John Saunders (Health & Safety Laboratory)
- 1 Introduction
- 2 Definition
- 3 The Hierarchy of Control
- 4 Types and Examples of Engineering Controls
- 5 Designing and Implementing Engineering Controls
- 6 Ensuring that Engineering Controls are effective and reliable
- 7 Conclusion
- 8 References
- 9 Further reading
The term ‘Engineering Controls’ covers a broad spectrum of possible interventions that are intended to reduce worker exposure, to chemical, physical and biological agents. This article will explain what ‘Engineering Controls’ are with respect to chemical and biological agents and how they fit into the hierarchy of controls. Examples are given of engineering controls along with some advantages and limitations. The importance of matching the control measure to the health risk and its reliability is also discussed along with commissioning. Once control has been achieved the article will explain why maintenance and checks are vital in order to maintain good control and therefore reduce worker exposure.
In the context of health and safety, an ‘Engineering Control’ can be described as a physical modification to a process, or process equipment, or the installation of further equipment with the goal of preventing the release of contaminants into the workplace (adapted from ). As can be seen from this broad definition there are a wide range of engineering controls, which could be applied. The control selected will depend upon the type of process, the nature of the contaminant source (its toxicity and release mechanism) and the route of exposure (inhalation, dermal, and ingestion). However, the reality is that no single engineering control in isolation will be successful; control is always a mixture of equipment and ways of working.
The Hierarchy of Control
The approach to controlling the chemical risk released from a process is rarely straightforward as there will always be a choice of control options – some easier to apply than others. However, the approach taken should be based on a priority list. This principal of priority is often referred to as the ‘Hierarchy of Control’. The European Control Hierarchy, as stipulated by Council Directive 98/24/EC  gives the priority order and is summarised below:
- Elimination of hazardous substances;
- Substitution by a substance less hazardous;
- Design of appropriate work processes and engineering controls and use of adequate equipment and materials, so as to avoid or minimise the release of hazardous chemical agents which may present a risk to workers' safety and health at the place of work;
- Application of collective protection measures at the source of the risk, such as adequate ventilation and appropriate organisational measures;
- Where exposure cannot be prevented by other means, the application of individual protection measures including personal protective equipment (PPE).
The directive goes on to state that hazardous chemical agents shall be eliminated or reduced to a minimum by:
- the design and organisation of systems of work at the workplace;
- the provision of suitable equipment for work with chemical agents and maintenance procedures which ensure the health and safety of workers at work;
- reducing to a minimum the number of workers exposed or likely to be exposed;
- reducing to a minimum the duration and intensity of exposure;
- appropriate hygiene measures;
- reducing the quantity of chemical agents present at the workplace to the minimum required for the type of work concerned.
It should be noted that this hierarchical approach is not unique to Europe and is adopted by safety professionals worldwide . From the above list it can be seen that engineering controls are integrated into steps 1 to 4. For example it can be argued that modifying a manufacturing process so as to eliminate the hazardous substance is a form of engineering control. However, it is common practice to associate engineering controls with steps 3 and 4: i.e. once elimination and substitution of chemical hazards have been considered. At times engineering controls may not offer adequate control and may need to be supplemented with other measures. Often this will take the form of PPE, which includes respiratory protection equipment (RPE). As can been seen from the priority list, PPE is the last step if all other interventions fail to offer sufficient protection. The problem with PPE is that it only protects the wearer. For RPE this is of particular concern as whilst the process operator may be protected from an airborne hazard, once it is released into the air it will inevitably pervade the workplace and therefore expose others who are likely to be unprotected. Furthermore for RPE to be effective it needs to be properly selected and correctly fitted, making training and user cooperation essential.
Types and Examples of Engineering Controls
It is not possible to list every different type and design of engineering controls, however they can be broadly divided into two types: non-ventilation and ventilation controls. Table 1 gives a broad range of examples of engineering controls, including both non-ventilation and ventilation (adapted from . This list is by no means exhaustive, but gives a flavour of the variety of controls available. Further examples of good practice can be found in the EU-OSHA report The practical prevention of risks from dangerous substances at work 
Table 1: Examples of engineering controls
|Process/Exposure source||Engineering control||Additional procedural control|
|Cleaning with solvent on rag||(i) Use a rag holder
(ii) Provide a small bin with a lid for used rags.
