The measurement properties of indices about workplace aggression initiated by the public, referred to as external workplace aggression, are under-researched. This study addressed how exposure to external workplace aggression is best measured and modeled in three types of emergency responders. The study inspected the factor structure and explored the addition of severity to an existing measure of frequency of exposure to workplace aggression, which addresses forms of physical aggression, threats, and verbal and nonverbal/nonphysical aggression (gestures) by people outside the organization directed toward employees. Self-reported data from 1,499 emergency responders, including emergency medical workers, firefighters, and police officers in the Netherlands, were analyzed using factor analyses in Mplus. In addition, the relationships between workplace aggression indices and a measure of the situational risk for violence were tested. Results show that the frequency index measured exposure to external workplace aggression better than the index combining the frequency and severity, and that factor structures of indices differed, regarding number and content of factors, between the three groups of emergency responders. An important implication is that researchers and policy employees can use a relatively simple measure to examine exposure to aggression in organizations.