Airflow Assessment
Airflow & IAQ · Airflow & IAQ overview
An airflow assessment goes beyond raw measurement. It interprets airflow data alongside building layout, process behaviour and operator practice to explain why the air is moving the way it is, and what to do about it. It is the bridge between numbers on a clipboard and decisions about ventilation, exposure control and indoor air quality.
What an airflow assessment reviews
An airflow assessment looks at the workplace as a connected air system. It reviews supply and extract volumes, pressure relationships between rooms, capture performance at sources, air patterns around occupants, and how building elements — doors, partitions, roller shutters, mezzanines — change the way air moves.
The assessor brings airflow measurements together with observations of process, occupancy and operator behaviour, and forms an engineering judgement about whether the ventilation is fit for purpose for the contaminant control and comfort objectives of each area.
Difference between airflow testing and airflow assessment
Airflow testing produces the data: velocities, volumes, pressures, directions. An airflow assessment interprets that data in the workplace context. Testing alone tells you what the air is doing; assessment tells you whether it is doing the right thing and what needs to change.
In practice the two activities are usually combined, but they are different deliverables. A testing report is a calibrated dataset; an assessment report is a prioritised set of findings and recommendations supported by that dataset.
How supply and extract balance is reviewed
A fundamental step in any airflow assessment is establishing whether the supply and extract volumes for each space are in balance, and whether that balance is creating the pressure relationships the building needs. The assessor measures supply airflow at diffusers and make-up openings, extract airflow at grilles and LEV discharge points, and compares the net difference against the intended pressurisation strategy.
Where extract exceeds supply, the space is under negative pressure. That can be desirable in contaminated process areas because it limits outward leakage, but it can also pull unconditioned air through poorly sealed doors, creating draughts and thermal discomfort. Where supply exceeds extract, the space is over-pressurised; clean air is pushed out through cracks, and contaminated air from neighbouring areas may be prevented from entering, but the accumulation of pollutant in dead zones can go unnoticed.
The assessor also tracks unintended make-up paths — air entering through loading-bay gaps, roller shutters, ceiling voids and poorly sealed ductwork — because these bypass the filtration and tempering that designed supply routes are supposed to provide.
Identifying dead zones, short-circuiting, draughts and pressure problems
Airflow pathologies are often invisible until they are traced with smoke, tracer methods or a systematic grid of velocity readings. The assessor maps the space to locate where air is not doing what the design assumes.
- Dead zones are identified by low or zero velocity readings at breathing-zone height, confirmed with smoke that lingers rather than dispersing. They usually occur behind obstructions, in corners, under mezzanines or where supply jets fail to reach.
- Short-circuiting is detected when smoke released at a supply diffuser is drawn directly into a nearby extract grille without passing through the occupied zone. Pitot traverses and temperature profiles help confirm the bypass path.
- Draughts are traced to their source: external doors, air curtains set too aggressively, cooling fans, or cross-flow from adjacent spaces. The assessor measures velocity and turbulence intensity at occupied positions and compares against comfort criteria.
- Pressure relationship problems are diagnosed by differential pressure readings across doorways and transfer openings, correlated with door-state observations. A pressure differential that collapses every time a forklift passes indicates a control failure, not a design success.
- Capture disruption at LEV hoods is observed with dust lamps and smoke to see whether draughts or cross-flows are deflecting the contaminant cloud away from the hood face.
What an assessment looks at
A workplace airflow assessment reviews several layered phenomena that together determine how air and contaminants move.
- Room air movement patterns and mixing effectiveness at occupied and process zones.
- Pressure cascade across the building and unintended transfer paths between rooms.
- Supply and extract balance and the effect of uncontrolled make-up air.
- Stagnant pockets, bypass routes and comfort issues in the occupied zone.
- Draughts and cross-flows that disturb capture at LEV hoods.
- Capture velocity and envelope at hoods, booths and partial enclosures relative to source location.
How findings are interpreted
Findings are interpreted against three reference points: the original ventilation design intent where it exists, HSE guidance such as HSG258 and HSG202, and the contaminant control or comfort objective for each space. Where design data is missing — common in older industrial buildings — the assessor reconstructs reasonable performance targets from the process and the exposure limits involved.
Recommendations are then ordered by exposure significance and practical feasibility. Quick wins such as rebalancing dampers, removing obstructions, repositioning hoods, or sealing leakage paths are flagged separately from larger capital actions such as new extraction plant or building rework.
Practical recommendations
A useful airflow assessment leaves the organisation with clear actions, not just a description of the problem. Typical recommendations include rebalancing supply and extract to restore intended pressure relationships, repositioning hoods or adding flanges to extend the capture envelope, sealing transfer paths between contaminated and clean areas, adjusting door discipline or installing air curtains, and scheduling filter or fan maintenance to recover lost extract volume.
Where the assessment reveals fundamental shortfalls — undersized extraction, unsuitable hood geometry, or general ventilation that cannot achieve the required dilution — those findings feed into a separate ventilation upgrade brief rather than being treated as routine maintenance.
Frequently asked questions
How is an airflow assessment different from a ventilation risk assessment?
A ventilation risk assessment is a higher-level review of whether the ventilation strategy is suitable for the hazards present, including paperwork, design, maintenance and user practice. An airflow assessment is the technical, measurement-led component that demonstrates whether the air is actually moving as the strategy assumes.
When should an airflow assessment be carried out?
After layout or process changes, when exposure or comfort complaints arise, before specifying ventilation upgrades, and as part of periodic ventilation reviews. It is also valuable in advance of an LEV test where the system has not been independently reviewed for some time.
Can the assessment cover the whole building?
Yes. Although individual processes are often the focus, airflow behaviour in one area affects neighbouring areas through pressure relationships and transfer paths. A whole-building view is usually more revealing than a series of isolated checks.
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