Due to the higher workload it produces, reducing the size of operational teams in the chemical process industry can have a negative effect on the ability to control abnormal situations, fatigue, etc. A lack of qualified operational personnel in unusual conditions and the resulting lack of process control can trigger a series of internal or external accidents, eventually leading to a major accident. This paper suggests a practical method to evaluate the safety critical staffing levels required to meet performance specifications for safety critical activities. For single plants as well as for clusters of chemical plants, the method also enables consultants and inspectors to consequently apply principles to assess those manning levels representing the last but one line of defense in the prevention of major accidents.