+86 13638632902
Jason@cleanroomequips.com
MENU
The functions of fume hoods can be summarised as follows: capture chemical contaminants at their origin, and contain vapours and aerosols inside the enclosure so they never reACH the worker's breathing zone. A laboratory fume hood is a personnel-protection device first.
This article explains what does a fume hood do in practice, how a fume hood ventilation system holds containment, how fume hood airflow requirements are defined and confirmed, and how ducted vs Ductless Fume Hoods differ.
It also covers cleanroom fume hood selection and fume hood maintenance across the whole service life of the unit.
ITbJpmIk6Bs
Core functions of fume hoods
The basic functions of fume hoods are directly connected to laboratory chemical exposure control. A laboratory fume hood creates a negative pressure cavity, draws air from the room through the sash opening, and exhausts contaminated air away from the operator.
Here are the main functions of fume hoods:
- Extract contaminants at their point of origin
- Contain chemical vapours and aerosols inside the working cavity
- Keep laboratory chemical exposure below occupational exposure thresholds [1]
- Separate personnel protection from product protection, which other devices perform
What a laboratory fume hood actually controls
A laboratory fume hood controls chemical hazards, not all the dangers present in the laboratory. Chemical fume extraction removes the contaminants that become airborne and does not eliminate those that remain on the working surface.
Some practical examples include:
- Vapour released during the preparation of a sample
- Fumes released during compatible digestion processes
- Aerosol produced during pouring and transferring of substances
- Smells, but only when the dangerous source is effectively controlled
Protection boundaries and common misconceptions
Many misunderstandings lead to incidents and accidents caused by equipment. Each card below separates a common claim about the functions of fume hoods from the documented fact.
CARD 1
Claim: the sash is a blast shield. Fact: the sash provides only limited splash protection.
CARD 2
Claim: the hood prevents explosions. Fact: ordinary hoods do not contain explosions.
CARD 3
Claim: HEPA filtration removes chemical vapour. Fact: HEPA removes only the particle phase.
CARD 4
Claim: the hood ensures sterility of materials. Fact: ordinary hoods do not prevent product contamination.
CARD 5
Claim: the hood replaces a biosafety cabinet. Fact: hoods and cabinets operate on different airflow principles.
CARD 6
Claim: the hood is permanent chemical storage. Fact: hoods do not serve as a storage area for chemicals.
How a fume hood ventilation system provides containment
xypR6f2YEbY
Containment is essentially the balance between inward airflow and disturbances. A fume hood ventilation system holds that balance through constant movement of air from the operator's side to the exhaust duct.
The process starts with baffles that extract chemical fumes from the rear and sides of the cavity. The exhaust system then transports chemicals away from the working area.
Even though the fan works properly, containment can be interrupted by air movements, opening doors and rapid operator motion. Proper use ensures effective chemical fume extraction; improper use negates it.
Airflow path and fume hood airflow requirements
Face velocity is the average speed of air at the sash level. It is not the only necessary criterion, and it is affected by baffle design, sash construction, heat load and draughts.
There are no universally safe face velocity values. Account for manufacturer criteria and a documented risk assessment when setting fume hood airflow requirements.
Exhaust volume follows the formula Q = v x A, where Q is exhaust volume flow, v is face velocity and A is the open area.
For example, a hood 1.2 m wide with the sash open 0.4 m gives A = 1.2 x 0.4 = 0.48 m².
If v = 0.5 m/s, which equals 1800 m/h, then Q = 0.24 m³/s = 864 m³/h.
Use measured opening dimensions and measured velocities when determining exhaust flow. Also distinguish hood exhaust volume from whole-room air change rates.
Operator practices that preserve containment
Work at the proper sash height
Check airflow monitoring devices before starting work
Keep instruments within the permissible working area
Do not block baffles, slots or the rear airfoil
Avoid sudden movements and draughts
Stop all operations if an airflow indicator gives a warning
Choosing ducted vs ductless fume hoods
Deciding between ducted vs ductless fume hoods should start from analysing the chemical inventory rather than the price. Containment must match what is actually being handled, in what quantity and for how long.
Verify whether exhaust ducting and replacement air are in place. Assess filtration constraints for the particular chemicals involved. After that, assess lifecycle maintenance, filter replacement and disposal.
An exhaust hood that cannot exhaust and cannot adsorb the chemical is not a hood, but merely a cabinet with a fan.
Exhausted and recirculating systems compared
When mixtures of unknown or variable composition are handled, ductless systems cannot be justified simply because they are convenient. Uncertainty about breakthrough makes them inappropriate regardless of filter claims. This is a common concern in QC laboratories that use different sample matrices over time.
