2.2 Risk Assessment Frameworks & Risk Matrix Calculations

Risk Assessment Frameworks & Risk Matrix Calculations

The Core Concept of Risk

While hazard recognition identifies the potential for harm, risk assessment evaluates the likelihood and severity of that harm actually occurring. In occupational safety, the fundamental formula for risk is:

Risk = Probability (or Likelihood) × Severity (or Consequence)

An OHST must be able to differentiate between hazard and risk. For example, a bottle of concentrated sulfuric acid is a hazard. The risk associated with that hazard is very low if the bottle is securely stored in an approved, locked cabinet. However, the risk becomes very high if the bottle is open on a busy workbench where it could easily be knocked over.

The Risk Assessment Matrix

A Risk Assessment Matrix is a visual tool used to evaluate and categorize risks based on their probability and severity. It allows organizations to prioritize hazards, ensuring that the most critical risks receive immediate attention and resources.

Defining Severity

Severity refers to the worst credible consequence should an incident occur. While definitions vary by organization, a standard classification system (often modeled after military standards like MIL-STD-882E) includes:

1. Catastrophic: Death, permanent total disability, significant environmental damage, or immense financial loss.
2. Critical: Permanent partial disability, severe injury or occupational illness, major system damage.
3. Marginal: Minor injury or illness resulting in days away from work, minor system damage.
4. Negligible: First aid injury or illness, minimal environmental or system impact.

Defining Probability

Probability assesses the likelihood that the hazard will result in an incident. Common classifications include:

• A - Frequent: Likely to occur continuously or frequently during the life cycle of the system/process.
• B - Probable: Will occur several times.
• C - Occasional: Likely to occur sometime.
• D - Remote: Unlikely, but possible to occur.
• E - Improbable: So unlikely, it can be assumed occurrence may not be experienced.

Constructing the Risk Assessment Code (RAC)

By combining the severity category and the probability level, a Risk Assessment Code (RAC) is generated. The RAC dictates the required level of management action.

Interpretation of RAC Levels

• High Risk (Unacceptable): Immediate action required. The operation must be halted until controls are implemented to reduce the risk to an acceptable level. Requires senior management visibility.
• Medium Risk (Tolerable with Controls): Action required to implement controls. The operation may proceed, but strict monitoring and administrative controls must be in place. Requires mid-level management review.
• Low Risk (Acceptable): No immediate significant action required, though continuous improvement efforts should seek to eliminate the hazard if possible. Managed by routine procedures.

Initial Risk vs. Residual Risk

A critical concept in risk assessment is the difference between initial and residual risk:

• Initial Risk (or Baseline Risk): The level of risk assessed before any new controls or mitigation measures are applied. It represents the inherent risk of the activity.
• Residual Risk: The level of risk remaining after all planned controls and mitigation measures have been implemented and verified.

The goal of the OHST and the safety program is to drive the residual risk down to an acceptable level (typically Medium or Low, depending on the organization's risk tolerance). If the residual risk remains High after all feasible controls are applied, the activity may need to be fundamentally redesigned or abandoned entirely.

Quantitative vs. Qualitative Risk Assessment

Risk matrices are generally considered semi-quantitative or qualitative. They rely on categorical data and expert judgment.

Quantitative Risk Assessment (QRA) uses complex mathematical models, failure rate databases, and statistical analysis to calculate numerical probabilities (e.g., a $1 \times 10^{-6}$ chance of fatality per year). QRAs are heavily used in high-hazard industries like nuclear power and chemical processing (e.g., Process Safety Management). While an OHST may assist in gathering data for a QRA, they are more frequently tasked with conducting standard qualitative matrix assessments for daily operational tasks.

The ALARP Principle

Many regulatory frameworks and international standards utilize the concept of ALARP—As Low As Reasonably Practicable. This principle acknowledges that zero risk is often unachievable. Instead, risks must be reduced to a level where the cost (in terms of time, money, or effort) of further reduction is grossly disproportionate to the safety benefit gained. Calculating ALARP requires a careful balancing act and a thorough understanding of both the risk matrix and the available control technologies.