Local exhaust ventilation

What is LEV?

LEV systems for which the control of hazardous substances (e.g. dust, fume, gases or vapours) relying upon their removal by exhaust ventilation.

In order to comply with Regulation 9 of CoSHH and HSG258 any dust or fume extraction system must be tested at least once in a period of 14 months.

Ventilation ducting

Most systems, but not all, have the following:

  • Hood: This is where the contaminant cloud enters the LEV.
  • Ducting: This conducts air and the contaminant from the hood to the discharge point.
  • Air cleaner or arrestor: This filters or cleans the extracted air. Not all systems need air cleaning.
  • Air mover: The ‘engine’ that powers the extraction system, usually a fan.
  • Discharge: This releases the extracted air to a safe place.

EFS can offer a comprehensive commissioning and balancing service

The purpose of Commissioning of LEV systems is to ensure that they are working in accordance with the design specification. This is particularly important for the handover process for Mechanical engineers. The commissioning report becomes the comparison data for future LEV assessors to compare future results against. In theory, it is the first LEV test as we also assess the performance of the hood. We carry out this work to the HSG258 guidance. The employer is responsible for effectively controlling exposure by means of adequate control measures, both ‘hardware’ such as LEV, and work practices.

This means:

• process-related equipment, eg seals, jigs, handling aids, as well as the LEV system;

• work practices, such as optimum work position, the angle and position of work tools and the correct use of the LEV

LEV installers and commissioners turn equipment supplied to a design into hardware that provides adequate control of the hazard as specified by the employer as the client. Effective commissioning requires the employer to work closely with the LEV supplier and LEV service providers. Service providers must tell the employer that installation and commissioning may interrupt production.

There are four stages to LEV commissioning:

• installation (if necessary), and verifying that the system was installed as designed;
• showing that the LEV system meets the specified technical performance;
• control effectiveness – demonstrating adequate control of contaminant clouds;
• reporting findings (qualitative and quantitative) as benchmarks for management and maintenance of LEV performance and subsequent examinations and tests. 263

The LEV commissioning report, together with the user manual, is the basis of the Statutory ‘annual’ thorough examination test. Many LEV systems will not have been commissioned or supplied with a user manual. In these cases, the employer will have little information on the required performance or how to maintain it. The LEV examiner may also have difficulties.

The design of any LEV system with more than one hood needs each branch to extract just the right amount of air. Installation involves much more than simply connecting up the ducts and turning on the fan.

Balancing means achieving the performance required at every hood in a system. This must be done either by the installer or by the commissioner.

The airflow in each branch is determined by:
• inlet or hood resistance;
• duct branch length, diameter, and flow resistance;
• flow conditions at the junction with the main duct.

Balancing is always required on installation, commissioning and on any reconfiguring of the LEV system. The correct balancing (and rebalancing) of a LEV system is a highly skilled activity, particularly a multi-branch system. Altering the airflow in one duct affects the flows in all of the other branches. It is often necessary to work through the whole LEV system and repeat the process at least once.

A common reason for an existing system to be seriously unbalanced at inspection is that somebody has isolated a redundant inlet, or added new hoods. In such cases, rebalancing is required, starting at each hood and branch, and making adjustments while working towards the air mover.

All machines in any product process must be regularly maintained, LEV systems must comply with regulations but more often than not the filter unit is often overlooked. This is a critical part of a LEV system. If not maintained then the filter unit can fail which results in downtime and significant cost.

It is often unknown that the filter units can actually present a higher risk to health, particularly if located inside a building as the filtration media can fail with time, which then fine dust returns into the workplace. If not identified or maintained then this can cause irreversible soft tissue damage. It is vital that the LEV system is maintained correctly.

We can take responsibility for the proper operation of your filter unit. We are able to service all makes and models of filter units. Our service engineers come from a technical engineering background and thoroughly understand the workings of any filter unit.

We can develop bespoke service packages tailored to your needs which could include. We offer either a single visit or a service contract. EFS can create an annual schedule of the necessary maintenance required to keep systems operating within the HSE and SEPA guidelines. This is included in our LEV Logbook.

EFS has an extensive range of consumables and spares used for LEV systems. EFS can either supply genuine or equivalent replacement parts for all major manufacturers filters Including, dust control equipment ltd, dce, donaldson torit, airmaster, dantherm, nederman, mikropul, american air filter, aaf, spencer holstead, lodge sturtevant, dustcheck, duscovent, intensiv, camfil farr, luhr, abb, ab dust control, tilghman wheelabrator, cibel engineering and many other dust collector suppliers.

