It's impossible for civil aircraft to comply with FAR/CCAR 25.831(g) under the assumption of loss of all conditioned airflow for flight in failure conditions, including descent and landing. It has been speculated that the fixed humidity level of 27 mbar appears to be a reasonable limit for altitude conditions around 10,000 feet. Unfortunately this humidity level is often exceeded at lower altitudes and near sea level for airport ambient conditions. The Mechanical System Harmonization Working Group(MSHWG) Final Report on FAR 25.831(g), dated July 31,2003 proposed action is to harmonize on a new, performance-based regulation for failure conditions not shown to be extremely improbable. The objective of this regulation is to preserve a tolerable environment by limiting the metabolic and environmental heat loads to passengers and crew during exposures to a potential heat stress event. Relative to the current FAR/CCAR 25.831(g), and considering the inapplicability of its humidity requirements, the proposed regulation does not reduce the current level of safety. A thermal analysis simulation for calculating body core temperature of passengers and pilots is presented and simulated body core temperature is compared with standard temperature value defined in MSHWG report.The computing results are applied to validate one aircraft complying with FAR/CCAR 25.831(g) according to the new regulation in MSHWG report.