Implicit physical environment assumptions made by safety critical cyber-physical systems, such as medical cyber-physical systems (M-CPS), can lead to catastrophes. Several recent U.S. Food and Drug Administration (FDA) medical device recalls are due to implicit physical environment assumptions. In this paper, we develop a mathematical assumption model and composition rules that allow M-CPS engineers to explicitly and precisely specify assumptions about the physical environment in which the designed M-CPS operates. Algorithms are developed to integrate the mathematical assumption model with system model so that the safety of the system can be not only validated by both medical and engineering professionals but also formally verified by existing formal verification tools. We use an FDA recalled medical ventilator scenario as a case study to show how the mathematical assumption model and its integration in M-CPS design may improve the safety of the ventilator and M-CPS in general.