It's time to learn about "the evacuation equation" from one of its creators. This week I interview prof. David Purser about the elements that go into the evacuation process that help us determine the fire safety of buildings. We go through the components of the Required Safe Evacuation Time (RSET) one by one: detection timealarm timepre-movementoccupant movement and congestion.We discuss how those were initially defined, what the assumptions behind establishing them as a part of the framework were, and what engineering scenarios they responded to. Prof. Purser explains the similarity behind the "human behaviour scenario" and the "design fire scenario" as both being inaccurate descriptions of the real world but valuable for design. We also discuss cases in which those simple assumptions perhaps did not hold, such as the complex case of the Grenfell Tower evacuation. We confront the harrowing reality of communication breakdowns and delayed responses, dissecting how they can dramatically alter the outcome of an emergency. This highlights that the methodology is not "ultimate" and it is up to the engineer to understand the complexities of the process and the weak aspects of it. The method described in this episode forms the core of PD 7974-6:2019 Application of fire safety engineering principles to the design of buildings - Human factors. Life safety strategies. Occupant evacuation, behaviour and condition (Sub-system 6). The method has been implemented in many local codes throughout the world and can today be considered a standard approach in the fire-safe design of buildings. In the episode, prof Purser mentioned "new data" on pre-evacuation time. In his IAFSS lecture, he mentioned some sources, including this paper by Lovreglio et al.  Cover image based on a figure presented in paper 10.3390/buildings13123038