A Martian settlement poses great technical and human challenges. Most studies focussed on the technical aspects of the problem. We address the human factors issues. A list of variables has been defined. In the human factors domain, each variable falls into one of the following categories: psychological, sociological, physiological and cognitive. A cross impact matrix is proposed to describe the impact of each variable onto the others. The main variables are energy, vital needs, diseases, Earth-Mars relationships and skills management. Several scenarios are analysed to discuss the interactions among variables and to identify the failure risks of the colonization process. Suggestions are proposed to eliminate the risk or to mitigate the impact.