Designing and managing local exhaust ventilation


Local exhaust ventilation (LEV) is one of the main measures used to control worker exposure to hazardous substances. It’s difficult to work out exactly how many systems there are in Britain, but the Health and Safety Executive carried out some research in 2006 and estimated that there were between 260,000 and 330,000 businesses that have LEV. Now some of these will be small organisations who will only have installed one system but many others will have several systems in place. So, if the HSE’s estimates are still valid, the total number of systems could be approaching half a million.

As with any engineering control (or other plant and equipment for that matter), if LEV is to work effectively it must be well designed and than managed properly.

Sadly, experience suggests that this is rarely the case. In practice

  • Many suppliers fail to consider the nature of the process and the behaviour of the contaminants when designing extraction hoods.
  • Inappropriate, “off the shelf” systems are installed
  • Proper commissioning is rarely undertaken.
  • Most suppliers don’t provide detailed instructions and other information needed by the client on use, maintenance and testing.
  • Once systems are installed many companies don’t carry out planned routine maintenance and testing.
  • Where testing is carried out it normally only comprises an annual “thorough examination and test” with no interim checks on condition or performance.
  • The standard of the “thorough examination and test” is often poor. Many testing companies only carry out flow and pressure measurements and do not evaluate the effectiveness of the system at controlling the contaminants.

The HSE recognised these problems and, following extensive stakeholder consultation, published updated guidance in HSG258 Controlling airborne contaminants at work: A guide to local exhaust ventilation.

I’ve uploaded a presentation to Slideshare about “Effective design and management of LEV” which summarises many of the key points from HSG258.

To attempt to improve awareness of good practice amongst managers a new one day BOHS Approved course – Practical Management of LEV Controls has been developed in conjunction with HSE. We’re going to be running it on 15th September 2011 in Chester. Cost £195.00 plus VAT per delegate (includes course documents, lecture fees and lunch). The above presentation gives a flavour of the course content.

To test or not to test?

I recently received the following query from a client:

“I have come across various items of equipment which appear to have integral LEV types of arrangement ……..  All have been introduced to reduce the level of dust or chemical that may have a deleterious effect on the user of the equipment.  I am unsure if all are considered strictly LEV and subject to the 14 month inspection period.  Having asked a number of the manufacturers of the equipment if the LEV has been designed  and confirmed to reduce exposure adequately, they often reply that it was included as a desirable attempt at reducing exposure.  Where does this leave me?  Should I be getting the LEV commissioned and deemed appropriate?  For the majority of equipment the requirement to adequately maintain it under PUWER has meant that it is regularly serviced, but is this enough in relation to the LEV component?”

I think we sometimes get hung up about LEV. In fact Regulation 9 of COSHH requires maintenance of ALL controls and a regular thorough examination and test of all ENGINEERING controls. (see COSHH Reg 9.2 which states

“ Where engineering controls are provided to meet the requirements of regulation 7, the employer shall ensure that thorough examination and testing of those controls is carried out”

The only difference for “LEV” is that the maximum interval is specified and some specific guidance on what this should involve and other aspects of managing the control is available. In fact the general  principles set out in HSG258 regarding the management of LEV (commissioning, maintenance and testing) are really relevant to all engineering controls. LEV is only singled out because it is a common control where these aspects are particularly important if the system is to be effective.

Servicing in accordance with PUWER is unlikely to properly address the LEV. It may ensure that the fan works, for example, but the most important aspect of LEV testing is to verify  that the contaminants are being controlled, and this will almost certainly not be done.

So I think the good practice set out in HSG 258 is relevant to the systems mentioned by the client, although the concept of “reasonable practicability” should be applied when deciding exactly what is needed and how urgently the issue is addressed. Some systems will be more important than others in respect to controlling exposure and it would be best to identify these and prioritise them. Also the amount of work involved will depend on the design of the system.

In the case of simple systems used to control low concentrations of contaminants  it may only be necessary to introduce  smoke testing and a few basic measurements in addition to what you are already doing. However, more complex systems used to control higher concentrations of more hazardous substances are likely to need a more substantial test.

Of course, this is only my personal view. A HSE Inspector may have a different interpretation!

Testing walk-in spray booths

Spray painting inside a walk-in booth (photo credit: HSE)

We recently received a query from a client who carry out paint spraying of isocyanate based two pack paints in a large walk-in type spray booth. They wanted us to carry out sampling to help them to decide when it was safe to enter the booth without their workers wearing their air fed masks. This sounds like a good idea. In fact it’s essential for workers to know when the paint spray has been completely cleared from the booth. Isocyanates are very potent respiratory sensitisers causing asthma in susceptible individuals. Anyone sensitised to isocyanates will experience asthmatic symptoms even if they are only exposed to very low concentrations. However, air sampling isn’t the best way to determine the “clearance time”.

