Dilution Ventilation
Strategy & Controls · Strategy & Controls overview
Dilution ventilation reduces the concentration of airborne contaminants by introducing fresh air and removing a corresponding volume of contaminated air from the space. It is a legitimate control for low-toxicity, widely distributed releases — but it is consistently misapplied where source capture is what the risk actually requires.
What dilution ventilation is
Dilution ventilation, sometimes called general ventilation, works on a simple principle: contaminants released into a space are mixed with a larger volume of clean air so that the average concentration in the breathing zone falls below the level of concern. It does not stop the release; it spreads it.
Dilution can be achieved through mechanical supply and extract systems, roof extract with controlled inlets, or in some cases natural ventilation through high- and low-level openings driven by buoyancy and wind. In every case the design must account for where contaminants are released, where workers stand, and how air actually moves through the space.
When dilution ventilation may help
Dilution is appropriate when the contaminant is of relatively low toxicity, when the release is small, intermittent or spread over a wide area, when no single source dominates, and when workers are not standing immediately downstream of a release point. Heat and moisture loads, low-level solvent vapours from many small sources, and residual fume after source capture are typical examples.
Used in the right setting, dilution is energy-efficient, low-maintenance and flexible. It is also valuable as a complement to local exhaust ventilation, removing the residual fraction that capture inevitably misses.
When dilution is not enough
Dilution is a weak control for highly toxic, sensitising or carcinogenic substances, for high-rate point releases, and for any contaminant where exposure must be kept as low as reasonably practicable. The reasons are physical: dilution takes time, mixing in real workplaces is imperfect, and workers close to the source breathe the undiluted cloud before any mixing can occur.
Relying on dilution where source capture is needed is one of the most common findings in HSE inspections and exposure investigations. The COSHH hierarchy of control is explicit that engineering controls — and within that LEV at the source — sit above general ventilation as a means of reducing exposure.
Fresh air supply, general ventilation and source capture
These terms describe a spectrum. Fresh air supply is the volume of outdoor air introduced to the building; general ventilation is the wider movement of air across a space for comfort and broad dilution; source capture is engineered removal of a contaminant at the point of release. They are complementary, not interchangeable.
A well-designed workplace usually combines all three. Source capture handles the dominant releases; general ventilation manages residual contamination, heat and odour; fresh air supply provides the make-up volume that allows extraction systems to work without depressurising the building.
Air change, mixing and contaminant spread
Air change rate (the volume of air supplied per hour divided by the room volume) is a useful design number but a poor proxy for actual control. Real workplaces have dead zones where air barely moves, short-circuit paths where supply air leaves through the nearest extract before reaching the breathing zone, and pressure imbalances that drag contaminated air into clean areas.
Effective dilution depends on how the air is delivered and removed, not just how much. Diffuser placement, throw, induction, extract location and the position of physical obstructions all influence whether the design air change rate translates into real exposure reduction.
How dilution ventilation is assessed
Assessment combines volume measurement with behavioural observation. Supply and extract volumes are measured at diffusers and grilles to confirm balance; smoke or tracer methods reveal whether air actually moves from clean zones to contaminated zones; CO₂ and temperature mapping identify under-ventilated areas; and exposure data — where available — links the ventilation pattern to actual worker risk.
Findings typically distinguish between problems of quantity (not enough air) and problems of distribution (enough air, wrongly delivered). The corrective actions are very different, and a measured assessment avoids spending capital on bigger fans when the real issue is a poorly placed diffuser.
Frequently asked questions
Can dilution ventilation replace LEV?
Only where the risk assessment supports it. Dilution is not an acceptable substitute for source capture where contaminants are highly toxic, sensitising, carcinogenic, or released in significant quantities at identifiable points. In those cases LEV is required and dilution serves only as a supporting control.
What air change rate is appropriate?
There is no single correct figure. Rates are set by the contaminant load, the heat and moisture load, occupancy and the workplace risk assessment. Quoted figures in design guides are starting points, not compliance targets — actual performance depends on how the air is distributed across the space.
Is natural ventilation acceptable?
It can be, for low-risk spaces with predictable releases. But natural ventilation is weather- and season-dependent and is not generally appropriate as the sole control for industrial processes with significant contaminant generation.
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