Improving air quality starts with choosing the right ventilation system
Good ventilation plays an important role in creating a safe and healthy workplace, but not all ventilation systems provide the same level of protection. Many businesses assume that opening windows or relying on an air conditioning system is enough to control airborne contaminants. While these methods can improve overall air quality, they are not designed to remove hazardous substances generated during work processes.
Understanding the difference between LEV vs general ventilation is essential for employers who work with dust, fumes, vapours, mists, or other hazardous airborne contaminants. Choosing the wrong type of ventilation could leave employees exposed to harmful substances and increase the risk of failing to meet legal duties under the Control of Substances Hazardous to Health (COSHH) Regulations.
This guide explains how both systems work, when each should be used, and why Local Exhaust Ventilation (LEV) is often the preferred solution for controlling hazardous substances in the workplace.
LEV vs general ventilation: General ventilation improves overall air quality by introducing fresh air and diluting contaminated air within a workspace. In contrast, Local Exhaust Ventilation (LEV) captures hazardous dust, fumes, vapours, and mists directly at their source before they can spread into the workplace. Where hazardous airborne contaminants are generated, LEV is often the most effective engineering control and is frequently required to help businesses comply with COSHH.
General ventilation is designed to improve the overall quality of air within a building by supplying fresh air and removing stale or contaminated air. Rather than targeting a specific source of contamination, it works by diluting airborne pollutants throughout the entire space.
General ventilation can be achieved in two main ways:
These systems are commonly used to improve comfort, regulate temperature, control humidity, and reduce odours in workplaces.
General ventilation is well suited to environments where hazardous substances are not produced, such as:
Although general ventilation improves indoor air quality, it is not designed to capture hazardous contaminants at the point where they are generated. As a result, harmful substances can continue to circulate throughout the workplace before being diluted.
Unlike general ventilation, Local Exhaust Ventilation (LEV) is specifically designed to control hazardous airborne contaminants at their source.
Instead of allowing contaminants to spread through the workplace, LEV captures them as they are produced before extracting, filtering, or safely discharging the air.
A typical LEV system may include:
Because contaminants are removed before employees breathe them in, LEV provides a much higher level of protection than general ventilation alone.
LEV systems are commonly used in workplaces where hazardous substances are generated, including:
Examples of LEV equipment include:
A properly designed LEV system should always be suited to the process it is controlling. This is why many organisations seek professional advice when designing or upgrading extraction systems to ensure they meet operational requirements and support compliance with COSHH.
Although both systems improve workplace air quality, they are designed for different purposes.
| General Ventilation | Local Exhaust Ventilation (LEV) |
| Improves overall indoor air quality | Captures contaminants at source |
| Dilutes contaminated air | Removes hazardous substances before they spread |
| Suitable for low-risk environments | Designed for hazardous work processes |
| Uses natural or mechanical airflow | Uses specialist extraction equipment |
| Supports comfort and fresh air | Protects employees from airborne contaminants |
| Does not replace LEV where hazardous substances are present | Helps businesses comply with COSHH requirements |
The key difference is that general ventilation manages the air within an entire room, while LEV targets the source of contamination before employees are exposed.
General ventilation is often suitable where workplaces do not generate hazardous airborne substances and the primary objective is maintaining comfortable indoor conditions.
Examples include offices, classrooms, retail environments, reception areas, and storage facilities where only low levels of dust or odours are present.
It may also be used alongside heating and cooling systems to improve thermal comfort and provide a continuous supply of fresh air.
However, employers should not assume that general ventilation alone provides adequate protection against hazardous substances. If work activities generate harmful dust, fumes, vapours, or mists, a COSHH risk assessment should determine whether additional engineering controls are required.
Local Exhaust Ventilation becomes essential when hazardous airborne contaminants cannot be adequately controlled through elimination, substitution, or general ventilation.
A COSHH risk assessment may identify the need for LEV in workplaces carrying out activities such as:
For example, opening workshop doors may improve airflow, but it will not prevent employees from breathing welding fumes released directly into their breathing zone. An LEV extraction arm positioned close to the welding process captures those fumes immediately, significantly reducing exposure.
