What's measured
For a ducted fume cupboard doing general work with hazardous substances, the accepted best-practice face velocity is around 0.5 metres per second at the sash, and if it drops below 0.45 the cupboard should come out of use until it is put right. The number on its own, though, no longer tells the whole story.
The short answer
For a ducted fume cupboard doing general work with hazardous substances, the accepted best-practice face velocity is around 0.5 metres per second at the sash, and if it falls below 0.45 the cupboard should come out of use until it is put right. But airspeed alone no longer tells the whole story of whether a cupboard protects the person in front of it.
The detail
Face velocity is the speed of air drawn in through the open sash. The widely used benchmark is 0.5 m/s, plus or minus ten per cent, with the sash set to a 500 millimetre opening - the maximum safe opening marked on many cupboards is simply the aperture at which that velocity is reached. Below about 0.45 m/s the inward pull is treated as too weak to contain reliably, and older guidance noted that below roughly 0.3 m/s a cupboard is unlikely to be effective at all. Some higher-hazard work, such as certain radioactive materials, calls for a higher velocity again.
The British and European standard, BS EN 14175, is careful here: it does not actually set a required face velocity. What it specifies is that during type-testing and commissioning the velocity across an empty cupboard should not vary by more than about ten per cent from the average, and it puts the real emphasis on containment measured with tracer gas. Too much face velocity is its own problem - it creates turbulence around the operator and the sash that can pull contaminant back out rather than hold it in.
What it means for you
In practice a thorough examination checks the face velocity across a grid of points at the sash, confirms the readings do not vary excessively, and increasingly looks at containment rather than airspeed alone. Performance is judged against what the manufacturer confirmed at commissioning, which is why the commissioning figures matter as much as any generic benchmark.
A fume cupboard is examined and tested on the same fourteen-month cycle as other LEV under COSHH. A reading that has crept down since the last test, or that varies wildly across the sash, is a sign the extract, the make-up air or the cupboard itself has changed - and a cue to investigate before someone is exposed.
The service behind the guide
We test fume cupboards to face velocity and containment against their commissioning performance, with a clear report and remedial actions where a unit falls short.
Questions
Best practice for general hazardous work is around 0.5 m/s at a 500mm sash opening, within about ten per cent. Below 0.45 m/s the cupboard should be taken out of use until remedial work is done.
No. The standard does not specify a required face velocity. It focuses on containment measured with tracer gas, and asks that velocity across an empty cupboard vary by no more than about ten per cent during type-testing and commissioning.
Yes. Excessive face velocity creates turbulence around the sash and operator that can actually pull contaminant back out of the cupboard, so more is not always better.
Like other LEV under COSHH, a fume cupboard must have a thorough examination and test at least every fourteen months by a competent person.
Face velocity alone does not prove a cupboard keeps fume off the operator. Tracer-gas containment testing measures what actually escapes at the sash, which is a more direct check of protection.
Phoenix Duct Clean · by the numbers
We test fume cupboards to face velocity and containment against their commissioning performance, on the fourteen-month cycle, with a clear report and remedial actions.