The use of local exhaust ventilation for reducing worker exposure

Introduction

Many industrial processes release airborne contaminants into the workplace. The inadequate control of these can allow them to enter the breathing zone of workers’ resulting in inhalation exposure. One method of minimising exposure is to apply extraction at the source of the contaminant generation, thereby removing the hazard before it enters the workplace air. This technique is usually referred to as local exhaust ventilation (LEV).

This article explains LEV, its relationship to the hierarchy of control, the different generic hood designs, including examples used in industry, and the steps that need to be taken to achieve successful and reliable control.

How LEV fits into the hierarchy of controls

If, following a risk assessment, there is a potential for workers’ health to be put at risk during a work activity, then the action to be taken should be based on a principal of priority. This is often referred to as the ‘Hierarchy of Control’ covering engineering controls and control measures as stipulated by Council Directive 98/24/EC ^[1]. This can be summarised as:

1. Elimination of hazardous substances
2. Substitution by a substance less hazardous
3. Use of engineering controls at source, including LEV
4. Administrative controls e.g. work procedures and organisational measures
5. Individual protection measures including personal protective equipment (PPE)

From the above, it can be seen that LEV should only be considered at step three, after the possibility of elimination and substitution have been addressed. However, the reality is that many companies assume that any potential situation where the generation of an airborne contaminant may take place requires the automatic application of LEV. This demonstrates a lack of understanding of the importance of the hierarchy of control and misses out important considerations, such as minimising the emission rate and process change.

What is LEV and what comprises an LEV system?

If, following the hierarchy of control, engineering controls have been identified as an appropriate measure to control an airborne inhalation risk it is likely that LEV will be selected. LEV is probably the most frequently applied engineering control, and a well-designed, applied and maintained LEV system should be capable of protecting workers’ inhalation risk. LEV can be defined as the removal of contaminants close to or at their point of origin by ventilation. By removing the airborne contaminant close to the source, the amount of air required reduces considerably when compared to dilution by general ventilation.

LEV systems are made up of many parts, however, most LEV systems comprise of the following main elements:

• Hood – this is the point where the contaminant laden air enters the LEV system. The hood design varies considerably from one system to the next. This will be addressed later in this article.
• Ducting – the ducting transports the contaminant laden air from the hood to the air cleaner, fan and finally the discharge point.
• Air cleaner – this filters or cleans the air.
• Air mover – this is usually a fan and moves the air through the system from hood to discharge point.
• Discharge – exhaust air should be discharged to a safe place. The most common method is vertical discharge to the outside of the building.

Figure 1 illustrates how the parts listed above are connected. However, it should be noted that not every LEV system will have all of the components shown in the figure. For example, some systems do not have air cleaners and rely on system dilution before discharging the extracted air to atmosphere, whereas other systems clean the contaminated air and return it to the workplace and therefore do not have a discharge stack.

It should be remembered that if the LEV system discharges air to the outside, replacement air will need to enter the workplace; this should be planned to avoid ‘starving’ the LEV system of air and to minimise draughts. Planning replacement air is an important element of an LEV system and is frequently overlooked.

One of the most critical, and least understood, elements of an LEV system is the hood. If the hood is badly designed, or is the wrong type, it will not be able to capture or retain the contaminated air; in this situation the rest of the LEV system is effectively redundant. However, given the critical nature of the hood, all too often little consideration goes into hood design and it is not unusual to find expensive LEV systems with hoods connected that are little more than ventilated boxes.

The reality is that good LEV hood design requires a thorough understanding of the process and the nature of the contaminant source to be controlled.

Processes, sources and the properties of airborne contaminants

Processes and contaminant sources

In this article the ‘process’ is defined as the task creating the contaminant, e.g. cutting a block of wood with a saw; and the ‘source’ is defined as the point of generation, e.g. the point at which the saw blade cuts the wood. Continuing with this example, the contaminant generated would be wood dust having a large particle size range (dependent upon the coarseness of the saw blade, the type of wood etc.).

How a contaminant is emitted from a process is of great importance to the application of LEV. For example, cutting wood with a hand saw will generate a dust cloud that is relatively ‘quiet’ and is not strongly directional. However, cutting wood using a circular table saw generates a very energetic directional jet of contaminant laden air. Consequently, the LEV applied to the above two examples will require different hood designs and extract volume flow rates.

