Nowadays, the Unified Modeling Language (UML) is becoming a "de facto" standard for object oriented environments and then it is perceived as a part of the culture of software engineers. However, the luck of rigorous semantic basis is a great shortcoming of UML which upsets its use to address safety problems. Still, taking benefit of the richness of UML to model critical systems is worthwhile because it is aimed at reducing the certification effort required for the design phase. Currently, we undertake works to delimit a well-defined UML core which could be used to model such systems. In this paper, we explore some critical aspects and recommendations of basic UML constructs which should be approached when dealing with safety.