LEV stands for local exhaust ventilation, a system designed to capture airborne contaminants like dust, fumes, vapors, and mists right at the source before workers breathe them in. Think of the hood above a welding station or the suction arm over a woodworking bench. Instead of letting hazardous particles spread through an entire workspace, an LEV system pulls contaminated air away from the worker’s breathing zone and either filters it or vents it outside.
How LEV Differs From General Ventilation
Opening windows or running ceiling fans to push fresh air through a building is general (or dilution) ventilation. It works by spreading contaminants out until concentrations drop to safer levels, but it never actually removes the hazard at its source. LEV takes the opposite approach: it intercepts contaminants the moment they’re created, before they ever reach the surrounding air.
General ventilation is fine for low-hazard vapors that are released evenly across a space. LEV becomes the better choice when materials are more toxic, when dust or particles are large enough to settle on surfaces, when emissions come from a specific point, or when workers are stationed right next to the source. In colder climates, LEV also saves energy because you’re exhausting a small, targeted stream of air rather than flushing and reheating an entire building.
The Four Main Components
Every LEV system, whether it’s a small benchtop unit or a factory-wide installation, has the same four basic parts.
- Hood: The entry point. This is the structure that encloses or partially surrounds the source of contamination. Its shape and position determine how effectively the system captures contaminants.
- Ductwork: A network of pipes or channels that carries the contaminated air from the hood to the cleaning and discharge stages.
- Air cleaning device: A filter, separator, or other unit that removes particles or chemicals from the airstream before the air is released.
- Fan: The air-moving device, typically a centrifugal fan, that creates the suction pulling air through the entire system.
Types of Hoods
The hood is the most critical part of the system because it’s where capture happens. There are three broad types, each suited to different situations.
Enclosing hoods surround the source as completely as possible, like a glove box or a spray booth. Because the contaminant is generated inside the enclosure, these hoods need the least airflow to work effectively. The more enclosed the operation, the lower the energy cost.
Capturing hoods sit near the source without surrounding it. A slot hood along the edge of a tank or a flexible arm positioned beside a soldering station are common examples. These rely on drawing air fast enough across the gap between the hood and the source to pull contaminants in before they escape. The closer the hood is to the source, the better it performs, because suction strength drops off rapidly with distance.
Receiving hoods (often called canopy hoods) are mounted above a process and catch contaminants that naturally rise, such as hot fumes or steam. They work well for heat-generating tasks but are not appropriate for toxic materials, because the rising plume passes through the worker’s breathing zone on its way up to the hood.
How Air Gets Cleaned
Once contaminated air travels through the ductwork, it reaches the air cleaning device. The two most common technologies are HEPA filtration and cyclone separation, and many modern systems use both in sequence.
HEPA filters use a dense mat of fibers, usually fiberglass, arranged in a random pattern that forces air through a microscopic maze. Particles get trapped through a combination of physical interception, impact, diffusion, and electrostatic attraction. A true HEPA filter captures at least 99.97% of particles down to 0.3 microns, which is roughly 200 times smaller than a grain of fine sand.
Cyclone separators work on a completely different principle. Dust-laden air enters a cone-shaped chamber at an angle, creating a spinning vortex. Centrifugal force flings heavier particles outward toward the chamber walls, where they lose speed and drop into a collection bin. Cyclones are excellent at removing larger debris but less effective with very fine dust.
Hybrid systems combine these approaches in stages: one or two cyclone stages strip out the bigger particles first, reducing clogging and extending the life of the downstream filters, followed by a pleated filter for fine dust and a final HEPA stage for the smallest particles.
Where LEV Systems Are Used
LEV shows up across a wide range of industries. Woodworking shops use it to capture sawdust at the blade or sander. Welding bays use extraction arms to pull metal fumes away from the welder’s face. Pharmaceutical manufacturing relies on enclosed LEV systems to contain potent drug powders. Nail salons, dental labs, paint shops, bakeries producing flour dust, and factories handling chemicals all use some form of local exhaust ventilation.
The common thread is any workplace where a process generates airborne substances that could harm a worker’s lungs, skin, or overall health if left uncontrolled.
Monitoring System Performance
An LEV system only protects workers if it’s actually pulling enough air. Pressure gauges, often simple manometers, are recommended on each hood duct so that the people using the system can check at a glance whether airflow is adequate. These gauges should display a clear indication of the acceptable pressure range.
For systems with filters, a differential pressure gauge across the filter shows whether the filter is becoming clogged or failing. When the pressure drop moves outside the calibrated range, it’s a signal to replace or clean the filter. Daily visual checks by operators, combined with these simple instruments, catch most performance problems before they become a health risk.
Legal Requirements for Testing and Maintenance
In the UK, the Control of Substances Hazardous to Health (COSHH) regulations require employers to maintain LEV systems in efficient working order. Beyond routine maintenance following the manufacturer’s recommendations, every LEV system must undergo a thorough examination and test by a competent person at least every 14 months. The employer is required to keep a copy of each test report for a minimum of five years.
All user checks, servicing, and maintenance should be logged in the system’s logbook. This paper trail serves two purposes: it demonstrates legal compliance during inspections, and it creates a performance history that helps identify gradual declines before they lead to worker exposure. In the United States, OSHA sets similar expectations through its general duty clause and industry-specific standards, though the specific testing intervals vary by regulation.