Third-degree burns

In the case of third-degree burns, both the epidermis and dermis layers are damaged; in turn, trauma to the blood vessels occurs. Due to this damage, no blood flows and the cells in the burn region start to die. This situation results in leathery skin, and it is very difficult to recover from this type of burn [122].

Fourth-degree burns

This is the most advanced degree of burn injury. In this stage, all of the skin layers are destroyed and the muscles, bones, and other structures beneath the subcutaneous tissue are also damaged. Healing from such burns is similar to third-degree burns; however, special medical care is required when severe damage occurs on the underlying muscles and bones of the subcutaneous tissue [122,124].

Skin burns models

Mainly, two types of models are used for predicting skin burns in the context of thermal protective clothing studies: (1) the Henriques burn integral equation and (2) the Stoll second-degree burn criterion [35,36,133,134]. These models are primarily used to evaluate the thermal protective performance of clothing and fabrics in terms of time required to generate skin burns on wearers’ bodies. For this evaluation process, thermal protective clothing and/or fabrics are exposed to high heat flux, and the heat flux histories transmitted through the clothing and/or fabrics (during and/or immediately after the thermal exposure) are used in the models to calculate the time required to generate burns on wearers’ bodies. Presently, these models have also been employed to develop various standard (eg, ASTM, NFPA, ISO) test methods to evaluate the thermal protective performance of clothing and fabrics, using full-scale manikin tests and bench-scale tests, respectively. For example, the ASTM F 1930 test standard was developed to evaluate the performance of thermal protective clothing under flash-fire exposure using the Henriques burn integral equation; similarly, the ISO 6942, ASTM D 4108, and ASTM F 1939 test standards were developed to evaluate the performance of fabrics under flash-fire flame and radiant heat exposures, respectively, using Stoll second-degree burn criterion [135-138]. In this context, it is notable that these models predict the time required to generate burns using the Henriques mathematical equation of human skin or experimental data obtained from a small number of animal or human experiments. Although much effort has been made to refine these predictions, they may not always be applicable in real life situations [36,139]. This is because human skin and its thermal properties vary widely from person to person (due to factors such as the age of the individual), and from body site to body site of an individual. These variations may result in different times to generate burns on human beings. In the following sections, both models are thoroughly discussed.

Henriques burn integral equation: In the mid-20th century, Henriques and Moritz [125,126,128] found that the destruction of the tissue layer located at the epidermis/ dermis interface in human skin starts when the tissue temperature of the epidermis rises above 44°C. Based on this finding, Henriques and Moritz [125] modeled the destruction rate of human skin as a first-order biochemical reaction using the Arrhenius relationship. To develop this model, they critically considered the time for which the temperature of the epidermis was above 44°C (Eq. 3.1). This Eq. (3.1) was further integrated to produce Eq. (3.2), where, Q = a quantitative measure of burn damage at the epidermis or at any depth in the dermis (dimensionless), P = preexponential factor (s^-1), ДБ = the activation energy for human skin (J/mol), R = the universal gas constant (8.315 J/kmol K), T = absolute temperature at the epidermis or at any depth in the dermis (K), and t=total time for which t is above 44°C (s). Based on Eq. (3.2), first-, second-, and third-degree burns occur at Q = 0.53, Q = 1, and Q = 1, respectively. In order to calculate the first- and second-degree burn time, T of the base of the epidermis layer is required; whereas, T of the base of the dermis is used to predict the third-degree burn time. In Eq. (3.2), T is usually calculated by Pennes’ bioheat transfer equation [Eq. 3.3, where, p = density of skin (kg/m³), c = specific heat of skin (J kg.°C), к = thermal conductivity of skin (W m/°C), x = skin depth (m), G = blood perfusion rate (m³/s/m³of tissue), p_b = density of blood (kg/m³), c_b = specific heat of blood (Jkg °C), and T_c = core temperature of human body (37°C)] [140]. Although Pennes’ (Eq. 3.3) equation was originally developed to simulate skin at normothermic condition, this equation incorporates the capacitive, conductive, and convective characteristics of skin tissue that become significant at hyperthermic condition; moreover, the metabolic heat generation is considered negligible at the hyperthermic condition [141-143]. Here, the values of thermal properties (k, p, c, x, G, P, and ДБ/R) of skin and blood in the Eqs. (3.2), (3.3) are set as per Table 3.1 [36,144]. Henriques and Pennes