We supply replacement components such as bag cages, Reverse jet, reverse pulse Controller (major suppliers plus ae autel, bpc, goyen, asco). Filter bags and envelope filters are supplied in many different materials such as polyester felt, nomex, tetratex, gortex, ryton, p84 with many different finishes.

EFS first step to design is consultancy, at this point we shall attend a meeting to understand your process and requirements with a full examination taking place. Designers are responsible for interpreting the requirements of the employer and advising on an effective LEV system which is capable of delivering the required control.

What suppliers and designers need to know:

Their role and legal responsibilities.
How to liaise effectively with the employer and installer.
Hazardous substances to be controlled.
The principles of LEV hood design.
How to apply hood design to the processes and sources requiring control.
How to design LEV for ease and safety of checking and maintenance.
The specifications for airflow, duct, filter, air mover, air cleaner, discharge, instrumentation and alarms.
The specification for in-use performance checks.
How to prepare a LEV user manual with schedules for maintenance and statutory thorough examination and test.
How to prepare a logbook for the system, recording checks, replacing parts etc

EFS use both internal and external consultants which are experts in Local Exhaust Ventilation. They have gained extensive knowledge and understanding within a variety of industrial processes through on-site experience.

Using this experience allows us all as a team to identify all the correct questions to efficiently move towards the best solution to your needs. The Process is key to understanding the containment, if we know the process then this makes it much easier to control the containment. Which is why we spend much attention to getting to know your process.

EFS’s initial consultations are free and you can contact us here, there is no obligation to proceed with proposals, although we are confident clients will be happy to continue with our services for many years to come due to our competitive prices and attention to detail. After consultation, the design process can proceed at the client’s request.

EFS LEV Test includes a site visit where we examine the systems and gain several readings. These readings will then be compared to the previous LEV test and the commissioning data to ensure the performance of the machine is has not dropped. Along with qualitative test which may be a dust lamp or smoke tube to confirm adequate control of exposure. The results will then be typed up into a report which includes photographs and a drawing of the system. This will be accompanied with a covering letter which will highlight any issues which require attention. Advice will be provided on recommended employee checks.

Planned Maintenance Schedules

Hoods and Enclosures / Source Control

Hoods have a wide range of shapes, sizes and designs. While they may look similar, they control contaminant clouds in three different ways. The ‘classification’ of hoods highlights their essential features and they fall into three basic categories: 1)enclosing hoods; 2) receiving hoods; and 3) capturing hoods. This classification applies in most circumstances. Sometimes hoods work in ‘mixed-mode’. Only when a LEV hood does not fit the classification does the supplier/designer need to consider design from first principles.

The type of hood or enclosure is influenced by the work being done. The hood or enclosure should not obstruct or cause ergonomic difficulties (e.g. manual-handling limitations or over-reaching). The hood/enclosure may need to be designed to capture/contain dust, fumes, mist, fibres, vapour or gas aerosols. The contaminant cloud or aerosol may be a slow release or a highly energised release caused by a power tool, for example. Dust from solids being dropped can temporarily overcome a system. The degree of containment around the emission point is crucial. The hood should be structured and placed at the emission point so as to entrain/contain the emission. For example, the airflow rate to a circular extraction duct with no hood attached will fall to about 10% of the in-duct flow rate at one diameter distance from the duct opening.

As the distance of the emission point from the hood increases, the LEV effectiveness decreases dramatically. There should be an indicator at the hood to show the system is performing correctly. Fume-hood cupboard and biological safety cabinet standards require visible indicators. These indicators can have numbers such as flow rate or negative pressure or colour-coded bands for acceptable ranges. Quantitative methods produce a reproducible measurement of performance. Measurements alone do not provide direct evidence of control effectiveness, but the records are available for future comparison, as benchmarks.

Pressure drop, or differential pressure, is the amount of static resistance experienced when operating a positive or negative pressure LEV. This pressure drop is typically measured across the filters in inches of water column (in. w.c.). Examples of some standard gauges used for this monitoring are a Magnehelic¨ gauge, Photohelic¨ switch/gauge and manometer. Pressure drop is a good indicator regarding the amount of dust that has collected on the filter media, and if continually monitored and logged, the condition of the filters themselves. New filters have the lowest pressure drop because of the inherent permeability of the media. As the filters develop a dust cake, some particulate embed themselves into the interstices of the filter media, and the pressure drop will increase accordingly. It is the filtering of the airstream through this accumulated dust cake that provides high-efficiency collection of fine particulate. In fact, the highest efficiency a dust collector can offer is just before the cleaning mechanism is initiated.

