In this study we take advantage of the Mixed Reality (MR) ability to effectively facilitate the visualization and assessment of potential hazards in authentic environments, thereby empowering workers to efficiently discern and mitigate risks. The study discusses the design, development and testing of an MR application that enhances worker safety training by creating immersive environments, visualizing real-time safety information, and connecting to IoT sensors and wearables. The developed MR application was integrated to an existing IoT infrastructure of a large construction site, to result in a safer working environment, increased worker awareness, improved training efficiency and better safety standards compliance. The main functionalities of the MR application are: i) enhanced visualizations for better understanding of potential dangers and safety procedures, leading to improved situational awareness; ii) real-time data monitoring for proactive monitoring, early hazard detection, and timely interventions; iii) documentation and reporting for simplifying incident reporting and safety inspections; and iv) collection of safety performance analytics for gaining insights on risk factors and improving overall safety.