Similarly, dust generated while machining hardwood can remain suspended in the air for long periods if only general ventilation is used. A dedicated dust extraction system removes the dust at source before it spreads throughout the workshop.
Employers should also remember that LEV is not simply a recommendation in many of these environments. It is often the most appropriate engineering control identified through a COSHH risk assessment and plays an important role in helping businesses comply with workplace health and safety legislation.
When hazardous substances are released into the air, removing them before they reach employees is far more effective than simply diluting them throughout the workplace.
This is why LEV is considered an engineering control under the COSHH Regulations. By capturing contaminants at source, it significantly reduces the amount of hazardous material entering the breathing zone and helps maintain a safer working environment.
Effective LEV systems can help control exposure to:
Controlling exposure is essential because repeated inhalation of hazardous airborne contaminants can contribute to serious occupational illnesses, including occupational asthma, chronic obstructive pulmonary disease (COPD), silicosis, and certain occupational cancers. Implementing effective source extraction helps reduce these risks while supporting compliance with workplace exposure limits and COSHH requirements.
Installing an effective LEV system is only part of meeting your legal responsibilities. Employers must also ensure the system remains in efficient working order throughout its lifespan.
Under the Control of Substances Hazardous to Health (COSHH) Regulations, LEV systems used to control hazardous substances must undergo a Thorough Examination and Test (TExT) at least every 14 months in most workplaces. Certain specialist applications may require more frequent examinations depending on the process and level of risk.
A Thorough Examination and Test typically includes:
Routine LEV Maintenance is equally important. Over time, filters can become blocked, ductwork may develop leaks, and extraction hoods can be repositioned, all of which reduce the effectiveness of the system.
Regular maintenance helps maintain airflow, reduce unexpected downtime, improve energy efficiency, and extend the lifespan of extraction equipment.
Employers should also ensure that anyone responsible for testing or maintaining an LEV system is competent and works in accordance with recognised guidance, such as HSG258: Controlling Airborne Contaminants at Work.
While LEV is one of the most effective methods of controlling airborne contaminants, it forms just one part of a wider occupational hygiene programme.
DevineAir also provides a range of workplace assessment services to help organisations identify and manage other health risks, including:
These services provide businesses with a more complete understanding of their working environment and support a proactive approach to employee health and safety.
Understanding the difference between LEV vs general ventilation is essential when selecting the right control measures for your workplace.
General ventilation plays an important role in improving overall indoor air quality and maintaining comfortable working conditions. However, where hazardous dust, fumes, vapours, or mists are generated, it should not be relied upon as the primary means of protecting employees.
Local Exhaust Ventilation provides a far more effective solution by capturing contaminants at source before they spread into the workplace. Combined with a suitable COSHH risk assessment, routine maintenance, and regular Thorough Examination and Testing, LEV helps businesses reduce employee exposure, comply with health and safety legislation, and create safer working environments.
At DevineAir, we help organisations design, test, maintain, and optimise LEV systems that meet the demands of their workplace. Alongside our specialist LEV services, we also provide occupational hygiene assessments to support long-term workplace compliance and employee wellbeing.
If you're unsure whether general ventilation is sufficient for your workplace or need advice on your LEV system, contact the DevineAir team. We'll help you assess your workplace, recommend the most appropriate control measures, and keep your ventilation systems performing effectively.
No. General ventilation improves overall air quality by diluting airborne contaminants, but it does not capture hazardous substances at their source. Where hazardous dust, fumes, vapours, or mists are generated, a COSHH risk assessment may identify LEV as the appropriate engineering control.
Not every workplace requires LEV. However, if a COSHH risk assessment determines that hazardous airborne contaminants cannot be adequately controlled through other methods, LEV is often necessary to reduce employee exposure.
HVAC systems are designed to heat, cool, and ventilate buildings for occupant comfort. LEV systems are specifically engineered to capture hazardous airborne contaminants at source before they can spread throughout the workplace.
In most workplaces, LEV systems used to control hazardous substances should undergo a Thorough Examination and Test every 14 months. Some specialist systems require more frequent examinations depending on the work being carried out.
LEV systems should be inspected and tested by competent professionals with the appropriate training, technical knowledge, and practical experience. Using qualified specialists helps ensure systems remain effective and compliant with current HSE guidance.
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