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July 4, 2011

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Airborne hazardous substances – Local hero

Dr Mark Piney presents a simple four-stage process of how to specify and apply reliable and cost-effective local exhaust ventilation.

Local exhaust ventilation (LEV) is often treated as an engineering bolt-on – the province of the maintenance department and, as such, a lower priority for the health and safety department. Its overriding function – to minimise people’s exposure to airborne hazardous substances and any risk to their health – can therefore be overlooked as part of the hierarchy of controls.

The costs to individuals, employers and society in general of work-related breathing and lung problems are considerable. A recent case in point is that of Yorkshire building restoration firm, William Anelay Ltd, which had to pay £36,000 in fines and costs because two of its stonemasons contracted silicosis. The company had LEV controls that were ‘thoroughly examined’ every year, and its health, safety and environment arrangements were audited annually by an external company, yet exposure to silica dust over many years led to permanent health damage for two workers.1

The British Occupational Health Research Foundation has estimated that “new cases (of asthma) over a ten-year period cost society £1.1 billion. In terms of human cost, some workers are left severely disabled, causing early retirement, with others forced to change jobs”.2

In comparison, implementing reliable LEV controls can be extremely straightforward and cost-effective. Consider the following four-step procurement  model.

Step 1 – Processes and sources

Before even thinking about exposure control, it is crucial to understand which work processes emit airborne contaminants, and how they do so. For example, circular saws, spray guns, free-falling powders, sack emptying, and compressed-air devices can all cause movements of air and any contaminants contained within it – the HSE website contains a wealth of information, including downloadable video footage, of these and many other work processes and sources.3

To determine process-induced air movement, various non-expensive and simple methods can be employed. For example, a dust lamp can illuminate invisible or fine dust, and show escape of particle clouds from an LEV enclosing or receiving hood. Smoke tubes, or tracers, can also be used to demonstrate leakage from hoods, as well as highlighting the movement of contaminated air away from a process, or source, and showing the movement and impact of draughts.4

Having identified the key processes and sources, the next thing to consider is how they can be modified to reduce their emission rate. Often, processes can be modified to make them less ‘energetic’, or the materials being used can be substituted by less hazardous alternatives. An old example is the change from sand blasting to grit blasting of castings, while a more modern example would be the change from rosin-cored solder, which can cause asthma, to non-rosin cored solder, which doesn’t.5

After changing the work process, and substituting the material being used, implementation of LEV is the next control in the hierarchy. But it can only be applied to work processes that are ready for it. Often, the production process and sequence are taken as givens and LEV is ‘applied’ to all points at which emissions occur. This can be an expensive way of applying LEV; moreover, exposure control is usually ineffective, because LEV is shoehorned in and/or it is applied to very energetic or large sources, with not much hope of successful control.

There are two sensible options:

1    Re-arrange production to bring all the significant sources together and apply LEV at this point; or
2    Segregate the high-emitting processes in one place and control exposure with LEV (in this case, operators will often need supplementary personal respiratory protection).

Step 2 – Specification

Until 2008, when the HSE published Clearing the air,6 there was no guidance on how to buy LEV controls, or write a specification. This meant employers usually deferred to the expertise of the LEV suppliers, whose levels of competence vary, and many of whom are tied to a particular product range. If what you need is in that range, fine, but often employers get what is ‘in the catalogue’, regardless of whether it is suitable or not.

It is therefore crucial that employers think through and write down their LEV specification, in which they must make their needs clear to any prospective supplier. The specification must include process information and require the following six issues to be addressed in any quotation:

1    What type of LEV system is being proposed, and how does it control emissions and exposure?
2    How will the supplier show that the LEV effectively controls exposure?
a    If there is a standard LEV design for a standard process that is known to be effective, the supplier can quote the appropriate standard. There is general HSE guidance available,7,8 but the more specific the design detail, the better. Don’t be fobbed off with talk of ‘hoods’ spoken of in vague terms.
b    It’s probably best to specify the LEV performance in terms of an exposure benchmark. Leave the supplier to explain how the benchmark is going to be met.
c    Qualitative performance tests –  for instance, dust-lamp or smoke tests – may be adequate, especially where LEV control is clearly very effective. Problems occur where there is ‘some’ emission/exposure.
d    Air velocities or volume flows, by themselves, are not measures of control effectiveness but are often what suppliers are comfortable with, and what they tend to quote on. Pin the supplier to exposure control – that’s what the LEV is there to do!
3    The agreed control measure should be clearly explained in any supplier quotation, and clearly described in the purchase order. It should also be made plain that the supplier will not be paid in full until the agreed measure of control success is met.
4    Cost and functionality should be specified. Well-designed systems often move less air, which reduces running costs, while a system that is easy to check and maintain is also preferable.
5    As the LEV system will need to be managed and maintained on a day-to-day basis a user manual and some training will be required, so these should also be specified.
6    Simple airflow indicator(s) are a very powerful and easy way for users to check that control airflow is being maintained, and so should be specified.

Get at least three quotations, compare them against the specification, and follow up references and testimonials. For those buying a big, or bespoke system, it may well be worth going to another site to judge the quality of a supplier’s work, and speak to the employer and employees.

