C is for causality

As Fred walks to his work area on the construction site, his phone rings. The site rule is that mobile phones should be used in the mess room only, but he looks to see who is calling, and his foot catches the edge of building materials left on the path. He falls, injuring himself. What is the cause of the accident?


Words: Bridget Leathley

In 1931, US industrial safety pioneer Herbert W Heinrich suggested that “an injury invariably results from a complicated sequence of factors, the last one of which being the accident itself”. Heinrich presented his idea as a line of dominos – the final piece, representing the injury, fell because the previous one, tripping on the building material, did too. The trip occurred because the injured person created an unsafe condition – that of not paying attention to his environment – and they did this because of their “ancestry and social environment”. In our example, Fred is part of the generation that finds it hard not to respond to their phones, and the work environment around him has not tempered this impulse.

In the 1980s Frank Bird, inventor of the safety triangle, updated the model to place lack of management control as the first domino in the sequence – for example, lack of control over building materials storage. Other theories suggested that a single row of dominos couldn’t represent the multiple contributing factors in an accident.

By the end of the 1990s, psychologist James Reason’s Swiss cheese model (SCM) had become popular, showing an accident trajectory with a sequence of barriers which could prevent an accident but which, due to latent conditions, allow an accident to happen. This is represented by slices of cheese (the barriers) each with holes representing failings. A “hole” in each barrier allowed the accident to occur, and fixing any one of them would have prevented the accident. For convenience, in analysing such accidents we then say there were multiple causes, usually categorised as immediate (tripping), underlying (obstacle on path, not paying attention) and root causes (the failure of supervision and management on the worksite which should have prevented the obstacle and discouraged Fred’s non-conformant behaviour).

Though multiple causation has become a convenient way of thinking about accidents, an opposing school of thought is summarised by the writings of Paul Difford, research director at the Institute of Industrial Accident Investigators, stating an accident has only one cause – in this case, Fred being distracted. Twenty people had walked down the same path and had managed to avoid the obstacle, hence the one cause is the distraction. It may be technically correct to call something a “cause” if it is chronologically the final step that led to the injury but, arguably, the single-cause approach is of limited use. People will be distracted, they will make mistakes, they will forget things, so an employer has to create a working environment that is as safe, and as forgiving of errors and violations, as reasonably practicable.

In civil cases, the claimant who has been hurt at work has to prove that a negligent action of the organisation (or of an employee) caused the injury. Asbestos cases show how complicated this can be. In the case of multiple employers that have exposed a worker to asbestos, a ruling in 2000 determined that damages should be split between all of them, proportioned according to the length of time the claimant worked for each employer. But later cases made this division of causality less certain. Some cases resorted to damages being awarded against a single employer that could be shown to have made a material contribution to the risk.

In criminal cases there is no need to prove that an act or omission caused an injury. The Health and Safety at Work Act requires employers to ensure, so far as is reasonably practicable, the health, safety and welfare at work of all their employees. The law does not directly trouble itself with whether the employer caused damage to health, safety or welfare, just whether the employer could have “ensured” those three standards.

It is a legal requirement to provide a safe place of work and a safe system of work, so though the single-cause school might argue that neither the obstacle on the path in our earlier example nor poor site management were the causes of the accident, the breach of the requirement to provide a safe workplace would be demonstrable through the accident occurring. If the final link in the chain was the employee being distracted by a phone call, this does not remove the liability of the employer for permitting obstacles on the path. The law applies the “but for” test – but for the obstacles, Fred would not have fallen over.

For safety and health professionals, it is constructive to have a theory that looks at any factors an organisation can influence to reduce risk, which means considering the whole web of immediate, underlying and root causes. 



Bridget Leathley is a freelance health and safety consultant, providing risk management support in facilities, retail and office environments.  She delivers face-to-face safety training including IOSH and bespoke courses, and contributes to e-learning courses through evaluations and design work.  She has been writing for health and safety publications since 1996.  

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