6.1 Heinrich's Domino Theory
Key Takeaways
- Heinrich's Domino Theory describes accidents as a chain of five sequential factors: ancestry/social environment, fault of the person, unsafe act or condition, accident, and injury.
- Classic figures taught in Basic HSE attribute roughly 88% of accidents to unsafe acts, about 10% to unsafe conditions, and about 2% to unpreventable acts of nature.
- Removing any single domino stops the chain—so practical prevention focuses on unsafe acts and unsafe conditions before injury occurs.
- Modern systems thinking complements Heinrich rather than erasing it: Level 1 exams still expect the five dominos and the 88/10/2 split.
- On Nigerian sites, identifying and correcting the third domino (unsafe act or condition) is the most actionable daily control for supervisors and workers.
6.1 Heinrich's Domino Theory
Domain 3 of the ISPON HSE Level 1 (Basic HSE) syllabus—Workplace Safety & Accident Prevention—opens with classic accident causation models. The model you must know first is Heinrich's Domino Theory. Herbert W. Heinrich, an industrial safety pioneer working from large insurance claim datasets in the early twentieth century, argued that occupational injuries are not random bad luck. They result from a chain of factors that topple one after another—like a line of standing dominos. If you understand the chain, you can interrupt it before anyone is hurt.
For Level 1, you need the five dominos in order, the classic percentage figures for causes, the prevention implication (remove one domino), and enough awareness of modern critiques to avoid treating Heinrich as the only possible view of safety—while still answering exam questions correctly.
Why Causation Models Matter
Without a model, people default to weak explanations: "It was God's will," "He was careless," or "Accidents just happen." Causation models push investigators and supervisors to look for identifiable factors that can be controlled. On a Nigerian construction site in Lagos, a Port Harcourt fabrication yard, or a manufacturing plant in Ogun State, that shift in thinking decides whether the organisation only pays compensation after injury—or prevents the next event.
Heinrich's model is simple, memorable, and still taught worldwide in entry-level HSE courses. Later models (such as the Swiss Cheese Model in the next section) add system layers; they do not cancel the need to know the five dominos for Basic HSE.
The Five Dominos
Heinrich described industrial accidents as a sequence of five factors. Each factor is a "domino." When one falls, it knocks over the next.
| Order | Domino (Classic Label) | What It Means at Level 1 |
|---|---|---|
| 1 | Ancestry and social environment | Background influences that shape habits, attitudes, and skills—family, education, community norms, earlier work culture |
| 2 | Fault of the person | Personal traits or failings that predispose someone to error—carelessness, recklessness, ignorance, bad temper, disregard of instructions |
| 3 | Unsafe act and/or mechanical or physical hazard | The immediate cause: what someone did wrong, or the unsafe condition that existed, just before the event |
| 4 | Accident | The unwanted event itself—fall, struck-by, contact with energy, release, collision |
| 5 | Injury | The harm outcome—cut, fracture, burn, fatality, or (in broader HSE use) damage or ill health |
Domino 1 — Ancestry and Social Environment
This first domino is the deepest background. Heinrich linked injury risk partly to the social world that formed the worker: upbringing, peer habits, literacy, and attitudes toward rules and authority. At Basic HSE depth, do not over-psychologise this step. The exam point is simply that personal history and environment influence behaviour—for example, a culture that mocks PPE use or treats "strong men" as people who ignore height protection.
In Nigerian workplaces, social environment can include pressure to "manage yourself" without complaining, fear of job loss if production slows, or informal-sector habits carried into formal sites. These influences do not excuse unsafe behaviour, but they help explain why training and leadership must reshape norms, not only punish individuals.
Domino 2 — Fault of the Person
The second domino is the personal factor: traits or states that make unsafe behaviour more likely. Examples taught at awareness level include:
- Willful disregard of rules
- Lack of knowledge or skill for the task
- Distraction, fatigue, or impairment
- Overconfidence ("I have done this a hundred times")
- Poor attitude toward safety instructions
Important exam nuance: "Fault of the person" is Heinrich's historical wording. Modern HSE still studies human factors, but mature systems also ask why the organisation allowed a poorly trained or fatigued person into a high-risk task. For Level 1, learn the classic label; for professional practice later, combine it with system thinking.
Domino 3 — Unsafe Act and/or Unsafe Condition
This is the critical immediate cause domino—the one supervisors and workers can usually see and fix today.
- Unsafe act — something a person does or fails to do that creates danger (removing a guard, standing under a suspended load, hot work without a permit, bypassing lock-out)
- Unsafe condition — a physical or environmental state that creates danger (oil on the floor, unguarded opening, damaged ladder, missing edge protection, defective electrical cable)
Heinrich's teaching emphasises that most accidents have an unsafe act or unsafe condition (or both) immediately before the event. Later chapters and Section 4.3 of this guide expand the distinction between acts and conditions; here, the point is that Domino 3 is where the chain becomes operationally preventable.
Domino 4 — Accident
The accident is the event: the body meets the energy or the process fails. Examples: a worker falls through a floor opening; a vehicle strikes a pedestrian; a wrench drops from height; a flash fire occurs during line-breaking.
Not every accident produces injury (near misses and property-damage-only events still matter), but in Heinrich's classic injury sequence the accident is the fourth domino that leads to harm.
Domino 5 — Injury
Injury is the final domino: the human harm (and, by extension in modern HSE talk, related loss). Once Domino 5 falls, the organisation is in emergency response, medical care, investigation, compensation, and learning mode. Prevention aims to stop the chain before this stage.