If it was possible to use a direct reading instrument that gives a reading of the instantaneous concentration it would be relatively easy to determine when the booth was cleared. Unfortunately there isn’t a direct reading instrument that can be used for isocyanates. Sampling has to be carried out using an integrating technique – that means that a sample is collected over a period of time, and, after it has been analysed in a laboratory, it is then possible to determine the average concentration present during the sampling period. To determine the  “clearance time” a series of samples would have to be taken one after another. This would be expensive (the analysis isn’t cheap) and the standard method for isocyanates is not particularly good. The samples are collected by bubbling air through a reagent dissolved in toluene and as a low detection limit would be required it’s difficult to take samples over a short period.

Consequently its better to use a different approach, releasing smoke into the enclosure and then timing how long it takes for it to be completely removed from the booth. This method is quicker, less messy and much less expensive. The test can also be used to check that there are no leaks from the booth – any smoke escaping should be clearly visible. The Health and Safety Executive explain how to carry out this test in their publication “Controlling isocyanate exposure in spray booths and spray rooms” which can be downloaded from their website here.

The following videos show a smoke test taking place in a typical booth.

Once the clearance time has been established it’s important to ensure that anyone who works in the booth is informed and a notice posted at the entrance to the booth, something like this:


The HSE would like to see automatic clearance time indicator fitted on walk-in spray booths, but in my experience few have them. This is something that should really be addressed by the manufacturers and suppliers of the equipment. A new booth should already have them fitted, and clearance times should be established during the commissioning of the equipment after it’s been installed.

Following on from the initial test checking the clearance time should also form part of the statutory thorough examination and test required for LEV systems under COSHH Regulation 9. All engineering controls deteriorate over time, so the time taken for the contaminants to clear is likely to increase, even with a well maintained booth.

Glove boxes

Glove boxes are often used in the pharmaceutical industry to control highly toxic “active” agents used in drug formulations. In principle they should provide a high degree of protection for the user. The contaminant is completely contained inside an extracted enclosure while the worker is outside. So when we’re carrying out a risk assessment and spot that the work is carried out inside a glove box we might assume that exposure will be well controlled. However, we have to be careful – sometimes the opposite is true.

positive pressure glove box

Looking at the above picture you can see that the gloves are sticking out of the enclosure. This is a good indication that the enclosure is under positive pressure. This means that contaminated air will leak out through any gaps or breaches in containment, potentially bringing contaminants out with it to which the operator will be exposed.

Such “positive pressure” booths are often used for quality considerations – to protect the product from contamination – and this is probably the case here.

Managing LEV systems – have things improved?


Yesterday I went over to the Health and Safety laboratories for a “stakeholder meeting” with HSE/HSL to discuss a potential new one day course on the management of local exhaust ventilation (LEV)systems. It was a good meeting and I feel that HSE were willing to listen and take on board suggestions that I and other stakeholders made.

The idea for this course has come about because, despite the publication of the HSG258, HSE’s updated guidance on LEV, there remain major problems with their design and management. Working as a consultant I visit many organisations and there are usually significant problems. I have done any proper research on this, but from my own experience I still find that:

  • badly designed systems are still being installed. Suppliers are still failing to consider the nature of the process and the behaviour of the contaminants when designing extraction hoods
  • some companies are still not carrying out routine maintenance and testing
  • where testing is carried out it normally only comprises an annual “thorough examination and test” with no interim checks on condition or performance
  • the standard of the “thorough examination and test” is often poor. Many testing companies only carry out flow and pressure measurements and do not evaluate the effectiveness of the system at controlling the contaminants

It’s disappointing that in the two years since the publication of HSG258 these problems are still widespread. It’s difficult for consultants to make a difference. We can only make recommendations that we hope or clients will implement. Sadly, they often don’t. In-house hygienists may be more successful, but I think that they can still hit problems where the maintenance department look after the LEV systems. More training of engineers would be helpful here, and this is one of the target audiences for the proposed course. However we have to get over the hurdle of getting them to realise that they need to know more about LEV.

Although the occupational hygiene profession can exert some influence, we’re too small to have a major impact. Other groups were involved in the development of HSG258, including major manufacturers and ventilation testing companies. Unfortunately, I don’t think that they have bought into the principles set out in the guidance.

HSE did a lot of work developing the guidance, involving key stakeholders, and there was a lot of enthusiasm when it was launched in 2008. They even trained up general Inspectors on LEV and we have seen an increase in the number of improvement notices on COSHH Regulation 9, albeit a modest one. However, I feel that the campaign has lost momentum.

Further action is needed by HSE if the standards of LEV design and management are to improve. More work probably needs to be done to get suppliers and LEV testing companies to buy in to the good practice set out in the guidance. But the key to improving standards is more action by Inspectors when they’re out on their visits. The campaign needs to be re-launched and reinvigorated. Sadly, with the change in Government I’m not optimistic that this will happen.