## 4 Discussion
Assuming that the resting skin blood flow in thermoneutral environments is approximately 250 mL/min [24], 2 s-q-m of the total skin surface, and taking into account that all skin blood goes through the deep vascular plexus, we can estimate that the perfusion rate of the deep vascular plexus is on the scale of 0.02 m\({}^{3}\)/s/m\({}^{3}\), which is close to our estimate for 25 \({}^{\circ}\)C ambient temperature (thermoneutral zone)--see the insert in Figure 3. Thus, we can expect that 0.02 m\({}^{3}\)/s/m\({}^{3}\) is a typical value for a thermoneutral environment. However, the perfusion rate may significantly exceed this basal level for exposed skin parts due to cutaneous vasodilation. For example, skin blood flow can reach 6 to 8 L/min during hypothermia [25]. Note that in our calculations and discussion, we refer to the perfusion rate at the deep vascular plexus. The average perfusion rate for the whole dermis sometimes reported in the literature will be more than 20 times lower. Our results also support the notion that the absolute temperature is less relevant than the relative temperature (or temperature gradient). The absolute temperature for the large skin patches is regulated on the systemic level. Conversely, the temperature difference between two nearby points (e.g., wound vs. periwound), or a contralateral point, can indicate local abnormalities (wet wound, inflammation, reduced perfusion, or callus). All results were derived under the assumption of complete acclimation and controlled conditions: room temperature, controlled humidity, and no extensive sweating (e.g., due to exercise). These assumptions are valid for most thermographic measurements in the healthcare environment. However, any departure from these assumptions may significantly impact the heat loss balance. Other factors also affect skin temperature. The primary factors are gender and body fat. In a recent study, [PERSON] et al. [26] found that women had lower skin temperature than men in the trunk and upper and lower limbs. They also found that body fat percentage affects trunk skin temperature most. In men, hand skin temperatures are positively correlated with body fat percentage. Facial skin temperature could be related to gender but not to body composition. These gender-related findings are probably valid for adults only, as [PERSON] et al. [27] found that out of 14 skin locations, the statistically significant gender-related difference in skin temperature was present in only one location (back of the hand) of infants and toddlers. They also found that the temperature of children's forehead and limbs increases with age, and their core and skin temperatures seem to become more dependent on BMI with age maturation. [PERSON] et al. [28] found that in obese individuals, the fingernail-bed temperature was significantly higher (33.9 \(\pm\) 0.7 \({}^{\circ}\)C compared with 28.6 \(\pm\) 0.9 \({}^{\circ}\)C; \(p\) < 0.001), while the abdominal temperature was considerably lower (31.8 \(\pm\) 0.2 \({}^{\circ}\)C compared with 32.8 \(\pm\) 0.3 \({}^{\circ}\)C; \(p\) = 0.02) than in normal-weight individuals. The model applies to any convection heat transfer mechanism (free convection flow and forced air). However, it is illustrated for free convection flow only as being practically relevant. To accommodate the forced air mechanism, minor adjustments to Equation (16) are required. Additionally, results were derived under the assumption of a flat surface. Given that the surface curvature of body parts, \(\kappa\), is much smaller than the reciprocal thickness of our model (\(\kappa\ll 1/(D+E)\)); it is hard to expect any meaningful effect of the curvature on the results' validity. However, the heat dissipation can be different near curved parts. Thus, the heat loss Equation (8) and boundary conditions Equations (11) and (14) may need to be revisited for highly curved surfaces like fingers and toes. In future work, we plan to validate our findings in phantom experiments (for epidermis thickness) and controlled experiments on healthy volunteers.