Ducted vs Recirculating Fume Cupboards: Which Is Right for Your Lab
James Cox
Project Manager
Estimated reading time: 8 minutes
When you're specifying a new fume cupboard, sash width and worktop material are the easy decisions. The one that actually shapes the project is whether the extract goes outside the building at all. A ducted fume cupboard exhausts continuously to atmosphere. A recirculating (also called ductless or filtered) cupboard cleans the air through a filter and puts it straight back into the room. Get that choice right and everything downstream, running costs, compliance, and what you're allowed to work with, follows naturally.
This guide covers how each type works, what the current standards require, and what tends to matter most once you look past the install and into the years of actually running the equipment.
How Each Type Works
Ducted
Air is drawn continuously through the sash opening, across the work surface, and out through ducting to a discharge point outside, usually a stack at roof level. Because that air leaves the building for good, it has to be replaced: a ducted system needs a source of make-up air, conditioned and brought back into the room, or you end up depressurising the lab and fighting doors and other extract systems. Ducted fume cupboards are covered by BS EN 14175, the multi-part standard we've covered in detail elsewhere on this blog, and are type tested to hold a face velocity of 0.3 to 0.5 m/s across the sash opening.
Recirculating
Air is drawn across the work surface the same way, but instead of leaving the building it passes through a filter stage, typically activated carbon for solvent vapours, HEPA for particulates, and is returned to the room. There's no duct run, no roof penetration, and no make-up air demand. The trade-off is that containment now depends entirely on the filter being the right one for what's actually used underneath the hood, and on that filter being changed before it saturates. Recirculating cabinets have their own dedicated standard, BS EN 17242:2025, published to cover exactly this type of equipment; it explicitly excludes conventional ducted cupboards, which stay under BS EN 14175.
What the Standards and COSHH Require
Two different standards, one shared testing duty
BS EN 14175 (all parts) sets design, safety, and testing requirements for ducted fume cupboards, and has done so for over two decades. BS EN 17242:2025 does the equivalent job for recirculatory filtration fume cabinets, covering design, manufacturing, and both type and on-site testing. It's a newly published standard, replacing the older BS 7989:2001, which has been withdrawn.
Whichever type you install, it counts as local exhaust ventilation (LEV) under COSHH Regulation 9, and both need a thorough examination and test (TExT) by a competent person at least every 14 months, with records kept for at least 5 years. A filter change schedule is not a substitute for statutory testing, and neither is a service visit that doesn't check performance against the system's designed duty.
What Changes Once the Cupboard Is Running
The specification decision is a one-off. What follows it isn't. Three things are worth weighing before you commit to either type:
Ongoing cost and dependency
- Consumables. A ducted system's ongoing cost is energy plus routine fan and duct servicing. A recirculating cabinet adds filter media on top of that: bought, monitored for saturation, and disposed of as hazardous waste, for as long as the cabinet is in service.
- Chemical inventory changes. A ducted cupboard doesn't need reassessing when the substances in use change. A recirculating cabinet does: the filter has to stay matched to whatever's actually going under the hood, which means revisiting the specification each time the inventory moves.
- How settled the standard is. BS EN 14175 has a long track record of type-test and on-site testing practice behind it. BS EN 17242 was only published in 2025, so the guidance and testing infrastructure around it is still comparatively new.
Filter Suitability: Why It's Not a Drop-In Swap
A carbon filter is built to adsorb specific groups of chemical vapours, not everything that might end up in the hood. A filter matched to common solvents won't necessarily do anything useful against a substance from a different chemical family, and once a filter saturates, contaminant starts passing straight through instead of being trapped: what's called breakthrough. A recirculating cabinet's protection depends on someone actually tracking what goes in the box and when the filter needs changing.
That's why institutions that use recirculating cabinets alongside conventional fume cupboards typically restrict them to lower-hazard, well-defined work rather than treating them as a like-for-like swap. Imperial College London's code of practice for fume cupboards, for example, keeps recirculating units out of the higher COSHH hazard groups and away from unsealed radioactive material, and puts responsibility for filter monitoring and statutory testing on the department running the equipment rather than central estates. That's one institution's policy rather than a blanket legal limit, but the underlying logic applies generally: recirculating suits chemistry you can name and characterise in advance, not an open-ended "whatever gets used in here."
When Ducted Makes Sense
A wide or changing chemical range
If the same cupboard sees different substances over its working life, especially unfamiliar or higher-hazard ones, ducted extraction removes the problem instead of depending on a filter being matched to every possibility.
New build or major refurbishment
Where the project already has plant space, riser access, and a route to a roof-level discharge point, the ducting and make-up air a ducted system needs is usually a manageable part of the design rather than a constraint.
No filter dependency
There's no filter media to select, monitor for saturation, or replace and dispose of as hazardous waste. Extraction runs continuously to atmosphere, and containment doesn't rely on someone changing a cartridge on time.
When Recirculating Makes Sense
No viable route to atmosphere
Listed buildings, mid-floor labs, or spaces with no realistic duct or riser run make a recirculating cabinet the practical option: there's simply nowhere for a ducted discharge to go.
A faster, lighter install
No ducting design, no roof penetration, and no make-up air system to plan around can shorten a project meaningfully, which matters where programme is tight.
Low-risk, well-defined chemistry
Teaching or QC labs working from a known, limited, lower-hazard chemical list are a natural fit, provided the filter is matched to that list and monitored properly.
Ducted vs Recirculating at a Glance
| Factor | Ducted | Recirculating |
|---|---|---|
| Governing standard | BS EN 14175 (all parts), established over two decades | BS EN 17242:2025, newly published |
| External ducting / roof discharge | Required | Not required |
| Make-up air supply | Required, to replace exhausted air | Not needed, air stays in the room |
| Ongoing consumable | None beyond routine servicing | Carbon/HEPA filter media, replaced and disposed of as hazardous waste on schedule |
| Adapts to new chemistry | Yes, without reassessment | Only if the filter is re-matched to the new substances |
| Suited to | Wide, unknown, or higher-hazard chemical use | Known, lower-hazard, well-characterised chemical use |
| Site flexibility | Needs a viable duct/riser route to atmosphere | Can go almost anywhere with power and floor space |
| Statutory testing (COSHH Reg 9) | TExT at least every 14 months | TExT at least every 14 months |
Across most of these factors, ducted carries a heavier upfront installation but fewer ongoing dependencies once it's running. Recirculating carries a lighter install but an ongoing filter and monitoring commitment for as long as the cabinet stays in service. Which matters more depends on how long you expect to be running the equipment, and how much that route to atmosphere is actually worth to you.
Getting the Decision Right
This isn't a decision to make on cost or lead time alone. Map the chemical inventory and hazard classification of what will actually go under the hood, and check what route (if any) exists to get ducting out of the building. Weigh that against the recurring cost and monitoring commitment a filter regime represents over the years you'll actually be running the equipment, not just at commissioning. Switching from one type to the other later usually means re-specifying from scratch rather than adapting what's already installed, so it's worth getting right at the planning stage. If you're sizing ductwork for a ducted system, our ductulator tool is a useful starting point once you know the route.
Specifying a Fume Cupboard?
Our engineers can assess your chemical inventory, site constraints, and building layout, and help you weigh up the right extraction solution for the long run.