Occupational Health and Safety Models

(for incorporation in Course Units)

Current national OHS initiatives have focused on the strategic benefit of including OHS content in tertiary courses. As well as the intrinsic educational value of such material, it also contributes to the skill-base of graduates. 

The University’s OHS Policy Committee would seek the commitment of academic lecturers to integrate relevant OHS models, concepts and case studies into their course units presented within the University's curriculum.  This initiative aims to encourage: 

• the recognition of the academic value of the major OHS models and concepts,
• the explicit inclusion of these models into course theory units and practicals,
• the improved understanding and practical application of these models by graduating students, with the object of further improving the good OHS performance of the University and, in the fullness of time, that of the wider community.

1.  Risk Management (Reasonable Practicability) Model. 

The Occupational Health and Safety (Commonwealth Employment) Act 1991 requires [Section 16 (1)] employers to “take all reasonably practicable steps to protect the health and safety at work” of their employees. Other jurisdictions in Australia have similar OHS requirements. 

The legal judgement as to what is “reasonably practicable” is based on a consideration of the following general issues:

• severity of the hazard,
• probability of the risk,
• current knowledge regarding the hazard and the risk,
• availability of suitable hazard control/elimination methods,
• cost of such control/elimination methods. 

Thus the legal test of “reasonable practicability” basically calls on management to apply the risk management model (see Fig. 1) to control the hazards in their workplace to an acceptable level of risk. This is the approach taken by the University (http://info.anu.edu.au/hr/OHS/OHS_Information/OHS_Management_Plan.asp).

Figure 1    Risk management model

2.  Hazard and Risk.

A hazard is a physical entity (i.e. an energy or an environmental agent) which could produce injury or disease in people (or environmental damage, damage to property, etc.).

Hazards that need to be managed within University disciplines include “everyday” hazards such as:

• electricity,
• plant and machines,
• heights,
• confined spaces,
• vehicles,

and the more esoteric hazards like:

• chemical materials (including compressed gases and cryogenic liquids),
• biological organisms,
• ionizing radiation,
• lasers,
• pressure vessels and systems,
• fieldwork.

General ANU guidelines are available giving information on the identification, appropriate control practices, and residual risk assessment for such hazards (http://info.anu.edu.au/hr/OHS/OHS_Policies.asp)

Risk is the chance or likelihood of the hazard, under the circumstances of its use, producing an injury, disease, etc. Thus, risk is a number between 0 (no risk) and 1 (a certainty). Risk is influenced by such factors as:

• nature of the hazard,
• quantity of hazard,
• energy of the hazard,
• use to which the hazard is put,
• work system / operator factors,
  • leadership / teamwork
  • design
  • documentation
  • training
  • work practices
  • job selection
  • environment control
• system auditing.

3.  Safety and Health.

 Safety is freedom from the danger of injury. A cut to the body or impact from a falling object are common experiences that are easily recognised as safety hazards. The pain and the injury that immediately result from the cut or the impact are obvious. Also obvious is the potential for more serious harm, even death, to be produced should the cut sever an artery or the object falls on an unprotected head. Thus safety refers to the freedom from hazards that may produce immediate injury to the body.

Safety involves keeping control of excessive energies, for example:

• gravity and mass energy,
• self-generated (i.e. by the person) energy,
• object energy,
• machine energy,
• vehicles and mass energy,
• electrical energy,
• fire and thermal energy,
• chemical energy, etc.

in such a way that exposure to people is eliminated as far as is reasonably practicable. If the fragile human body is exposed to an overwhelming energy, the result is a potentially disabling or fatal injury.

Health is commonly recognised as the absence of disease in the body. Disease can be caused by infection from a biological organism but it can also be a long-term outcome from exposures to the other factors routinely present in our environment, namely

• chemical substances,
• physical agents - such as noise, heat, radiation,
• biological agents,
• mechanical / ergonomic factors,
• psychosocial factors.

If the energy involved in any such exposure is high enough (e.g.  during a major chemical spill), then such an exposure becomes a direct safety hazard. But at the low energy levels normally encountered, such environmental factors need to be evaluated and controlled as far as is reasonably practicable to ensure the risk of producing a disease is minimised.

4.  Safety Risk:  Injury Causation Model and Error Agencies.

The appropriate focus for safety practice is the anticipation (and control) of

• hazards that already exist in the workplace,
• errors in the work system that may generate new hazards.

