Whether a local exhaust ventilation (LEV) system can truly protect workers’ health, the demands of employers are indeed important, but the more direct determining factor lies in the professional capabilities of suppliers and designers. Employers can state their need to control welding fumes, but how to transform this demand into an effective engineering system – where to place the exhaust hood, what route the pipes should take, what size of fan to choose, and what precision of filter material to select – all these technical decisions are in the hands of suppliers and designers.
An unqualified design will result in the equipment operating while pollutants still spread to the breathing zone of workers as usual. This is not only a waste of investment but also a dereliction of duty to the health of workers. Therefore, the HSG258 guideline issued by the UK HSE has specially set aside a chapter to detail the knowledge and skills that LEV suppliers and designers must master.
I. Legal Liability: An Inescapable Professional Bottom Line
LEV suppliers and designers must first clarify their legal status and corresponding responsibilities.
In LEV projects, suppliers and designers act as technical advisors and implementers for the employer (system owner). According to the COSHH regulations (Control of Substances Hazardous to Health Regulations), the employer bears the ultimate legal responsibility for controlling workers’ exposure, but suppliers and designers also bear equally serious responsibilities at the technical level – the equipment they provide must be “fit for purpose”, that is, verified to effectively control pollutants.
The specific contents of legal liability include: the provided LEV system must undergo commissioning and acceptance testing to prove that it meets the design performance; suppliers must provide complete user manuals and logbooks; the system design must facilitate subsequent inspection and maintenance. If the system delivered by the supplier has design flaws or insufficient performance, resulting in damage to workers’ health, the supplier will also bear legal consequences.
Therefore, when undertaking projects, suppliers and designers must possess the corresponding capabilities. HSG258 clearly states that every link in the LEV supply chain – from design to installation to testing – must be carried out by competent personnel. This is not a vague requirement but can be quantified: designers need to master the basic knowledge of fluid mechanics, ventilation engineering, and occupational health; testers need to be trained and certified by professional institutions.
II. Effective Communication: Understanding the Employer and Coordinating with the Installer
The LEV project involves three parties: the employer (who sets the requirements), the designer (who formulates the plan), and the installer (who implements it). Effective communication among these three parties is the prerequisite for the project’s success.
The first task for the designer is to understand the employer’s true needs. Employers are often not ventilation experts and can only describe problems with vague terms like “a lot of dust” or “a pungent smell”. The designer’s job is to convert these ambiguous descriptions into technical parameters – the types and concentrations of pollutants, the locations and sizes of pollution sources, the operation methods of the process, and the space limitations of the workstations. This information needs to be actively sought by the designer rather than passively waiting for the employer to provide it.
Communication between the designer and the installer is equally crucial. A well-designed system can have its performance significantly compromised if the installer makes arbitrary changes – such as deviating from the designed pipe routing, increasing the number of elbows, or shifting the position of the exhaust hoods. HSG258 emphasizes that installers must not make any changes to the system design without the designer’s consent.
The results of communication should be documented in writing: clear technical specifications, detailed construction drawings, and explicit acceptance criteria. Verbal promises cannot serve as a basis for resolving subsequent disputes.
III. Hazardous Substance Identification: What is the Object of Control?
Not knowing the enemy makes it impossible to choose the right weapon. Before starting the design, the designer must clearly identify what hazardous substances need to be controlled.
The information on hazardous substances should include: the chemical composition and physical form of the substance (dust, smoke, vapor, droplets), toxicity level and occupational exposure limit (OEL/WEL), particle size distribution (determining deposition location and filtration difficulty), generation rate and concentration range.
The requirements for the LEV system vary greatly among different substances. For example:
– Dry dust (woodworking dust, silica dust): Use cartridge or bag filters, and the filtration precision should match the particle size.
– Welding fume (submicron particles): High-efficiency filter materials (such as membrane-coated filter media) are required; ordinary filter paper is insufficient.
– Oil mist (metalworking fluid mist): Coalescing filters are needed, which separate based on the principles of impact and coalescence.
– Organic vapors (spray painting exhaust): Cannot be removed by physical filtration; activated carbon adsorption or catalytic combustion should be used.
In addition, pollutants of different natures must not be discharged together. If two substances may undergo a chemical reaction when mixed (such as the generation of heat and salt from the mixture of acid and alkali waste gas, or the formation of highly toxic hydrogen cyanide from the mixture of cyanide and acid), independent LEV systems must be set up. The national standard GB/T 35077-2025 “Mechanical Safety – Safety Requirements for Local Exhaust Ventilation Systems” also emphasizes that pollution sources with a risk of mixture should be treated independently.
IV. Design of Exhaust Hoods: The First Line of Defense for Capturing Pollutants
Exhaust hoods are the most crucial component of a LEV system – they are the entry point for pollutants into the system and the key factor determining the capture efficiency. A well-designed exhaust hood can capture pollutants the moment they are generated; conversely, a poorly designed exhaust hood, even with the most efficient fans and filters at the back end, cannot prevent pollutants from spreading into the breathing zone of workers.
HSG258 classifies exhaust hoods into three types: enclosing hood, receiving hoods, and capturing hoods. Designers need to select the appropriate type based on process conditions:
Enclosing hoods: Completely enclose the pollution source, offering the highest control efficiency. They are suitable for highly toxic substances or high-concentration dust generation points. The drawback is that they impose significant operational constraints.
Receiving hoods: Utilize the kinetic energy of the pollutants (thermal updraft or jet force), with the exhaust hood placed along the movement trajectory to passively receive them. They are suitable for thermal processes (furnaces, ovens) and high-speed jet operations (sandblasting).
Capturing hoods: Actively draw in pollutants through the suction of a fan, and are the most widely used. They are suitable for most scenarios such as welding, grinding, woodworking, etc.
The core of the design of the exhaust hood is to establish a sufficient control wind speed – that is, at the point of pollutant release, the airflow velocity generated by the exhaust hood must be sufficient to overcome the initial kinetic energy of the pollutant and the interference of the lateral airflow in the workshop, and draw the pollutant towards the hood opening. The value of the control wind speed depends on the toxicity of the pollutant and the release conditions. It is particularly important to note that the control wind speed must meet the requirements at the farthest end of the pollutant release point – the dust-producing point farthest from the exhaust hood is the benchmark for judgment. If only the wind speed near the exhaust hood opening is measured and the farthest point is ignored, the actual capture efficiency may be much lower than expected.
In the layout of exhaust hoods, the following basic principles should be followed: exhaust hoods should be as close as possible to the pollution source; the suction direction should be consistent with the direction of pollutant escape; exhaust hoods should not be placed near doors and windows; the layout should prevent polluted air from flowing through the breathing zone of workers. In addition, the shape and size of the exhaust hood should match the geometric characteristics of the pollution source – for long and narrow dust-generating surfaces, slot-type suction ports should be used; for point sources of pollution, circular or bell-shaped suction ports can be adopted. A common engineering experience is that the size of the exhaust hood should not be less than 1.5 times the area of the pollution source, and the distance between the hood opening and the pollution source should not exceed 1.5 times the equivalent diameter of the hood opening. Beyond this range, the capture efficiency will drop sharply. These seemingly simple principles are often overlooked in actual engineering – the result is that the equipment is installed and the fan is running, but the smoke and dust still float in front of the workers’ eyes.