Table 3.1 Values of thermal properties for Eqs. (3.2), (3.3) [35,144]

equations together have been widely used to evaluate the time required for different degrees of skin burns; this is because both Henriques and Pennes equations are accurate for hyperthermic conditions [141-143]. Contextually, it is notable that many researchers have made considerable efforts to refine these equations by considering the heat exchange between skin tissue and blood vessels, as well as between blood vessels (arteriole and venous) [145-147]. However, this refinement may not be applicable in the context of thermal protective clothing studies because this heat exchange diminishes at high heat flux due to the high temperature gradient between skin tissue and blood. Additionally, it has been found that temperature profiles of arteriole and venous vessels are uniform, and thus, the heat exchange between them is negligible. Overall, the main advantage of the Henriques burn integral equation is that this model is applicable to predict first-, second-, and third-degree burn time at any heat flux; however, this prediction method is more involved and requires the use of a computer and specialized software [35,36,134].

It is clear from the earlier discussion that the burn time prediction from the Henriques burn integral equation depends upon various thermal properties of skin and blood. Many researchers found that these thermal properties have different degrees of effect on the prediction, with the thermal conductivity of the epidermis and dermis having the most significant effect [148-152]. Torvi and Dale [144] also suggested that these thermal properties have negligible impact on the prediction for short-duration (~6—7 s) intensified thermal exposures, but have a significant effect on the prediction for the long-duration (>7 s) intensified thermal exposures. In this context, it has been observed that at slightly elevated temperatures of thermal exposure, the permeability of the cells of skin tissue and capillaries increases, and can cause edema in the skin. If the intensity (heat flux) of thermal exposure is sufficient, higher temperatures can cause the cell walls to lose structural integrity and rupture, resulting in massive fluid loss; if either the intensity or the time of exposure is high, the cumulative water loss from the body can drastically alter the thermal properties of skin and blood, ultimately affecting the skin burn time prediction [148-153].

Stoll second-degree burn criterion: In the late 1950s and early 1960s, Alice Stoll, Leon Green, and Maria Chianta conducted burn injury studies (on pigs, rats, and sailors of the US Navy) at the United States Aerospace Medical Research Department, Naval Air Development Center in Pennsylvania [129,154,155]. Based on their studies, Stoll and Chianta [131] established a range of heat exposure time and its corresponding heat flux to generate second-degree burns on human bodies. In this range, some data were based on observed exposure times required to produce second- degree burns on blackened human skin subjected to incident heat fluxes from 4.2 to

• 16.8 kW/m², whereas other data were theoretically determined for heat fluxes from
• 16.8 to 41.9 kW/m² [129]. For the range of exposure time and its corresponding heat flux, the temperature rise (AT°C from initial temperature 32°C) of a copper calorimeter (with iron-constantan thermocouple) was later recorded and a relationship plot between exposure times, temperature increases, and heat flux was developed as the “Stoll Curve” (Fig. 3.2) [156]. By employing the Stoll Curve, the temperature rise of the copper calorimeter under a particular heat exposure is used to calculate heat flux or time required for second-degree burn injuries on human bodies. The main advantage of this model is that it is simple and does not require any sophisticated numerical calculations to predict skin burns. However, this model is limited to predicting the burn time within a certain range of heat flux. Therefore, the prediction of burn time beyond this range of heat flux is questionable. This model is also inapplicable in the case of “nonrectangular” or “nonsquared” heat flux histories [131]. Additionally, this method is limited to predicting the time required for second-degree burns, and thus, the prediction of other degrees of burns (first- and third-degree) is impossible [133,134]. Presently, it has been suggested that the Stoll second-degree burn criterion should incorporate the thickness of human skin in the calculation of the time required for second-degree burns, as the thickness of human skin varies from individual to individual [139]. Furthermore, Stoll’s original work measured second- degree burn times on the forearms of her subjects (sailors of the US Navy), where skin layers are relatively thin [129,131,154,155]; this may affect results related to other properties of skin covering the rest of the human body [139].

Fig. 3.2 Stoll Curve.

Modified from S. Mandal, G. Song, Thermal sensors for performance evaluation of protective clothing against heat and fire: a review, Text. Res. J. 85 (1) (2015) 101-112.