These two issues are shown in the injury causation model of safety reproduced in Fig. 2. The model was developed by Wigglesworth, A teaching model of injury causation and a guide for selecting countermeasures, Occupational Psychology 46, 69-78 (1972).

Figure 2       Injury causation model of safety

The excessive energy is generally already present in the work environment, but is stored and safely controlled until it is released by an error in the work system, including the people. For example, gravitational energy and fall from a ladder, machine energy and entrainment of loose clothing.

 The injury causation model usefully documents the possibility of the error producing the uncontrolled energy release (i.e. the accident) either directly or via the generation of a hazard which has the latent potential to generate a direct injury (or an indirect injury via an energy release) while the work system is otherwise operating in a normal, error-free way.

Although an error occurs randomly in time, the occurrence of the error event will have been contributed to by one or more of an:

• unsafe act,
• unsafe condition,
• unsafe system.

These categories of error agency have traditionally been used to analyse the factors contributing to an error event.  In recent times, however, the “unsafe condition” category has been separated into 'environment', 'equipment' and 'design' components, to give five general categories of error agency, as summarised in Fig. 3.

 Figure 3       Categories of error agency

5.  Health Risk:  Occupational Hygiene Model.

Control of an environmental agent that has the potential to produce a risk to health can be achieved by,

• eliminating the agent,  or
• reducing the exposure/dose produced by the agent to as low as reasonably practicable.

The options available to control emissions emanating from a source of an environmental agent are summarised in the occupational hygiene model, given in Fig. 4.

Figure 4       Occupational hygiene model

There are six domains where interception of, or reaction to, an environmental agent can occur; as shown by the boxes in Fig. 4:

Source: The source has the potential to release the environmental agent into the work environment. Can the source be eliminated or substituted with another less hazardous material? Can the total process be changed to improve performance and, simultaneously, reduce the environmental hazard?

Emission: If the source is a chemical or biological material, can contamination from the source be controlled at the point of emission by means of total enclosure (e.g. biological safety cabinet) or by the use of local exhaust ventilation (e.g. chemical fume cupboard)?

Transmission Path: Can the environmental agent be controlled along its transmission path, by a physical barrier, by exhaust or supply ventilation, or by separating the source from the people by means of a very large distance? This last option results in a reduction in exposure by natural dilution for materials and by the inverse-square intensity reduction for physical agents (like sound and radiation).

Absorption: Can the significance of a potential exposure be reduced by absorption, which will minimise the passage of the environmental agent into the body? Personal protective equipment is an example of this control strategy. Another example is the use of an agent which lacks absorption or penetration properties. This is demonstrated in the use of non-respirable fibres in insulation material, and the use of soft b radiation (which does not penetrate clothing or the skin) in tracer research.

Receiver: One aspect of receiver control is to set a minimum contact time in which a person is exposed to the agent. In a heat stress situation an important control strategy is to limit the time in each hour that a worker spends in the hot environment. Similarly in the case of occupational strains, great emphasis is placed on task variation on an hour-to-hour basis to limit the time spent in any one postural state. For those agents having an occupational exposure standard, receiver control action is to withdraw the worker from the process when exposure or dose limits are reached. Lead workers are traditionally transferred into 'low-lead' areas if their blood lead level reaches an established limit.

Effect control seeks to identify the presence of biological dysfunction. It is particularly useful for the mechanical, ergonomic, and psychosocial factors where no quantifiable exposure standards have been established. For such factors, the detection of dysfunction in its early stages is an important mechanism for the identification of excessive exposures and/or the presence of the intolerant individual.

6.  Hierarchy of Controls (Engineering/Administrative).

Control measures vary in their effectiveness (i.e. of minimising exposure to safety or health hazards). Those methods that are potentially the most effective are placed at the top of a preferred hierarchy of control options and used first, where practicable, in designing the hazard control system.

The preferred hierarchy of control is broadly segregated into engineering or administrative measures:

Engineering methods achieve control without the need for active participation by the workforce and, as such, are generally considered the more effective.

Administrative control methods rely on active management leadership and workforce participation to be effective.

Note: the establishment of safe work procedures, and the training and supervision of staff in such procedures, is an essential control element in all workplaces.

The hierarchy of control can thus be listed as follows:

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