## 2 Materials and Methods
### Methods--Theoretical Background  
#### 2.4.1 Body and Environment  
Thermal discomfort that arises while using helmets is directly related with body thermoregulation. The head plays an important role in human-body thermoregulation. Studies show that the human head shows high heat sensitivity [45] and unique biology due to which, under forced convection, the head can contribute up to one-third of total metabolic body-heat dissipation [29]. Helmets are constructed from high-strength materials like polymers such as plastics or composites and inner foam material to protect users from impact-related injuries. However, helmet shells made from high-strength materials act as an insulating layer for heat and moisture transfer from the head, resulting in thermal discomfort. Hence it is vital to understand the influence of helmet design on the thermoregulation of the human head. To understand the thermal factors influencing thermal comfort, the thermophysiological response of the human body must be understood. The core temperature of the human body is maintained at approximately 37 \({}^{\circ}\)C, which indicates that there exist heat-transfer mechanisms between body and environment such that heat is dissipated when in surplus and retained when in deficit. The relationship  
Figure 4: Surface temperature of thermal manikin measured at 39 locations (locations shown in Figure 1c) by temperature sensors during test. between heat generation in the body and transfer to the environment is dynamic, resulting in a heat balance that is described by the heat-balance equation [46]:  
\[\mathrm{S=M-W-(E_{res}+C_{res}+E_{sk}+C_{sk}+K+R),} \tag{1}\]  
where M (W) is the body metabolic rate and W (W) is work rate, quantifying the rate of heat production. Heat loss/gain is the result of evaporation (E) (W), convection (C) (W), conduction (K) (W), and radiation (R) (W). Evaporation and convection occur through respiration (res) (W) and skin (sk) (W), and S (W) is heat storage in the human body. Heat storage/dissipation by the human body plays a vital role in maintaining energy balance in the body. However, the rate of heat production is not always equal to the rate of heat dissipation, resulting in changes in thermal balance. A basic understanding of what this concept would be, if heat storage S = 0 or S \(\sim\) 0, i.e., generated heat is approximately equal to dissipated heat (M = W + heat-transfer modes), energy balance is attained, which results in thermal balance and vice versa. Heat transfer through conduction (K) is studied in combination with convective components since heat-transfer-through-conduction mode is relatively small due to little or no direct contact between head and helmet, and because of the much bigger contributions from evaporation, convection, and radiant exchange. Researchers have often used dry (or sensible) heat transfer and wet (or evaporative) heat transfer when describing the heat-transfer mechanisms between head and environment. Dry heat transfer is the combination of heat transfer by convection (natural and forced) and radiation. The convective component is primarily influenced by the \(\Delta\)T that exists between head surface and environment. Convective heat transfer is defined by [47]  
\[\mathrm{\dot{Q_{cs}}=h_{c}A(T_{sk}-T_{a}),} \tag{2}\]  
where, \(\mathrm{\dot{Q_{cs}}\) (W) is the convective heat transfer per unit of time, A (m\({}^{2}\)) is the surface area of the object, h\({}_{c}\) (W/m\({}^{2}\).K) is the convective heat-transfer coefficient, T\({}_{sk}\) (K) is the temperature of head surface and T\({}_{a}\) (K) is the temperature of air/fluid.