving aid by creating a film on the skin. This film created by shaving soaps allows the blade to slide with decreased resistance along the outer epidermal skin surface without cutting into the vascular (blood vessel-possessing) deeper dermal skin layer. In general, soap results from the treatment of... 

When surface-barriers or a surface-resistance or skin effect to sorbate gases occurs, the overall characteristic functions can be indicated by [3,43,44]... 

The parameter a = kJ ID is the ratio of the rate of chemical transfer from the soil to the SC (a process that can be characterized by a mass transfer coefficient which has units of cm s ) to the rate of mass transport through the SC (i.e., DIL). Thus, when a > 1, the SC resistance is most important on the other hand, for a < 1, mass transfer from the soil to the SC limits the dermal absorption rate. When the mass transfer resistance from the vehicle is insignificant, the concentration of chemical on the skin surface (i.e., C ) is equal to (see Equation 11.2). By comparison, when the mass transfer resistance witliin the vehicle is not small relative to that in the SC (i.e., a is not very large), then at steady state is reduced relative to as specified by Equation 11.3. That is. 

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