Properties of airborne contaminants

Airborne contaminants can be generated as aerosols, gases, and vapours. Aerosols (defined as liquid or solid particles suspended in a gas – usually air, e.g. dust, fume, and mist ^[2]) can be generated over a vast size range, but the size ranges relevant to human health are; (i) inhalable, which as the name suggests, can be inhaled and can be up to 100 microns in aerodynamic diameter and (ii) respirable, which are particles that can enter the deep lung and are up to approximately 10 microns in diameter. Large particles, generally greater than 100 microns in diameter, will settle out quickly, often close to the contaminant source. However, smaller (finer) particles, for example those in the respirable range settle so slowly that rather than moving through the air, these particles have no independent motion and instead travel with the air in which they are suspended. They are therefore capable of pervading the whole workplace if they are not controlled at source. Fume falls into this category as these are very fine particles (less than 1 micron in diameter), whilst mists are liquid particles and fall into the same size fractions of interest as with solid particles, noting that the size distribution of liquid particles suspended in air can change with time due to evaporation.

One of the main problems visualising airborne particles is that under normal lighting conditions, respirable particles are generally invisible to the naked eye. Likewise, inhalable particles are only partially visible and therefore the extent of the contaminant cloud is likely to be unknown or at best, underestimated. However, the extent can be revealed by the intentional release of smoke at the contaminant source, which allows the size, shape and an indication of the contaminant cloud speed to be identified. Alternately a powerful lamp can be used to provide forward scattering of light to make the small particles visible, often referred to as a dust lamp ^[3]. Whichever method is used to determine the extent of the airborne contaminant cloud, the information is essential for applying suitably designed LEV.

Gases and vapours are formed by evaporation of a volatile liquid. Similar to particles, gases and vapours readily mix with air but at a molecular level and, like particles, they move with the air in which they are suspended.

When we are considering the control of airborne contaminants for health reasons, in any of the above states, the density of the contaminant material is not an important factor, although it is frequently mistaken to be. Studies have shown that dust from 'heavy' (i.e. dense) materials do not necessarily fall to the ground and that small particles, even of high-density material such as lead dust can remain suspended in the air^[4]. For particles (dusts) a critical factor is the size of the particles as it is this which determines whether or not they are breathable and likely to remain in the lungs if inhaled. For gases and vapours it is not the weight of the molecules themselves that dictate the effect of the contaminant cloud. Again, solvent vapours mix with the air and remain mixed in it rather than 'falling' to the ground. Therefore low-level LEV installed to control a ‘heavy vapour’ is often, but mistakenly, applied to control exposure and for the reasons above will fail. Importantly, LEV should be applied to contain and capture the vapour/air mixtures before they mix with the room air.

A qualifying statement does need to be added to the above; if large vessels containing solvents were to evaporate rapidly, there would be large quantities of vapour generated that would not have chance to mix fully with the surrounding air and in these scenarios a fire and explosion risk would need to be mitigated.

Hood classification system

General

LEV hood designs come in all shapes and sizes, which make it challenging to recognise how each hood works and why some hoods appear to perform better than others. Therefore, it is common practice to group hoods according to key design parameters ^[4][5][6]. Classification of hoods allows designers, maintenance staff, testers, and workers to understand how they work and what the limitations are. It also helps any critical assessment of LEV performance. However, as is usually the case not all hood designs fit conveniently into the following classification and some hoods work as a mix of two types. Nevertheless, the vast majority of hoods fall into one of the three following hood types:

• Enclosing
• Captor
• Receiving

Enclosing hood

Enclosing hoods are the most effective form of LEV hood. This is because the source is placed inside the hood. Enclosing hoods can be total or partial; an example of a total enclosure is the glove box, in this scenario the worker is physically separated from the contaminant source and exposure should be eliminated. Partial enclosures are more common as they allow access for the worker and are therefore more practical. An example of a partial enclosure is a fume cupboard. Fume cupboards have an adjustable transparent sash opening, which allows access to the interior of the fume cupboard when setting up experiments and can be partially closed when experiments are taking place inside. Importantly a sash can separate the workers breathing zone from the interior of the enclosure.

