Gravitational Hazards

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Physical Hazards: Gravity


The term ‘slips, trips and falls’ is treated almost as a single word in the workplace context with, in some cases, differentiation between a ‘fall on the same level’ and ‘a fall from a height.’ Such occurrences rank among the most significant causal factors in workplace injury and death in Australia, and there have been only relatively minor reductions in the injury rate in the last 10 years. Hazard identification, risk assessment and development of control strategies require an understanding of the physics of gravitational energy and the mechanisms of causation. This chapter uses injury statistics to examine the extent of the problem and the agencies of injury of slips, trips and falls (on the same level and from heights), and due to being hit by falling objects. It facilitates understanding of gravitational hazards with an overview of the relevant physics of gravity and the normal gait of a person, and examines the different mechanisms of a slip, a trip and a misstep. The importance of building design in prevention of injuries related to gravitational hazards is highlighted through a discussion on causation and scenario examples for control. The chapter concludes by discussing the role of the generalist OHS professional in preventing injuries from gravitational hazards.

Keywords gravity, slip, trip, fall, misstep, falling objects,


Physical hazards have been defined as “sources of potentially damaging energy” (Viner, 1991, p. 42). This chapter on gravitational hazards concerns the effects of unintended exposures to energy associated with gravitational forces in the workplace. Gravity – defined in this context as the force of attraction by which bodies tend to fall towards the centre of the earth – results in falls by people who have lost their balance for various reasons, as well as falls of unsecured objects.

The World Health Organisation defined a fall as “an event which results in a person coming to rest inadvertently on the ground or floor or other lower level” (WHO, 2010). Generally, falls occur after people lose their balance due to experiencing a slip, a trip or a misstep, or due to a failure or absence of an expected or required support mechanism. Injurious falls can occur on the same level – referred to as ‘slips, trips or falls on the level’ (STFL) – as well as from one level to another (i.e. falls from heights).

This chapter addresses falls on the same level and falls from heights, and incidents that involve people being struck by, or attempting to control, falling objects. Musculoskeletal injury can be caused by stresses generated within the body as a result of the often very rapid movements made during balance-recovery manoeuvres after an initial slip, trip or misstep. While a large proportion of such injuries are likely to be recorded in work injury statistics under body-stressing rather than in the fall-related categories, they should also be considered within the spectrum of hazards associated with gravity. As gravity acts on the mass of an object to give it weight, it is directly involved in hazards associated with manual-handling activities. However, such hazards (other than those associated with attempts to catch or restrain falling objects), are addressed in other chapters of the OHS Body of Knowledge.

Historical perspective

While it has long been known that falls are one of the most common causes of accidental death and injury - occupationally, domestically and recreationally - the first International Conference on slipping, tripping, and falling accidents was held at the University of Surrey in 1982 as a joint venture by the UK Medical Commission on Accident Prevention and the University of Surry Robens Institute of Industrial and Environmental Health and Safety. Much of that conference was dedicated to research reports on slip resistance of floors and foot wear but also cautioned that “a too simplistic approach to slip prevention can be, of itself, a hazard”. (Davis, 1983).

In recognising that fall-prevention research is a critical aspect of implementing effective occupational safety improvements, the US National Institute for Occupational Safety and Health (NIOSH) has had fall prevention as a strategic research priority for at least the past 15 years, and has conducted a program of laboratory- and field-based research to identify fall risks and develop prevention strategies and technologies. Regular international conferences are held covering topics such as: epidemiology; design of buildings and of equipment; slip resistance of floors and shoes; use of ladders; gait and movement; personal factors and risk of falls; forensic analysis of injury events; and training including physical training. (CDC, 2010).

The importance of investigating ways to prevent falls has long been recognised by the International Ergonomics Association (IEA), which has a Technical Committee on Slips, Trips, and Falls and holds biennial conferences focussed on preventing these types of incidents (see CGSTF, 2011).

Extent of the problem

People are continually exposed to gravitational hazards, virtually from conception. While the risk is quite low that foetal damage might occur in utero if an expectant mother falls (especially in the first trimester), such incidents comprise one of the known causes of prenatal mortality. At the other end of the age spectrum, falls all too often precipitate rapid decline in the health and independence of elderly people, with hospitalisation, complications and untimely death being all too common outcomes. Despite this ‘whole of life’ exposure to fall hazards there appears to be widespread complacency concerning the magnitude of the hazard, as reflected in the disproportionately low levels of preventative action directed at falls when compared to preventative measures relating to other potential hazards with higher profiles. For example, in Australia from July 2002 to June 2005 there was an annual average of 343 deaths and 105,968 hospitalisations caused by falls in buildings; in contrast, 110 deaths and 3,300 injuries were due to fires in one year (Ozanne-Smith, Guy, Kelly & Clapperton, 2008). Factors that likely contribute to the generalised complacency about fall hazards include the very common experience of surviving, generally without apparent ill effect, the inevitable tumbles associated with infancy and childhood, the frequent and largely automatic recoveries made after disturbances to balance once the complex skills of ambulation have been mastered; and, the lack of physical damage resulting from a large proportion of falls.

That non-injurious falls are such a pervasive aspect of the human condition appears to have overshadowed the fact that falls are also a major cause of harm. Globally, only road traffic accidents cause more deaths and injuries than falls (Ozanne-Smith et al., 2008). Workplace injury statistics justify a greater focus on identifying hazards and controlling risks associated with gravitational risk than often occurs.