|(i) Check controls are used
(ii) Safe disposal of waste
|Dust spills from damaged sacks||Portable vacuum cleaners with HEPA filter||(i) Ensure vacuum is maintained and available for use
(ii) Safe emptying of vacuum cleaner
|Cutting-fluid mist from a lathe||Put an enclosure around the lathe and extract and filter the air and discharge to a safe place
(Protective gloves will also be required)
|(i) Train workers (e.g. It takes time for the mist to clear from the enclosures and this clearance time must be known)
(ii) Check and maintain fluid quality
(iii) Test and maintain controls
(iv) Carry out health checks
|Dust from disc cutter on stone worktop||(i) Carry out the process in an enclosure fitted with extraction, filter and extract to a safe place||(i) Test and maintain controls
(ii) Train workers
(iii) Carry out health checks
|Transfer of volatile liquids||(i) Pumping rather than pouring
(ii) Tight fitting lids to minimise evaporation
|Regular checks and maintenance (e.g. Check for damage to lids seals)|
|Evaporation of liquid from an electroplating tank||A layer of plastic balls floating on the surface to reduce both evaporation and mists||Check and maintain controls|
Non-ventilation engineering controls
Non-ventilation controls have the capability to reduce or eliminate process emission rate, for example the use of well-fitting lids to liquid containers. They can range from enclosures, seals, jigs and handling aids. However, engineering controls are frequently assumed to involve some form of ventilation control. This is unfortunate, as whilst ventilation controls can be effective and are the most commonly applied control to airborne contaminates, dismissing other non-ventilation engineering controls can be a costly mistake both in terms of financial and health cost.
There is probably no single reason why non-ventilation methods of control are overlooked, but perhaps the two main reasons are:
(i) the perceived need to alter the process and the potential ramifications this entails, whereas ventilation is often seen as something that can be retrofitted to any process, with little or no process modification. The reality is that this approach often leads to poor and erratic control. Designing a ventilation system to effectively control airborne contaminants requires specialist advice. All too often ventilation systems are badly designed and installed, poorly maintained and therefore frequently fails to provide adequate control;
(ii) Ventilation is seen as a low cost option, which is not true. As well as the capital cost required to buy and install a ventilation system there are associated running costs. The latter includes power (required to drive the air mover plus that required to heat/cool air that is brought into the workplace to replace the air extracted) and the maintenance costs, such as replacement filter units.
Unlike non-ventilation control, ventilation is unlikely to affect the emission rate from a process; rather it is designed to control the contaminant once it has been released.
As mentioned in above, ventilation controls are probably the most widely used method to control airborne contaminants in the workplace and can be divided in to two types: general ventilation and local exhaust ventilation.
General ventilation is the introduction of clean air into the workplace that eventually replaces the contaminated air. General ventilation can be subdivided into two further types: dilution ventilation and displacement ventilation.
The aim of dilution ventilation is to uniformly mix the clean air that is continually introduced in to the workplace with the contaminated air in order to dilute the contaminant concentration to an acceptable level. Whilst this is an accepted form of engineering control, its application is limited to low toxicity sources that are usually diffused throughout the workplace and where the workers are a sufficient distance from the source(s).
Displacement ventilation is where air is introduced with the aim of replacing the contaminated air by clean air with little or no mixing. In practice this is difficult to achieve, particularly over large areas and therefore needs specialist assistance. Both of the above types of general ventilation tend to use a significant amount of air, which usually needs to be heated or cooled; consequently this type of ventilation is an expensive solution.
Local Exhaust Ventilation
Local Exhaust Ventilation (LEV) is designed to capture, receive or contain the airborne contaminant at source before it has chance to enter the workers breathing zone or mix with the workplace air. As the control is applied as close to the source as possible, considerably less air is required when compared to general ventilation and consequently LEV is typically more effective in both terms of effectiveness and cost. For this reason LEV is preferred to general ventilation and should be considered to be higher in the hierarchy of controls.
Designing and Implementing Engineering Controls
Whilst elimination or substitution of the chemical hazard is the most effective solution, it is recognised that this is not always possible or straightforward. Often the process relies on the chemical in question and therefore elimination or substitution will not be an option. Consequently engineering controls are applied that either:
- Reduce the emission rate from a process, usually by process modification (non-ventilation controls); or
- Capture or containment of the emission once it has been released from the process, usually by enclosing and air extraction (ventilation controls).
An example of reducing the contaminate emission rate is by the application of water to a stone cutting disk. This significantly reduces the emission of dust, creating instead liquid slurry. It should be noted in this example disposal of the liquid slurry may create a further hazard. An example of removal of the contaminant once it has been generated would be by the application of local exhaust ventilation. This would be achieved by enclosing the process as much as possible and extracting the airborne contaminant with a relatively low volume, high velocity extraction system. Interestingly these two forms of engineering control have similar effectiveness .