The Classic 88% / 10% / 2% Figures
Basic HSE courses worldwide—and ISPON Level 1 outlines—still teach Heinrich's approximate split of accident causes:
| Cause Category | Approximate Share (Classic Heinrich) | Level 1 Meaning |
|---|---|---|
| Unsafe acts | ~88% | Most accidents are linked primarily to what people did or failed to do |
| Unsafe conditions | ~10% | A smaller share are driven mainly by physical/mechanical hazards |
| Unpreventable / acts of nature | ~2% | A very small share are considered practically unavoidable (for example, certain natural events) |
Memorise these figures for the exam. Questions may ask which category accounts for the large majority of accidents, or whether most injuries are "unpreventable." The correct Basic HSE answer is that the large majority relate to unsafe acts, with a minority mainly from unsafe conditions, and only a tiny fraction treated as unpreventable.
How to Use the Percentages Wisely
These numbers are teaching tools, not precision science for every Nigerian site in 2026. Different industries, data definitions, and modern taxonomies produce different statistics. For Level 1:
- Use 88/10/2 when the question tests Heinrich's classic teaching
- Still control both acts and conditions—10% of events is still a huge number of injuries across an industry
- Never use the 88% figure as an excuse to blame workers only while ignoring missing guards, poor design, or production pressure
The Prevention Principle: Remove One Domino
The most powerful implication of Domino Theory is simple:
If any one domino is removed, the chain stops and the injury does not occur.
You cannot easily rewrite someone's entire social background on a Monday morning toolbox talk. You can:
- Train and supervise to reduce personal factors that lead to error (influence Domino 2)
- Eliminate unsafe acts through coaching, clear procedures, and fair enforcement (Domino 3)
- Eliminate unsafe conditions through inspection, maintenance, and engineering (Domino 3)
- Separate people from energy so that even if an event starts, injury is less likely (intervening before Domino 5)
Heinrich's practical message for supervisors was to focus heavily on Domino 3—unsafe acts and unsafe conditions—because that is where daily control is strongest. Modern hierarchy of controls (taught in Chapter 5) fits this idea: fix the workplace and the system so Domino 3 never stands ready to fall.
| Domino Targeted | Example Control on a Nigerian Site |
|---|---|
| Social / cultural influences | Leadership that praises reporting and PPE use; no mockery of safe behaviour |
| Personal factors | Competence checks; fitness for work; fatigue management; clear instructions |
| Unsafe act | Stop-work when someone climbs without fall protection; enforce permit rules |
| Unsafe condition | Repair oil leak, reinstall guard, close floor opening, replace damaged sling |
| Accident → injury path | Soft landings, barriers, exclusion zones, emergency readiness if event still occurs |
Critiques at Awareness Level (Do Not Discard Heinrich for the Exam)
Modern safety science adds important corrections. Know them at awareness level:
- Over-focus on worker "fault" — Classic wording can encourage blame culture if managers stop at Domino 2 and ignore management system failures.
- Linear chain vs complex systems — Real incidents often involve multiple concurrent failures (procedures, design, supervision, commercial pressure), not one neat line of five stones.
- Latent conditions — Problems created long before the day of the accident (weak maintenance budgets, unclear roles, poor contractor control) may matter as much as the final unsafe act.
- Near misses and high-potential events — Modern practice values learning from events that did not complete Domino 5.
Exam stance for Level 1: These critiques complement Heinrich; they do not erase the syllabus need for the five dominos and 88/10/2. The Swiss Cheese Model (next section) is the natural partner model that emphasises system barriers. Good candidates can explain Heinrich accurately and recognise that systems thinking deepens prevention.
Nigerian Site Scenario — Applying the Five Dominos
Scenario: On a multi-storey building site in Abuja, a formwork carpenter falls from an incomplete platform and breaks his leg.
| Domino | Application to the Scenario |
|---|---|
| 1 Ancestry / social environment | Peer culture on the gang treated harness use as "slow"; new workers copied the habit |
| 2 Fault of the person | The carpenter overestimated experience and chose speed over clipping on |
| 3 Unsafe act / condition | Unsafe act: working at height without fall protection. Unsafe condition: incomplete platform with missing edge boards |
| 4 Accident | Fall from height onto lower deck |
| 5 Injury | Fractured leg; lost-time injury; medical evacuation |
Where to break the chain (practical prevention):
- Complete edge protection and platforms before access (remove unsafe condition)
- Enforce harness and anchor points with supervision (remove unsafe act)
- Induct and coach against "hero" culture (influence earlier dominos)
- Stop-work authority for incomplete access (system support)
If only Domino 3 had been controlled—safe platform or proper fall protection—the fall and injury would likely have been prevented even if personal overconfidence remained. That is Domino Theory in action.
Linking Domino Theory to Daily Level 1 Behaviours
A Basic HSE worker uses Heinrich when they:
- Spot an unsafe condition and report or correct it immediately
- Challenge or stop an unsafe act (standing under a load, smoking near flammables, removing a machine guard)
- Refuse the myth that "almost all accidents are unpreventable"
- Support investigation that looks beyond "who to blame" toward what act or condition completed the chain
- Accept that training, attitude, and culture matter—but still prioritise fixing the immediate cause
Exam Focus
Expect questions that ask you to list or order the five dominos, identify which domino is the immediate cause, recall ~88% unsafe acts / ~10% unsafe conditions / ~2% unpreventable, and apply the rule that removing one domino prevents the injury. Scenario items often place the answer at Domino 3. If a question mentions system barriers and latent failures, you may be moving toward Swiss Cheese—but do not abandon Heinrich's sequence when that is what the stem requires.
In Heinrich's Domino Theory, which sequence correctly orders the five factors from first to last?
According to the classic Heinrich figures taught in Basic HSE, approximately what share of accidents is attributed primarily to unsafe acts?
A supervisor on a Warri fabrication yard finds a missing machine guard and has it reinstalled before the next shift. Which Domino Theory principle does this best illustrate?