Heatwaves are responsible for many deaths due to hot weather. Natural ventilation is promoted as a cooling solution to reduce the impacts of climate change because of its energy-saving potential. This paper addresses the prediction of heat-related health risk situations in naturally ventilated dwellings, where indoor thermal conditions depend on external climate and occupant behaviour. Thus, the operating limits of a dwelling vary according to its dynamic thermal response to indoor and outdoor stresses. To secure a building design, a method to improve the prediction of heat-related health risks is proposed that links predicted percentage dissatisfied (PPD) and wet-bulb globe temperature (WBGT) indicators to distinguish a moderately warm situation from a heat stress situation (WBGT > 29 degrees C). Simulations are performed for a dwelling during the 2015 heatwave in Paris (France) to test the method. In the example, natural ventilation was found to reduce the indoor temperature, but there remains ambiguity regarding the ability of natural ventilation to provide moderate conditions. According to the scope of the ergonomic standards, "moderate thermal environment" and "heat stress" temperature ranges are not contiguous. Bridging of this gap requires establishing thermal comfort ranges and heat stress thresholds adapted to people according to their acclimatisation level and their sensitivity to heat. (C) 2018 Published by Elsevier B.V.