Fundamentally risk is subjective; it is not possible to define what level of risk is acceptable dispassionately and objectively. Any two risk scenarios are inherently different from one another due to people’s inherent understanding and acceptance of different types of risk, their emotions, memories, hopes and fears.
Factors that affect risk perception include the following:
- Degree of control.
- Familiarity with the hazard.
- Direct benefit.
- Personal impact,
- Natural vs. man-made risks,
- Recency of events,
- Effects of the consequence term, and
- Comprehension time.
In spite of these difficulties, it is still necessary to have a clear understanding as to what levels of risk are acceptable. After all, if a facility operates for long enough, it is certain - statistically speaking - that there will be an accident. Therefore, process safety professionals need guidance as to what level of "acceptable safety" is required. This is tricky. Regulatory agencies in particular will never place a numerical value on human life and suffering because any number that they propose would inevitably generate controversy. Yet working targets have to be provided, otherwise the facility personnel do not know what they are shooting for. Nor can a regulatory body, a professional society or the author of a book such as this can provide an objective value for risk.
Individuals and organizations are constantly gauging the level of risk that they face in their personal and work lives, and then acting on their assessment of that risk. For example, at a personal level, an individual has to make a judgment as to whether it is safe or not to cross a busy road. And there is the further complication of the subjectivity of risk. Someone who is strongly opposed to having a chemical plant near their home may happily choose to go bungee jumping at weekends.
In an industrial context managers make risk-based decisions regarding issues such as whether to shut down an equipment item for maintenance or to keep it running for another week. Other risk-based decisions made by managers are whether or not an operator needs additional training, whether to install an additional safety shower in a hazardous area, and whether a full Hazard and Operability Analysis (HAZOP) is needed to review a proposed change. Engineering standards, and other professional documents, can provide guidance. But, at the end of the day, the manager has a risk-based decision to make. That decision implies that some estimate of "acceptable risk" has been made.
One company provided the criteria shown in the Table for its design personnel.
|Fatalities per year (employees and contractors)
|>5 x 10-4
|<5 x 10-4 and >1 x 10-6
|Broadly tolerable risk
|<1 x 10-6
Their instructions were that risk must never be in the 'intolerable' range. High risk scenarios are "tolerable", but every effort must be made to reduce the risk level, i.e.,to the "broadly tolerable" level.
Please move to ALARP and Acceptable Risk to learn more.