Table 1: Ducted versus ductless configurations
| Configuration | Contaminant destination | Suitable applications | Critical limitations | Verification needs |
|---|---|---|---|---|
| Ducted | Contaminants travel through a purpose-built outdoor exhaust; treatment depends on the pollutants and the discharge requirements | High-volume solvents, acids, unknown mixtures | Duct route, fan capacity and replacement air required | Duct pressure, face velocity, tracer-gas containment |
| Ductless | Recirculates air through adsorption media back into the room | Low-volume, known, filter-compatible chemicals | Adsorption capacity and breakthrough must be controlled | Filter compatibility, breakthrough monitoring, replacement schedule |
| Either | Neither arrangement automatically suits a cleanroom | Cleanroom use requires separate assessment | Room pressure and iso class can be affected | Room balance and pressure cascade tests |
| Configuration | Suitable applications | Critical limitations |
|---|---|---|
| Ducted | High-volume solvents, acids, unknown mixtures | Duct route, fan capacity and replacement air required |
| Ductless | Low-volume, known, filter-compatible chemicals | Adsorption capacity and breakthrough must be controlled |
| Either | Cleanroom use requires separate assessment | Room pressure and ISO class can be affected |
Chemical-specific selection and special hazards
- Identify the chemical types used and the maximum quantity used in one session
- Confirm filter capability for the whole mixture, not a single marker compound
- Evaluate flammability and material compatibility
- Strong acids may require dedicated waste management and safety measures
Containing a powder requires low-turbulence, directed airflow that carries particles to the filter without re-entrainment. A fume hood ideal for solvent vapour may therefore be unsuitable for a potent powder.
Note that adsorption filter capacity is limited, and filters can release previously trapped compounds when breakthrough occurs. Any general claim of universal filtration should be disregarded.
Integrating cleanroom fume hood selection with GMP facilities
Cleanroom fume hood selection is an interface problem from an engineering perspective. Because the hood removes air from the cleaned room, it directly affects the pressure relationship and contamination control in that room.
Balance chemical containment against particle containment for the process undertaken. Follow GMP requirements, and do not confuse gmp grades with ISO classes.
Deiiang Cleanroom Technology Co., Ltd. supplies the containment, exhaust, filtration and monitoring hardware that a fume hood depends on, and the cleanroom is designed around it. Deiiang does not manufacture fume hoods itself.
Room balance and contamination-control interfaces
Account for fume hood exhaust in the makeup air balance
Examine the influence of sash movement and door opening
Determine the pressure cascade under actual operating conditions
Calculate air change rates from documented room data
Follow EU GMP Annex 1 where necessary
Example: a room of 6 m x 5 m x 3 m has a volume of 90 m³. At 25 air changes per hour, supply is 90 x 25 = 2250 m³/h.
A single 864 m³/h fume hood exhaust equals about 38% of that supply. This volume must be compensated without changing the pressure cascade.
A pro-tip from Jason.peng notes that designers tend to disregard the pressure transient caused by rapid sash movement. The building management system must account for that instantaneous volume change.
Deiiang product specifications and verification evidence
Table 2: Requirement versus verified Deiiang catalogue specification
| Requirement | Verified Deiiang specification | Project state | Evidence | Acceptance decision |
|---|---|---|---|---|
| Branch flow regulation | Constant-air-volume valve; installed level within 2 per mille, 1.5 x valve width of straight duct upstream | Six hood branches on one roof fan | Single-valve trial run with final opening angle recorded | Accepted when branch flow matches design value |
| Reverse flow prevention | Backdraft damper; leakage below 2% standard and below 0.5% tight-seal, per JB/T 7228-94 | Fan shutdown events | Leakage test performed with the duct system | Accepted on tight closing at fan stop |
| Duct pressure monitoring | CCY11 differential pressure transmitter; 0.25% FS accuracy, 4-20 mA output, IP54 | Low-flow alarm per hood | Calibration data and alarm trip test | Accepted when the alarm trips before face velocity leaves tolerance |
| Control logic | SIMATIC S7-200 SMART CPU ST20; 12 digital inputs, 8 digital outputs, 24 V DC | Fan sequencing, damper position, alarm logic | PLC program and interlock test record | Accepted on documented interlock verification |
| Filtration | H13; 99.97-99.99% at 0.3 µm; V-bank combined activated carbon rated 3700 m³/h | Exhaust polishing duty | Filter scan test certificate | Accepted on test certificate and measured pressure drop |
| Room structure | Huijun hollow MGO Panel; 1150 mm effective width; 50 or 100 mm; 60-minute fire protection | Laboratory enclosure | Panel test record | Accepted on documented class and fire rating |
| Requirement | Verified Deiiang specification | Acceptance decision |
|---|---|---|
| Branch flow regulation | Constant-air-volume valve; installed level within 2 per mille, 1.5 x valve width of straight duct upstream | Accepted when branch flow matches design value |
| Reverse flow prevention | Backdraft damper; leakage below 2% standard and below 0.5% tight-seal, per JB/T 7228-94 | Accepted on tight closing at fan stop |
| Duct pressure monitoring | CCY11 differential pressure transmitter; 0.25% FS accuracy, 4-20 mA output, IP54 | Accepted when the alarm trips before face velocity leaves tolerance |
| Control logic | SIMATIC S7-200 SMART CPU ST20; 12 digital inputs, 8 digital outputs, 24 V DC | Accepted on documented interlock verification |
| Filtration | H13; 99.97-99.99% at 0.3 µm; V-bank combined activated carbon rated 3700 m³/h | Accepted on test certificate and measured pressure drop |
| Room structure | Huijun hollow MGO panel; 1150 mm effective width; 50 or 100 mm; 60-minute fire protection | Accepted on documented class and fire rating |
As a case study, a university analytical-chemistry teaching laboratory runs six ducted bench-top hoods on one roof fan. Project-reported commissioning found all six branches holding design volume within a few percent, with the low-flow alarm tripping before face velocity left tolerance.
