The global rise in interest towards robotics and artificial intelligence is increasing the technology acceptance among companies. This further encourages manufacturing companies to invest more in robotics on their factory floor. A robot manipulator can be sufficiently mobile and dexterous to operate alongside a human as would any other colleague. However, a human-centric viewpoint is needed in the design of the work cell to provide optimal working conditions for humans and thereby enhance employee performance. We identified a set of factors required for human comfort during cooperation with robots. These factors were divided into two main groups: mental and physical. Both mental and physical factors were based on scientific work reviews, robotics standards, and recognized human factors via a case study. These factors together are the basis for a comfort zone concept in human-robot collaboration. This concept forms design principles for developing the physical work environment of the future.