The ASHRAE Effective Temperature (ET*) describes the temperature of a standard environment at 50% relative humidity in which the total skin heat exchange (dry and latent) is equal to that in the real environment. The significance of ET* is reexamined here in terms of basic thermodynamics and transport theory. Dry-bulb (DBT) and wet-bulb (WBT) temperatures measure the potential of the environment to exchange dry heat and total heat with wet surfaces through convection.. The Lewis Relation links the coefficients of heat and mass transport. By analogy, operative temperature (t_o) and ET* measure the potential for dry heat and total heat exchange by convection and radiation with a human whose skin is partially wet and protected by clothing. Here, the Lewis Relation must be modified by the nondimensional factor i_m, a function of the air layer and clothing transfer coefficients. This analogy substantiates the use of ET* as a more accurate index of the energy transfer potential and enthalpy for the total environment surrounding the skin than either t_o or WBT.

Units: SI