This theory implies that the accident is the result of a single root cause. The domino theory is a very simple process, considering only a single chain of events. This may be considered a
“Accident Prevention is both science and art. It represents, above all other things, control – control of man performance, machine performance, and physical environment. The word ‘control’ is used advisedly because it connotes prevention as well as correction of unsafe conditions and circumstances.” (Heinrich and Granniss, 1959, 4)
In the mid-1920s, a series of theorems regarding accidental injury were developed (Heinrich and Granniss, 1959, 4). These theorems established that:
- Industrial injuries result only from accidents.
- Accidents are caused directly only by the unsafe acts of persons or exposure to unsafe mechanical conditions.
- Unsafe actions and conditions are caused only by faults of persons.
- Faults of persons are created by environment or acquired by inheritance.
Heinrich chronologically reversed the order of these theorems so that the injury is the last to occur. This created the ‘domino sequence’, which he presented to the Down River Section of the Detroit Safety Council (Heinrich and Granniss, 1959, 14).
The domino sequence explained
The domino sequence is an event-based or sequential accident causation model. It is used to explain a chain of events that occur in a specific order to result in an injury.
Heinrich’s five key factors leading to an injury are:
- Social environment
- Fault of the person
- Unsafe acts or conditions
- Accident
- Injury
An unexpected event initiates a sequence that leads to an accident, which then results in injury to a person. The injury is invariably caused by an accident, and the accident is always the result of the factor that immediately precedes it (Heinrich and Granniss, 1959, 14). Therefore, to prevent an accident, safety measures must target the central factor: unsafe acts or conditions.
As the name suggests, the best way to understand how these factors interact is to visualize them as a set of dominos. Like a physical set, the fall of the first domino triggers the fall of the entire row. By preventing the unsafe act, the chain is broken, the accident is avoided, and no injury occurs.
By preventing the ‘unsafe act’ from taking place an accident has been avoided and no injury has occurred.
Applying the model: A practical example
A major disadvantage of this theory is the implication that an accident is the result of a single root cause. The domino theory is a highly simplified process that considers only a single chain of events. In reality, an accident is usually caused by a range of interacting factors that do not necessarily happen in a fixed, logical order.
Sequential models struggle to explain accident causation in complex OHS systems (Qureshi, 2008). Because they trace back to a single root cause, they are often used simply to assign blame.
“Typically, in these types of (sequential) models, causal factors in an accident which are not linked to technical component failures were classified as human error as a kind of catchall or garbage can” (Hollnagel, 2001).
To illustrate this, consider a simple incident analyzed using the domino sequence.
Incident: An employee in the receiving area of a retail warehouse store was told by his supervisor to quickly move a stack of wooden pallets to the outside loading area because a collection truck was arriving soon.
| Sequence Factor | Incident Details |
| Social environment | Request by supervisor to action task ‘quickly’. |
| Fault of person | Did not use provided PPE (riggers gloves). |
| Unsafe act or condition | Moved pallet using bare hands and ignored safe working practice. |
| Accident | Sustained grazes to hands and dropped the pallet. |
| Injury | Broken toes on right foot due to the dropped pallet. |
Looking at the incident in this fixed logical order makes it difficult not to assign blame. The fault lands either on the supervisor for issuing an unsafe direction, or on the employee for ignoring safe working practices and equipment.
Limitations and the blame game
When you consider each factor in the example, it is clear they all played a part. The root cause is not as simple as the first step in the process. By removing any of the three initial factors, this specific injury could not have occurred. As Heinrich explained, “if the series is interrupted by the elimination of even one of the several factors that compromise it, the injury cannot possibly occur” (Heinrich and Granniss, 1959, 16).
Using the domino sequence quickly points to human failure (unsafe acts) as the heart of the problem, suggesting controls must be directed there (Heinrich and Granniss, 1959, 4).
However, this highlights the model’s limited scope. It only identifies the seemingly clear and direct causes, like the employee using bare hands. Heinrich did not claim that removing this single factor would eliminate all workplace accidents. Instead, he argued that prevention must first address the immediate conditions and circumstances that create similar accidents (Heinrich and Granniss, 1959, 4).
The primary advantage of the domino model is speed. It provides a quick, simple framework to determine immediate reasons for an accident.
The primary disadvantage is its failure to effectively analyze environmental or systemic factors external to the unsafe act. Using this model for all incident reviews will not uncover the true, underlying reasons why those unsafe acts occurred in the first place.
As Dr. Andrew Hopkins notes in Lessons from Longford (Hopkins, 2000): “Operators will inevitably make errors sooner than later, if it is possible for them to do so. From a prevention point of view it is better to focus on factors further back along the causal chains which put operators in a position where it is possible for them to make critical errors.”
This reality heavily outweighs the benefits of relying solely on the domino theory for modern accident prevention.
References
Qureshi, Z.H. 2008. A Review of Accident Modelling Approaches for Complex Critical Sociotechnical Systems, DSTO-TR-2094, Defence Science and Technology Organisation. http://www.dsto.defence.gov.au (accessed September 2, 2010).
Heinrich, H.W., and E.R. Granniss. 1959. Industrial Accident Prevention. United States of America: McGraw-Hill Book Company Inc.
Hollnagel, E. 2001. Anticipating Failures: What should Predications be About? In The Human Factor in Systems Reliability – Is Human Performance Predictable? RTO Meeting Proceedings 32, RTO-MP-32, January, Research and Technology Organisation, North Atlantic Treaty Organisation, Cedex, France: RTO/NATO.
Hopkins, A. 2000. Lessons from Longford: The Esso Gas Plan Explosion. Australia: CCH Australia Limited.