In terms of LEV hoods, it is essential to be clear about what works, and what doesn’t. The HSE guidance uses a simple classification of hoods, and their design principles: they are either enclosing, receiving or capturing. If you know which type you are dealing with you can assess whether or not it is working.

And LEV hood design really matters. The main reason LEV fails to control exposure and protect people’s health is incorrect choice/design of hood. If the wrong hood is installed, the airborne contaminant isn’t contained, received, or captured properly. So, getting the hood design right is key to exposure control success. Unfortunately, it’s often the thing that gets the least attention. LEV suppliers tend to concentrate on the fan and filters, because these cost the most and are where they make their margins. Ductwork and hood design tend to be done by rote, and often are simply assumed to ‘work’.

The HSE guidance contains some useful LEV hood-design rules of thumb. They are:
1    Maximise the enclosure of the process and sources.
2    Make sure the hood is as close to the process as possible.
3    Position the hood to take advantage of any directional airborne-contaminant cloud movement.
4    Physically separate the airborne-contaminant cloud from the workers’ breathing zone, using a transparent shield.
5    Minimise eddies at the hood entrance, and turbulence within the hood.
6    Use ergonomic principles when designing and applying LEV hoods.
7    Prototype LEV hoods and get employee and employer feedback.
8    Use simple methods to judge effectiveness.
9    Match control effectiveness against potential degree of overexposure.
Suppliers do not tend to go into this degree of detail, so employers should insist, in their LEV specification, that suppliers follow HSE LEV hood-design principles.

It is also important that employers understand the limitations of what tends to be the most commonly used hood – the capturing hood. With these, the process to be controlled takes place outside the hood, which needs to generate sufficient air flow at and around the process to ‘capture’ and draw in the airborne-contaminant cloud.

They are popular for a range of reasons – they can be applied with no changes to processes or work methods, they are what a lot of suppliers sell, and have become almost the default definition of ‘local exhaust ventilation’ – but, in many circumstances, they simply do not work. To avoid ending up with an ineffective and unreliable hood, follows the nine design rules listed above (see also panel, right).

Stage 3 – Installation and commissioning

Once an effective design has been determined and agreed upon the system must then be safely and accurately installed. Sometimes, the installer may want to modify the design owing to unforeseen building or process constraints; if any changes are to be made, they must be agreed with the system designer.

Having installed the system and sorted out any immediate ‘snags’ the LEV controls need to be commissioned. This is done against the original design quotation and it is at this point that the supplier needs to demonstrate that exposure control is effective enough, and meets any benchmark, or other agreed performance standard. This is also the point at which to calibrate the air-flow indicators, and for the supplier to provide training in the use of the system manual.

Stage 4 – Maintenance

LEV systems are relatively delicate mechanical devices and will degrade, initially imperceptibly, if they are not regularly checked and maintained. The employer should appoint a ‘responsible person’ who follows the instructions in the user manual, arranges the checking and maintenance, and keeps records. The employer will also need to check that operators are using the LEV correctly, as they were trained to do.

The final step in the procurement/review process is the mandatory thorough examination and test (TExT). Many of these are done as a ‘commoditised’ service, where the examiner is severely time-constrained. Few examiners will turn round to a client and tell him or her that they have bought the wrong type of LEV and it doesn’t work!

A TExT should be done well and used by the employer as an ‘audit’ of the LEV maintenance over the year. Examiners vary greatly in their competence, thoroughness and price, and it is well worth shopping around, or even considering, with the right training, doing TExTs in-house.

The more savvy and critical employers – assisted by their advisors – are in procuring LEV, the more effectively and reliably exposure will be controlled.9 With time and effort, the problem of airborne hazardous substances even in ‘dangerous trades’, such as stonemasonry work, will become a thing of the past.

1 4702297.York_firm_William_Anelay_
2    BOHRF leaflet: ‘Occupational asthma – a guide for employers, workers and their representatives’ –
3 index.htm
4    The HSE’s LEV Topic Inspection Pack, ‘Assessing and inspecting Local Exhaust Ventilation systems’, explains these, and other methods –
5    HSE (1997): Controlling health risks from rosin (colophony)-based solder fluxes (INDG249) – indg249.pdf
6    HSE (2008): Clearing the air – a simple guide to buying and using LEV (INDG408) –
7    HSE (2008): Controlling airborne contaminants at work: a guide to local exhaust ventilation (HSG25) –
8    COSHH Essentials – particularly the process-specific guidance –
9    The British Occupational Hygiene Society has approved a new one-day course, entitled ‘Practical management of LEV’, aimed at health and safety practitioners, building and facilities managers, and production and design engineers –

Dr Mark Piney is a former HSE Principal Specialist Inspector.

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I was surprised that there was no direct reference to the COSHH Regulations which apply to the use of LEV.
“Stage 3” – Commissioning (Initial Apprasial) of LEV is necesary to comply with Reg 6, and subsequently,
“Stage 4” – Thorough examination and testing, (based on the “performance benchmarks” set in the Commissioning (Initial Appraisal)), is necessary to comply with Reg 9.
If both of the above have not been carried out, for any LEV, the owner/user is in breach of the Regulations.