However, high differential pressures can cause bleed thru or blinding of the filter media. Therefore, it is suggested not to exceed the manufacturer’s recommended operating pressure drop. Keeping a daily log of a LEV`s differential pressure, from the time the filter media is new, will provide the opportunity to diagnose problems that may occur (i.e., an increase in dust emissions, reduced ventilation air at the dust source, shortened bag life, etc.). Following an initial seasoning or conditioning period of filters, the pressure drop should stabilise into a consistent operating range relative to the cleaning cycle, application and style of equipment. Therefore, at subsequent filter changes, this operating range can be predicted. Deviation from this historical level will alert an operator to investigate the cause of such an occurrence.

Any method used by the equipment to dislodge accumulated dust cake from the filter media is its cleaning system. This may be reverse air, shaker or pulse clean. Regardless of the style of cleaning, it is imperative that this system function properly at all times. Without an effective cleaning system, dust will continue to build on the bags. The resultant will be an increased pressure drop and reduced volume of ventilation air at the pick-up points. Further, airstream velocities within the ductwork will decrease and cause drop-out of dust in the ducts. This may choke the entire system and render it ineffective. As indicated on our Inspection Logs, cleaning systems require more than just periodic monitoring. It is suggested that all components of the system be regularly inspected, and corrections made in a timely manner. Besides the items noted on the attached logs, refer to your OEM’s Installation & Maintenance Manual to include other items specific to their equipment.

The hopper on a LEV is not to be used for storage of the collected product, unless originally designed to do so. Storing material in a hopper can lead to bridging of the dust, or it may set up as a solid mass requiring considerable labour and down time to correct. Material build up, if not discovered in time, can fill a hopper to its inlet and plug the unit. Further, with low density materials, the airstream may sweep the dust into the filter section, ruining filters and clogging the dust collector. It is strongly recommended that whatever method is used for material discharge (rotary valve, screw conveyor/pneumatic conveyor, etc.) it should be inspected frequently. This inspection should also be followed at shut down and filter changes.

Any particulate that can be seen discharging from the exhaust stack is considered visible emissions. These emissions are an indication that there is a breach in a seal or a broken (torn) filters. In either case, the leak must be found and corrected immediately. Not only will the emission cause a health concern and damage to property outside the plant, monetary fines imposed by the local authorities may also result. In addition, a fan located downstream of the collector can be damaged from abrasion or become imbalanced if this condition is not corrected quickly. The exhaust from a dust collector should be continually monitored and checked off in the Inspection Log. Besides visual inspections, one may consider incorporating a “Broken Filter Detector” into the clean air ductwork. Should a filter begin to fail or there be a leak in a filter seal, the particles that bypass the media will be detected. Typically, these detectors use triboelectric or scattered light technologies. These devices can be wired to an alarm horn, siren or flashing light for an immediate acknowledgement of an upset condition.

In a LEV, an exhaust fan is needed to accelerate ventilation air from the point of pick-up, through the ductwork and LEV filter media, and out the exhaust stack. A fan is selected to accommodate each application with respect to volume and pressure drop throughout the system. This pressure drop is calculated by evaluating the static resistance of the LEV, all ductwork and pickup points/hoods. Should an exhaust fan experience loose or worn belts or an imbalanced impeller, it will not exhaust the volume of air it was originally designed to handle. Without adequate ventilation air, a dust collection system will not operate effectively. Thorough fan inspections are to be performed on a semi-annual basis. However, any time unusual vibration, squealing, or other obvious variances from standard operation is observed, the original manufacturer is to be contacted for their evaluation and comment.

The most important item in a baghouse is the filter media because it allows for the accumulation and support of a dust cake. This dust cake is what provides high filtering efficiencies during operation. Periodic inspections of the filters is mandatory. Inspect the clean air side of the LEV for leaks. Should the pressure drop within a dust collector become extremely high, relative to historical data, it may be caused by excessive dust cake or blinding of the filters. Excessive dust cake is evident when visually inspecting the filters (when the dust collector is presumed to be clean) and finding them covered with a layer of the collected dust. Should this occur, one could suspect that the cleaning system is not functioning properly. However, if the dust cake has hardened to the filters and will not dislodge easily, the most probable cause is moisture in the LEV. Moisture in a dust collector may have resulted from dew point excursions, high moisture content in the process gas, in the compressed air supply, or a leak in the collector or ductwork that allowed water to enter the dust collector. The other obvious cause of high differential pressure may have been caused by blinding of the filter.

