What is the first action a nurse should take when responding to a disaster or mass casualty event?

Mass casualty events overwhelm normal resources and require triage — sorting clients to allocate limited care effectively. The START (Simple Triage and Rapid Treatment) system is widely used: clients are categorized in less than 60 seconds each into four groups identified by colored tags: Red (Immediate) — life-threatening injuries requiring immediate care to survive (severe bleeding, airway compromise); Yellow (Delayed) — serious injuries but stable, can wait for care; Green (Minor) — walking wounded, minor injuries; Black (Deceased/Expectant) — dead or so severely injured that survival is unlikely given resource constraints. The goal is greatest good for the greatest number. This contrasts with day-to-day triage where the most severely injured client receives the most aggressive care. Mass casualty triage requires accepting that some clients who might survive with full resources may be tagged black or yellow because resources cannot be diverted. Repeat triage occurs as conditions change.