In FOI projects related to human vulnerability, the focus is often on determining expected injuries or other effects on humans in various types of incidents. There are, within FOI, existing models available to both determine the precise injuries and predict outcomes and severity for single or multiple injuries. In many cases, more than one injury occurs per person, and each injury contributes to the person's overall condition. This creates a need to aggregate multiple injuries to reflect the total severity. This report aims to describe the challenges of aggregating multiple injuries and propose measures to improve FOI's models for this purpose. The report outlines the most commonly used methods for aggregating injuries and assessing their combined severity. The strengths and weaknesses of these models are discussed, and within this context, shortcomings in FOI's existing models are identified, and suggested improvements are discussed.