L is for latent

The word “latent” is an adjective describing something that lies dormant or hidden until the circumstances are suitable for it to manifest itself. The noun “latency” refers to either the state of having something dormant or the period of time before the manifestation is likely.


The Health and Safety Executive (HSE) uses the adjective to describe failures made by “designers, decision makers and managers” and contrasts latent failures with active failures, the errors made by “those with hands-on control of the system”. The HSE explains that the timescales are different – active failures occur immediately before the accident and are therefore synonymous with “immediate cause” while latent failures will be dormant for some time. HSE references equate them as both “root causes” and “underlying causes”.

The HSE says: “Latent failures provide as great, if not a greater, potential danger to health and safety as active failures. Latent failures are usually hidden within an organisation until they are triggered by an event likely to have serious consequences.”

The idea of latent failures was made popular in James Reason’s Swiss cheese model. This suggests that, given we know that active failures will occur, systems are built with multiple layers of defence. These safeguards include procedural and administrative controls, as well as engineering controls such as monitors, alarms and physical barriers. If all the safeguards were perfect, all active failures would be caught, and accidents prevented. However, because of latent failures or latent errors there are holes in each layer of safeguarding. When the holes in each barrier line up, an active failure will cause an accident.

In 1990, Reason’s solution to reduce accidents was to “find ways of identifying and neutralising these latent failures before they combine with local triggering events to breach the system’s defences”. Using examples of the MS Herald of Free Enterprise ferry disaster in the Belgian port of Zeebrugge and the King’s Cross tube station fire in London, both in 1987, Reason concluded that the tragedies could have been prevented by fixing the latent failures of “poor design, conflicting goals, defective organisation and bad management decisions”.

The Swiss cheese model has been widely used, but also widely criticised for its overuse and over-application. The visual image and the language of underlying and root causes imply that latent errors are obvious, and if only you look hard enough, you can find them and fill them in. Some critics also complain that the model places too much emphasis on looking for earlier failures by managers and supervisors, as if any decision implicated in an accident must be culpable.

Latent failures provide as great, if not greater, potential danger to health and safety as active failures


In later papers, Reason switches to using the term “latent condition”. In Revisiting the “Swiss Cheese” Model of Accidents, a 2006 paper for Eurocontrol, written by Reason with Erik Hollnagel, the authors (bit.ly/2HvZ59B) explain that latent conditions cannot always be identified and fixed: “All top-level decisions seed pathogens into the system, and they need not be mistaken… The existence of latent conditions is a universal in all organisations, regardless of their accident record.” In other words, a decision was not necessarily wrong at the time it was taken, but it leaves a latent condition that, should other circumstances occur, would result in an accident. The key to accident prevention is to better understand the latent conditions – both with a view to designing them out when practical, and planning around them when not. In an accident investigation, the concept is useful in understanding the context in which an individual took an action that resulted in the incident.

Latent health conditions

Latent is also used to refer to health conditions that do not appear the first time someone is exposed to a health hazard. Asbestos can have latency periods of up to 50 years. Although further use of blue and brown asbestos was banned in the UK in 1985 – white following in 1999 – deaths from mesothelioma have continued to rise because, once exposed, little can be done to prevent the disease occurring eventually. Notably, mesothelioma deaths are continuing to increase in people over 75 – who in general will have been retired for ten years or longer but were exposed at work before 1985. Rates are decreasing in those below 70 since younger workers are less likely to have been exposed.

Other health conditions with latent effects include cancers from chemicals and radiation. Although many of the chemicals that lead to bladder cancer are now banned, incidences of the disease continue to occur due to the latency period, which is similar to that of asbestos. Diesel engine exhaust is another factor in bladder cancer so, where this is poorly controlled, we are building up further problems for the next 50 years.




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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