The heat exchange between the human body and the thermal environment (Fig. 2.2) can be described in the form of the energy balance equation which is nothing but the first theorem of thermodynamics applied to the body's heat sources
(metabolism and environmental), and the various avenues of heat loss to environment (Büttner 1938):
M - W -\Qh (Ta, v)+ Q * (Tart, v)] - [Ql (e, v)+ Qsw (e, v);] - ßRe (Ta, e)± S = 0 (2.1)
M Metabolic rate (activity) W Mechanical power
S Storage (change in heat content of the body)
Peripheral (skin) heat exchanges:
Turbulent flux of sensible heat Radiation budget
Turbulent flux of latent heat (diffusion water vapour) Turbulent flux of latent heat (sweat evaporation)
Respiratory heat exchanges:
QRe Respiratory heat flux (sensible and latent)
Thermal environmental Parameters:
Ta Air temperature T , Mean temperature mrt r v Air speed relative to the body e Partial vapour pressure
The meteorological input variables include air temperature Ta, water vapour pressure e, wind velocity v, mean radiant temperature Tmrt including short- and long-wave radiation fluxes, in addition to metabolic rate and clothing insulation. In Eq. (2.1) the appropriate meteorological variables are attached to the relevant fluxes. However, the internal (physiological) variables (Fig. 2.1), such as the temperature of the core and the skin, sweat rate, and skin wettedness interacting with the environmental heat exchange conditions are not explicitly mentioned here.
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