The data in Table 1 demonstrate that between 2000–01 and 2006–07, slips, trips and falls caused approximately 20% of serious occupational injuries (i.e. those that result in at least one week being lost from work) to Australian workers. Slips, trips and falls accounted for 5%–10% of the subset of those injuries that proved fatal. Falls were identified in a regional study as the most common cause of injuries leading to persons of most ages attending public hospitals for emergency treatment (SSWAHS, 2009). In 2006–07, nearly 64% of fall-related occupational injuries involved STFLs, 30% involved falls from heights, and 7% involved stepping, kneeling or sitting on objects (Safe Work Australia, 2006–07). Sprains and strains were the largest single category of injuries caused by slips, trips or falls (50%), while 20% of injuries resulted in fractures of some type. In the same year, 21 people died as a result of experiencing a slip, trip or fall at work. A total of 45% of workers who experienced slips, trips or falls took 2–12 weeks off work due to their injuries (Safe Work Australia, 2006–07).

Although national data on the direct costs of workplace slips, trips and falls are not available, estimates of the likely direct public hospital costs of fall-related hospitalisations (based on data presented in Ozanne-Smith et al., 2008) indicate that the annual average of 28,136 occupational falls that occurred over the seven years from 2000–01 to 2006–07 would have resulted in more than 299,000 public hospital bed days. At an average estimated cost per bed day of approximately $1000, those falls would have involved annual direct public hospital costs in excess of $299 million. This figure increases to more than $1.1 billion if non-occupational falls are included. Workers' compensation cost estimates (based on data presented in WHSQ, 2007) suggest that the national cost of occupational falls approaches $130 million annually.

The experience is similar overseas, as demonstrated by international data. For example, the U.S. Bureau of Labor Statistics reported a total of 5,657 fatal work injuries for calendar year 2007. Of the fatality cases, 847 were associated with falls. In addition, of the 1,078,140 non fatal occupational injuries and illnesses involving days away from work in 2008, there were 260,610 cases associated with slips and falls. The National Safety Council [2002] estimated that some 200,000 to 300,000 disabling injuries are caused by work-related falls each year, and that compensation and medical costs associated with employee slip and fall incidents were approximately $70 billion/year (Stout and Hsiao in CDC, 2010). The U.S. Bureau of Labor Statistics cites slipping, or loss of footing, as the primary event involved in those falls. In addition, fatalities caused by falls represent about 12% of the accidental death toll in the USA.

There are differences between the key agencies of injury for falls from a height and slips, trips and falls on the same level (Safe Work Australia, 2006–07) (Table 2). For example, almost three-quarters of STFL occurring in 2006–07 were attributed primarily to environmental conditions, whereas environmental conditions were nominated in less than half of all falls from heights.

Table 3 provides an industry breakdown of claims for occupational incidents related to gravitational hazards (Safe Work Australia, 2006–07). Manufacturing and Construction industry workers incurred the largest proportion of incidents related to gravitational hazards (i.e. a total of 27% of damaging incidents caused by slips, trips and falls and 39% caused by impacts from falling objects). According to these data, slips, trips and falls in construction are equally split between falls from a height and falls on the same level whereas in manufacturing 70% of claims for slips, trips and falls are on the same level. Retail trade and property and business; which together account for 18% of claims for slips, trip and falls and 20% of persons being hit by falling objects; also have significantly more claims for slips, trips and falls on the same level than for falling from a height but in transport and storage 59% of claims are for slip, trip or fall on the same level compared with 41% for falls from a height.

While more people sustain injuries as a result of falling than are injured or killed due to being struck by falling objects, the latter mechanism is still significant. According to national occupational injury statistics (Safe Work Australia, 2000/01 - 2007/08), some 125 deaths and 36,670 serious injuries that occurred over that period were due to falling objects impacting with workers. Annually for the same period, such incidents caused approximately 2–8% of occupational fatalities and approximately 3–4% of serious occupational injuries. More than 75% of injuries caused by falling objects were attributed to falling materials, substances, hand tools and appliances (Table 2).

In summary, data on the causes of deaths and injuries consistently identify falls as one of the most significant causal factors of workplace injury and death. Considering that the statistics also demonstrate associations between increasing age and risk levels, it is likely that the incidence will continue to increase due to an aging population. In evidence-based approaches to identifying hazards and controlling risks, the scientific investigation of gravitational hazards and the implementation of suitable prevention measures warrant higher priority than they are commonly afforded.

Understanding gravitational hazards

Controlling gravitational hazards requires an understanding of the mechanisms of causation of slips, trips and falls, and the physics of gravity that contribute to the mechanism of the loss of balance and the seriousness of the resultant injury. These issues are addressed in this section under the headings of physics, slips, trips, missteps and falls from heights.


In general, gravitational hazards arise when a potentially unstable or unsecured object is involved in a damaging impact due to the manner in which gravity causes the object to fall to a lower level if the instability is realised or the object is dropped (e.g. when a tool is dropped from a height, or a person trips and falls). The damaging forces that might ensue when a fall is interrupted arise when the energy associated with the momentum of the falling mass is transformed into another form of energy, primarily through processes such as:

• Absorption (e.g. the body is punctured, crushed, lacerated, shaken or knocked off balance) • Deformation of the body posture against either natural stiffness or muscle effort (e.g. when a person attempts to arrest a falling object using muscle power).(Viner, 1991).

The magnitude of the forces associated with gravitational hazards is one of the key determinants of the risk of injury – the higher the forces the greater the risk. The consideration of forces associated with (i.e. causing or resulting from) moving objects is referred to as kinetics. Stationary objects that can fall have potential energy (PE). The magnitude of that energy (in joules) may be calculated as follows: