A critical injury is one serious enough to place a person’s life in jeopardy, cause unconsciousness, or result in a major permanent change to the body such as the loss of a limb or eyesight. The term carries specific meaning in both medical and legal contexts, and the exact definition matters because it triggers different reporting requirements, treatment protocols, and legal obligations depending on the setting.
The Legal Definition
In workplace safety law, “critical injury” has a precise definition. Ontario’s Regulation 834, one of the most widely referenced standards, defines a critical injury as any injury of a serious nature that:
- Places life in jeopardy
- Produces unconsciousness
- Results in substantial loss of blood
- Involves the fracture of a leg or arm (but not a finger or toe)
- Involves the amputation of a leg, arm, hand, or foot (but not a finger or toe)
- Consists of burns to a major portion of the body
- Causes the loss of sight in an eye
The distinction between, say, a broken arm and a broken finger is intentional. Regulators draw the line at injuries that fundamentally affect a person’s ability to function or that signal a dangerous workplace condition requiring investigation. Notably, the Ontario Ministry of Labour interprets “fracture of an arm” broadly enough to include fractures of the wrist and hand bones, not just the forearm or upper arm.
How Hospitals Measure Severity
Emergency medicine uses numerical scoring systems to classify how badly someone is hurt. The most common is the Injury Severity Score (ISS), which runs from 0 to 75. An ISS below 9 is considered mild, 9 to 15 is moderate, 16 to 24 is severe, and 25 or above is classified as profound. If any single injury is rated unsurvivable, the score automatically jumps to 75. Major trauma, the threshold where outcomes worsen significantly, generally starts at an ISS above 15.
For brain injuries specifically, doctors use the Glasgow Coma Scale (GCS), which rates a patient’s ability to open their eyes, speak, and move on command. Scores range from 3 to 15. A score of 3 to 8 indicates a severe injury and is often used to define coma. Patients scoring 8 or below typically require surgical placement of a device to monitor pressure inside the skull.
These two scales work together. The ISS captures overall body trauma while the GCS zeroes in on brain function, and both factor into decisions about where patients are transferred and how aggressively they’re treated.
What Triggers a Trauma Team Activation
When a critically injured patient arrives at a hospital, specific physiological signs determine whether a full trauma team is called. Updated European guidelines published in 2025 lay out clear thresholds: blood oxygen levels below 90%, breathing rate below 10 or above 29 breaths per minute, systolic blood pressure below 90, heart rate above 120, or a GCS score of 12 or lower. Body temperature dropping below 35°C (95°F) after trauma also qualifies.
Certain injury patterns automatically activate the team regardless of vital signs. These include a flail chest (where multiple ribs break in a way that destabilizes the chest wall), mechanically unstable pelvic fractures, penetrating wounds to the torso or neck, traumatic amputation above the wrist or ankle, and spinal cord injuries causing loss of sensation or movement. Burns covering more than 20% of the body at deep partial thickness or worse also meet the threshold, as does uncontrolled hemorrhaging that requires blood products to keep blood pressure stable.
The same guidelines recommend a lower threshold for activating trauma teams for older patients, whose bodies tolerate severe injury less well and whose vital signs can appear deceptively normal even when they’re in serious trouble.
How Triage Works in Mass Casualties
In disasters or large-scale accidents, first responders use a color-coded triage system to sort patients quickly. Red tags mark the most critical survivors: anyone breathing faster than 30 times per minute, who has no pulse at the wrist (or whose fingertip color takes more than 2 seconds to return after being pressed), or who can’t follow simple commands. These patients get treated first. Black tags are reserved for people who are not breathing even after the airway is opened, meaning resuscitation is unlikely to succeed given available resources. The mnemonic responders use to remember the red-tag cutoffs is “RPM: 30-2-can do.”
Survival Rates for the Most Severe Injuries
For the most critically injured patients, those with ISS scores between 50 and 75 (the highest possible range), survival sits at roughly 48%. A 20-year study ending in 2021 found mortality for this group was 51.7%, and that number didn’t improve significantly over the two decades despite advances in trauma care. Mortality fluctuated between 46% and 60% depending on the year. This is specifically for polytrauma patients, meaning people with severe injuries to multiple body regions simultaneously.
For patients in the severe but not extreme range (ISS 16 to 24), survival rates are considerably better, which is why rapid triage and getting to the right hospital matters so much. The difference between a Level I trauma center with full surgical capability and a smaller facility can be the deciding factor for patients on the edge.
Workplace Reporting Requirements
When a critical injury happens at work, employers face strict reporting timelines. In the United States, OSHA requires that a workplace fatality be reported within 8 hours. An in-patient hospitalization, amputation, or loss of an eye must be reported within 24 hours. The fatality reporting window applies if the death occurs within 30 days of the work-related incident, while the 24-hour window for hospitalization, amputation, or eye loss only applies if the event happens within 24 hours of the incident itself.
These timelines exist so regulators can investigate the scene before conditions change. In Canadian provinces with critical injury definitions like Ontario’s, similar rules require preserving the scene and notifying the relevant ministry immediately by phone, not just filing paperwork. Failing to report on time can result in significant fines for the employer, regardless of whether the employer was at fault for the injury itself.