Operator-position noise also met the laboratory dB(A) target. The duct-component and control-hardware designs are attributed to Jason.peng of Deiiang.
Qualification and performance verification
Verification indicates whether a specific system has accomplished its intended task and supported laboratory chemical exposure control. It is different from a claim made in a catalogue.
First define intended use and acceptance criteria, then gather and report performance and commissioning data. Link the evidence to the relevant standards [3][4]. Reverification is needed whenever a change may affect containment.
Testing the installed hood and room interface
Inspect installation, exhaust connections and damper direction
Map airflow at each specified sash position
Use smoke visualisation and appropriate containment tests
Perform alarm tests and check relevant interlocks
Check representative operating and room conditions
Ensure tracer-gas testing to ASHRAE 110 runs with all cleanroom devices operating [3]
gmp documentation and acceptance decisions
- Trace requirements through corresponding IQ, OQ and PQ records
- Justify site-specific acceptance limits in writing
- Record calibration status and test conditions
- Apply FDA 21 CFR Parts 210 and 211 appropriately [6]
- Reference PIC/S GMP or ICH Q7 where applicable [7]
- Separate test methods from mandatory performance levels
Risk-based fume hood maintenance
Fume hood maintenance preserves containment across the service life, not only at handover. Frequency must follow documented risk, chemical severity and duty cycle.
Decide who performs each inspection and who escalates a malfunction. Control exposure during servicing. Keep service and retest logs so trends can be analysed [2].
Routine inspections and planned servicing
Check sash, cables, runners and structural condition
Check airflow indicators and alarms in operation
Examine fans and ducts with the corresponding caution
Track ductless filter loading and replacement
Process contaminated components as applicable waste
Follow manufacturer recommendations and site procedures
Responding to failures and system changes
Alert: all containment failures require work to stop until the hood is proven again.
- Stop work immediately after any containment failure
- Secure hazardous materials as soon as it is safe
- Inform responsible safety and facility personnel
- Investigate airflow breakdown and breakthrough concerns
- Reverify airflow after significant modifications
- Resume work only once officially authorised
Frequently Asked Questions
The questions below answer common queries about the functions of fume hoods and about fume hood maintenance intervals.
In practical terms, what does a fume hood do?
A fume hood pulls contaminated air away from the user and retains hazardous chemicals until they are released to the outside. In a pharmaceutical QC laboratory, the hood protects the analyst working with solvents while the exhaust carries vapour outdoors.
Does a laboratory fume hood protect sterile products?
No. A regular laboratory fume hood protects only the user, and incoming room air may contaminate an open product. Aseptic protection should therefore be chosen separately, usually a biosafety cabinet or laminar-flow device.
Can a ductless fume hood handle every laboratory chemical?
No. There is no chemical that is compatible with every filter, so the specific chemicals and the load must be checked. Breakthrough monitoring and scheduled replacement must be carried out and cannot be skipped.
What determines acceptable fume hood airflow?
Applicable specifications and a documented risk assessment determine it, evaluated at the actual sash position and room conditions. Containment testing matters more than face velocity testing alone.
How often should fume hood maintenance and testing occur?
Use applicable regulations and manufacturer recommendations, then set specific intervals based on documented risk. Retest after any change that could affect containment.
Can a fume hood affect cleanroom pressure and air changes?
Yes. Fume hood exhaust changes the air balance of the room, so the replacement air must support the pressure regime. Verify the effect under representative operating conditions, not in an empty static room.
References
- [1] OSHA 29 CFR 1910.1450, Occupational exposure to hazardous chemicals in laboratories.
- [2] ANSI/AIHA Z9.5, Laboratory ventilation.
- [3] ASHRAE 110, Method of testing performance of laboratory fume hoods.
- [4] EN 14175 (parts 1-7), fume cupboards.
- [5] SEFA 1, Laboratory fume hoods recommended practices.
- [6] FDA 21 CFR Parts 210 and 211, Current good manufacturing practice.
- [7] PIC/S GMP Guide; ICH Q7, Good manufacturing practice for active pharmaceutical ingredients.
- [8] ISO 14644-1, -2, -3, cleanrooms and associated controlled environments.
- [9] EN 1822 / ISO 29463, High-efficiency filters (EPA, HEPA, ULPA).
- [10] ISO 16890, air filters for general ventilation.
- [11] NFPA 45, Fire protection for laboratories using chemicals.
- [12] GB 50243-2016, Code for acceptance of construction quality of ventilation and air conditioning engineering; JB/T 7228-94, Damper leakage classification.
