Objectives: To revise the criteria used in the present 'Required Sweat Rate' standard ISO 7933 (1989) for the prediction of the maximum duration of work in hot environments. Methods: Review of the literature and in particular, of the bases for the present criteria. Results: A new method is proposed, to take into account the increase in core temperature associated with activity in neutral environments. The prediction of maximum wetness and maximum sweat rates are revised, as well as the limits for maximum water loss and core temperature. Conclusions: An improved set of maximum values and limits is described, to be used in the revised version of the ISO 7933 standard. Due to the major modifications to the 'Required Sweat Rate' index and in order to avoid any confusion, it is suggested that the revised model be renamed the 'Predicted Heat Strain' (PHS) model.