Generally the effectiveness of enclosing hoods increases as the area of the opening decreases, in addition, reducing the area of the openings often reduces the volume flow rate requirements and hence running costs. By their design, enclosing hoods are more robust against draughts and are less vulnerable to poor work practices. A partial enclosure is illustrated in Figure 2.

Captor hood

Captor hoods (also known as ‘External’ or ‘Capturing’ hoods) are probably the most common type of hood found in the workplace, yet they are equally the most misused and misunderstood. For all captor hoods the contaminant source is placed outside of the hood and therefore the hood has to generate sufficient air flow immediately around the contaminant source to draw it into the hood, this zone can be referred to as the capture zone or envelope. Figure 3 shows an illustration of a captor hood including the capture zone. An example of a capture hood used in industry is the moveable hood frequently applied to control aerosols welding fumes]]. Another example is the smaller hoods used to control solder fume. Sometimes captor hoods are integrated into hand tools, for example on-tool extraction applied to welding torches and soldering irons. In both these examples the hood is small and positioned a fixed distance from the source (welding arc and the tip of the solder iron) and therefore does not need positioning each time the hand tool is moved.

The Achilles heel of the captor hood is the limited size and reach of the capture zone. Inside the capture zone the airborne contaminant will be captured and removed by the hood. Outside of this zone the capture efficiency falls rapidly to zero. The size of the capture zone is dependent upon a number of parameters and reduces in size as:

• the source becomes more energetic;
• the LEV system flow rate falls;
• disturbing draughts increase;
• the size of the hood decreases.

From the above it can be seen that the size of the capture is process dependent. For these reasons, captor hoods are not suitable for energetic sources or where there are significant draughts in the workplace which cannot be suppressed.

Captor hoods come in two types: fixed and moveable. With a fixed hood the work piece is brought to the hood, with a moveable hood the hood is positioned in the desirable place by the operator. Moveable captor hoods are a popular design and ubiquitous throughout industry. This is largely because it is relatively easy to retrofit a process with a moveable hood, however, they are frequently positioned where space allows close to a process rather than at the correct position to effectively capture contaminants. It is critical if the worker is to minimise their exposure that they understand the limited distance that the hood can be placed from the source.

Receiving hood

As with captor hoods, the contaminant source is positioned outside of the hood. However, rather than capturing the contaminant, extraction relies on the contaminant being propelled into the hood either by the energy of the process or buoyancy effects. The classical example of a receiving hood is a canopy hood over a hot process (see Figure 4). The rising plume of air is intercepted by the hood which then has to empty as quickly as it is filled. This latter requirement is one of the main reasons why receiving hoods fail; often the extraction flow rate is less that the rate of contaminated air entering the hood resulting in leakage around the perimeter of the hood.

For effective control, receiving hoods should be:

• placed as close to the source as possible;
• large enough to intercept the whole contaminant plume;
• shielded from draughts, particularly in the case of slow rising plumes of hot air, which can easily be deflected by cross draughts.

It should be remembered that receiving hoods should only be applied when the contaminant source has a directional flow (created by either the energy of the process or by buoyancy) and the plume does not pass through the workers breathing zone.

Other key elements of an LEV system

Background

This article has focussed on the design of the LEV hood and how the different types of hoods work. This information is important not only because it is critical to the LEV designer but also to the company purchasing the LEV system and the worker using it. This is because the hood is the component of the LEV system that they tend to interact with on a day-to-day basis. Nevertheless, there are other key elements to an LEV system, as shown in Figure 1, that are critical to ensure that the contaminated air is removed from the hood for cleaning and discharge.

Air cleaner

The selection of the air cleaner depends upon a number of parameters, namely;

• the chemical that needs to be separated from the airstream;
• for aerosol laden air, its size distribution;
• the degree of separation required (for example, this may be dictated by environmental regulations).

There are a wide range of air cleaners available and it is critical that the employer takes advice from a competent LEV designer. Adequate, regular maintenance and cleaning of the air cleaner is also crucial to the good functioning of the LEV.

Air mover

For an LEV system, the air mover will almost certainly be a fan. As with air cleaners, fan design vary and their selection depends upon the amount of air moved and importantly the system pressures. As with air cleaners it is critical that the employer takes advice from an LEV designer or a fan manufacturer.

