A Guide to Understanding the Risk Assessment Matrix
Key concepts, methodology and practical use in occupational health and safety.
How does a 5x5 matrix work?
Occupational health and safety most commonly uses a 5x5 matrix that combines the two main components of risk:
- Likelihood (1-5): How likely it is that the hazardous event will occur (from "extremely rare" to "very frequent").
- Severity (1-5): How serious the injury or damage would be if the event occurs (from "negligible" to "fatal / catastrophic").
Multiplying these two factors (L x S) gives you the risk level (1-25), which tells you how urgently you need to act.
Risk levels and required actions
Depending on the resulting score, risks are usually divided into several categories:
- Low risk (1-4): Acceptable risk. No additional measures are needed, but the risk should be monitored.
- Medium risk (5-12): Risk reduction measures need to be planned and implemented within a reasonable time frame.
- High risk (15-25): Unacceptable risk! Work must stop immediately until measures to reduce the risk are in place.
The hierarchy of controls
When you identify a high risk, apply measures to reduce it in the following order: 1. Elimination of the hazard (most effective), 2. Substitution (replacing something hazardous with a safer alternative), 3. Engineering controls (guards and safety devices), 4. Administrative controls (worker rotation, procedures) and finally 5. Personal protective equipment (PPE) as the last line of defence.