Knowing the conditions for a system to undergo thermal explosion is of utmost importance for many applications. A critical condition that accounts for reactant consumption and covers most practical situations, including low activation energy reactions is presented. Our solution applies to cylindrical reactors of any radius to height ratio. In the case of films, it is shown that thermal explosion is virtually impossible. A new criterion to define the boundary of thermal run- away based on heat balance is introduced. This new definition of criticality allows us to check the accuracy of the non- stationary model to describe the critical condition. The nonstationary model is the base of most approaches