In this paper, a novel human body exergy consumption formula was derived strictly according to Gagge's two-node thermal transfer model. The human body exergy consumption calculated by the formula was compared with values calculated using Shukuya's formula for a typical office environment. The results show that human body exergy consumption calculated by either of these formulas reaches a minimum under the same thermal condition. It is shown that this is in accordance with expectation. The relation between human performance and human body exergy consumption was studied by analyzing the data obtained in simulated office environments in winter. The results show that human body exergy consumption and human performance are inversely as operative temperature changes from 17 to 28 degrees C or human thermal sensation changes from -1.0 to +1.4, and that optimum thermal comfort cannot be expected to lead to optimal human performance, as has so often been assumed. According to the second law of thermodynamics, it makes sense that optimal human performance coincides with minimum human body exergy consumption and that this should occur under thermal conditions in which human thermal sensation is close to "slightly cool". (c) 2012 Elsevier B.V. All rights reserved.