Local Exhaust Ventilation Experts In Scotland

As experts in Local Exhaust Ventilation, we offer a professional service covering all your LEV needs in Scotland & all surrounding areas.

Still got a question?




(Control of Substances Hazardous to Health)

COSHH is the law that requires employers to control substances that are hazardous to health. Most businesses use substances or products that are mixtures of substances. Some processes create substances. These could cause harm to employees, contractors and other people. Sometimes substances are easily recognised as harmful. Common substances such as paint, bleach or dust from natural materials may also be harmful.
Under regulation 9 of CoSHH, all Local Exhaust Systems (LEV) should be thoroughly examined & tested at least once every 14 months by a competent person. This is to ensure that your system remains effective at control.
Please note that testing should be carried out more frequent for certain processes. For further information refer to Table 18 on Page 83 of HSG258.

The HSE have a publication called controlling airborne contaminants at work – also known as HSG 258. It is available to purchase as a hard copy or download for free from the HSE website http://www.hse.gov.uk/pubns/books/hsg258.htm . It is written for the suppliers of LEV goods and services as well as employers and managers in medium businesses. It describes the principles and good practice of design, installation, commissioning, and testing cost-effective ‘ventilation controls’.
The book contains information about: the roles and legal responsibilities of suppliers, and of their clients as employers; competence; principles of good design practice for LEV hoods and their classification; ducts, air movers, air cleaners; and system documentation.


Employers need to ensure that LEV systems continue to work properly. There are several ways of checking this, such as using an anemometer, dust lamp or smoke tracer – with the work process running. But the simplest way is probably to use an airflow indicator. This will give the operator a simple indication that the hood is working properly. It becomes critical when the operator has to adjust a damper to get adequate airflow. The airflow indicator must indicate simply and clearly when the airflow is adequate.

EFS can supply and install a range of airflow indicators from standard monitor gauges or electronic indicators which is calibrated. Once the airflow reduces by 20% this shows the operator that the airflow in not adequate enough to control the substance.

Why is Thorough LEV testing important?

The purpose of a Thorough Examination and Test is to look very closely at the whole system and ensure that:
• It is suitable to adequately control the dust or fume.
• It offers adequate protection for the operator.
• It is in good physical condition.
• Your operators are using the equipment correctly.
Any failures highlighted by your LEV Examiner that are quickly addressed could prevent operators being diagnosed Respiratory diseases later on in life.

LEV Routine Checks By Employees

The people who carry out routine checks of the LEV system are usually employees or supervisors as they are working with the equipment daily, but may be service providers.
What employees need to know to carry out routine checks:-

The parts of a LEV system and their function.
How the LEV system should be used.
How to recognise a damaged part.
Simple checks that the LEV system is delivering its design performance and is effectively controlling emissions and exposure.

Why must my LEV contractor be competent?

HSG258 call for the client to ensure that their LEV inspector is competent. This is to ensure that your LEV inspector can spot the underlying issues and make the correct recommendations. The requirement for competence for suppliers of goods and services means that the extent and depth of their knowledge and capability must be sufficient to assess and solve the problems they are likely to meet.

What can I expect?
EFS LEV Test includes a site visit where we examine the systems and gain several readings. These readings will then be compared to the previous LEV test and the commissioning data to ensure the performance of the machine is has not dropped. Along with qualitative test which may be a dust lamp or smoke tube to confirm adequate control of exposure. The results will then be typed up into a report which includes photographs and a drawing of the system. This will be accompanied with a covering letter which will highlight any issues which require attention. Advice will be provided on recommended employee checks.

What do I need to do after the test?

• Read the covering letter of your report. We don’t expect you to understand the report but please read these pages which highlight any issues that require bringing to your attention. If you have any questions then please do not hesitate to contact us.
• Fix or repair any issues. If there are any issues highlighted then it is important to fix or repair these issues at the earliest opportunity.
• Log book. It is a requirement of HSG258 that you carry out regular checks of your system and record the results. The checks are to ensure that the system is of satisfactory condition in order to maintain acceptable system performance.
• Keep the report for 5 years. You are obliged to keep copies of this reports for at least five years. But don’t worry if you mislay or lose any copies, we keep copies too, and will issue a duplicate upon request, free of charge.