Ducting

Ducting connects the various elements of the LEV system, joining hoods, air cleaners and filters, and air movers as well as venting to the discharge point. The design of the ducting system can have a significant effect on the effectiveness of the LEV. The construction of the ducting (material, diameter, etc.) will depend on the nature of the contaminant to be removed (for example, an abrasive dust will require more robust ducting than non-abrasive material). Again, sound advice is essential to ensure an effective and efficient LEV system^[4].

Achieving effective and reliable LEV control

Specifying an LEV system

When purchasing an LEV system it is advisable to first produce an LEV specification ^[7]. This does not need to, and should not detail the engineering characteristics of the system, e.g. volume flow rate, air velocities and system pressures, as this is the task of the LEV designer. Rather it should state what is required of the LEV system, such as what reduction in exposure levels are expected, and therefore should include information on the contaminant you wish to control (for supplied substances see the manufacturers material safety data sheet) and the degree of control required.

It is recommended that a ‘user manual’ is requested that includes information of how to operate the LEV, check and maintain it. The user manual should also include commissioning test data (see Section 6.2). Furthermore, training should be provided to workers on how to use correctly the LEV system, as without this, unintentional worker misuse can occur resulting in ineffective contaminant control.

Depending upon the complexity and nature of the process the employer may need assistance developing a specification. However, this stage in LEV procurement is worth completing as mistakes at the specification stage will be costly to rectify later.

LEV commissioning

After installing an LEV system it needs to be commissioned to demonstrate it meets the design specification. This will require the installer/commissioner making a range of measurements, the number and type will depend upon the design of the hood(s) and the complexity of the LEV system. Typical ventilation measurements will include air volume flow rates, velocity measurements at the face of hoods and possibly inside the LEV ducting, static pressure measurements at a range of positions throughout the system. Whilst these data are critical, of equal importance is information that demonstrates that the system successfully captures or contains the airborne contaminant and therefore achieves its purpose of protecting workers’ health.

If the LEV is designed to an accepted standard then this step is relatively straightforward and may in fact be dependent upon aforementioned airflow measurements. However, this is usually not the case and some additional tests will be required, which will be qualitative and/or quantitative in nature. What tests are performed will depend upon the system and the toxicity of the contaminant being controlled. Typically qualitative tests will include smoke tests (using smoke tubes or if larger quantities of smoke are required, a smoke machine) carried out with the process running in order to visualise the airflow and ensure that the LEV system adequately removes the smoke whilst preventing it entering the workers’ breathing zone. Smoke will also help to identify:

• the size of the contaminant cloud;
• that containment is achieved within an enclosing hood;
• the size of the capture zone for a captor hood;
• any disturbing workplace draughts.

If the process releases particles, a dust lamp can be used [2],ref name="Vincent""> to achieve similar results. This technique does not require a surrogate to visualise the air movement.

Other than the airflow measurements, quantitative tests can include personal sampling to demonstrate worker exposure is being controlled. Containment testing can also be carried out using tracer gases, for example, containment testing of fume cupboards ^[8] or microbiological safety cabinets [9] ^[9].

Once good control has been demonstrated the ventilation measurement data need to be included in the user manual: these data become the benchmark against which future measurements are compared to ensure that control is being achieved; assuming that the work process does not change.

Checking and maintenance

If LEV systems are not checked or maintained they will inevitably fail; it is just a question of when rather than if. The user manual should include what checks should be done and when. It should also detail what maintenance is required and its frequency. A trained member of staff may be able to carry out all of the above.

Periodic testing

Periodically LEV systems should be tested to ensure they still meet the ventilation performance specification established during commissioning and detailed in the user manual. It is not usually necessary to repeat all of the commissioning tests, rather the tests that establish that the systems is still performing as expected, for example the measurement of volume flow rates, face velocities and static pressure measurements plus an assessment that the LEV system is still capturing/containing the contaminant and therefore protecting the worker health. This can be carried out by the competent employer/employee but can also be contracted out to an independent company.

Summary

All too often LEV fails to protect workers. However, well designed, commissioned and maintained systems can prevent workers contracting a range of respiratory diseases. But, it should be remembered that the hierarchy of control measures has to be respected and, in addition, no single engineering control in isolation will provided reliable and successful control; control is always a mixture of equipment and, importantly, working procedures.