The examples above illustrate that there is always more than one engineering control approach that can be applied to any process and therefore it is important that the various engineering controls are collated and their suitability assessed before a solution is selected. When assessing the controls the following need to be considered:
- Ease of use, and of course
- Financial cost.
Estimating the effectiveness of a control measure is not always easy and can easily be misjudged. Figure 1 illustrates simply that as potential exposure risk increases, the control effectiveness must also increase. Failure to do so results in what is often referred to as the ‘control gap’ and it is this gap that results in worker exposure. The mismatch between the effectiveness of the engineering controls and the process risk can occur for a number of reasons, ranging from a lack of appreciation of the extent of the exposure risk from the process, to an over optimistic belief in the capability of the control measure.
Source: Adapted from HSE training material
Before engineering controls can be contemplated it is necessary to understand how the contaminant is being released into the workplace. This requires a full understanding of how the process works and how workers interact with the process. Ideally workers should be involved in the design and selection of the controls, as they will be using them on a daily basis. Failure to do this often results in engineering controls that are unworkable resulting in poor exposure control. Depending on the process, other disciplines may also need to be consulted, in particular the engineering function, as their input may be required to ensure the more intricate nuances of the process are considered and understood (for example quality of the final product is not affected by the introduction of the control measure).
As well as considering the process, the reliability of any proposed control measures needs to be addressed. There is little point designing and installing what is judged to be an effective control solution that only works intermittently and is prone to malfunction.
Some engineering controls are more difficult and time consuming to apply than others and sometimes companies may have to resort to RPE equipment as a temporary control measure until the final engineering controls are design and implemented.
Additional benefits of good control
Clearly engineering controls are designed to reduce exposure and to assist companies in complying with health and safety regulations and occupational exposure limits. However, it is possible that they may help to reduce environmental pollution and, importantly can make an economic impact by reducing company expenditure on such items as product consumables. Two examples of this are given in EU-OSHA, Factsheet 33 - An introduction to dangerous substances in the workplace .
One of the examples was a printing facility that introduced covers on older, high-solvent, printing machines. This engineering control required some thought, but hardly any capital expenditure. As a result the solvent vapour levels were halved, saving 5,000 litres of solvent per week equating to €74,400 a year. A clear case of an engineering control not only reducing worker exposure but saving the company a significant amount of money.
Ensuring that Engineering Controls are effective and reliable
Why engineering controls often fail to protect workers
Engineering controls can fail for a variety of reasons. Often they are not as effective as envisaged and therefore fail to protect from the date they are installed. Even when initially effective their performance can gradually decline. This can be exacerbated by poor management, e.g. inadequate training. Therefore there are issues to consider in ensuring controls work effectively and go on working.
Once a control measure is designed and installed it needs to be commissioned. ‘Commissioning’ is proving that the engineering control is capable of providing adequate exposure control. The type of commissioning and the complexity depends upon the control measure. Probably the most complex commissioning process is that of LEV systems. Unfortunately LEV commissioning is frequently carried out incompletely or is inadequate. LEV commissioning tends to focus on the engineering parameters, such as system pressures and air velocities. Whilst this is an essential part of the commissioning process, a judgement on the effectiveness of the controls and the worker exposure needs to be taken. There are a number of qualitative and quantitative tools available to help the assessor judge control. An example of a qualitative assessment is the use of smoke tubes to visualise the air flow in and around an LEV hood in order to assess LEV performance. An example of a quantitative control is personal sampling to quantify worker exposure to a particular substance(s).
In isolation, an engineering control solution is destined to fail. They need to be integrated with other control measures, such as a ‘standard operating procedure’. It is highly likely that some form of training and supervision will also be required to ensure that the controls are correctly used and therefore control workers’ exposure.
Checks, Monitoring and Maintenance
Without regular checks and routine maintenance, the effectiveness of engineering controls will degrade gradually and inevitably fail. The time it takes for this to occur will depend upon the type of control measure. Engineering controls tend to degrade slowly with time and this often goes unnoticed. An example of this are poorly maintained LEV systems; often the workers can hear the fan impeller rotating, but do not realise that the volume flow rate of the system is imperceptibly falling with time resulting in a loss of control of the airborne contaminant. In this example the performance of the LEV hood could be continually monitored by the use of an air flow indicator, such as a pressure gauge.
All too often when companies realise they have an exposure problem, they immediately assume PPE is the only solution. Invariably this is not the case, and following the hierarchy of controls, engineering controls that are properly commissioned and maintained play an important role in reducing the workers exposure to the chemical risk in the workplace.
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- Perkins, L. J., Modern Industrial Hygiene: Volume 3 - Control of Chemical Agents, American Conference of Governmental Industrial Hygienists, Cincinnati, Ohio, 2012.
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