Fume-hoods are probably the most important pieces of protective equipment in the Chemistry laboratory.
Fume-hoods are designed to protect against hazards from toxic, dangerous or otherwise obnoxious material by dragging airflow away from the user and so preventing harmful vapours being inhaled or otherwise ingested.
Those in the Department of Chemistry are built-in, ducted fume-hoods that vent to the outside through outlets on the roof.
When To Use
A chemical fume-hood must be used for all chemical procedures involving:
- Any materials that have been assigned an Occupational Exposure Standard and where airborne concentrations of these materials might approach these limits.
- Any materials that have been assigned a Maximum Exposure Limit irrespective of the likely airborne concentration and other materials which are listed as carcinogens, as teratogens or as mutagens.
- Any material requiring a COSHH Special Assessment.
- Any material of obnoxious odour or stench.
All wet chemistry work is carried out in a fume hood when one is available in the lab.
Means of Ventilation
Ventilation is normally achieved by a single extractor fan powering several hoods in varying locations and on different building floors - this arrangement leaves all of these fume-hoods at risk following the failure of the single fan. The single fan also runs at a fixed speed so that extraction rate from any specific hood depends upon the open or closed state of sashes in the remaining hoods on the stack. Most of the Standard hoods have no warning of fan failure apart from physical indicators e.g. toy windmills or paper tissues rigged by the users. In the event of an Emergency Alarm, Standard fume-hoods remain in operation.
The draught in all fume-hoods is routinely tested with the front sash open 500 mm and the cupboards are labelled A-D depending on the flow that is found.
- Category A and B fume-hoods have sufficient draught to be used safely with most gases or vapours.
- Category C cupboards are safe to use with care but it would unwise to expect them to cope fully with a massive release of a dangerous gas or vapour.
- Category D fume-hoods should be used only for the release of tiny quantities of toxic substances or for the storage of toxic substances in fairly leak-tight containers.
The grade of fume-hood that is available must be considered when any work is being planned.
While fume-hoods are designed to protect the user against hazards from other sources e.g. from toxic or obnoxious material or from flammable materials such as solvents, their misuse can lead to them affording less protection than expected or being hazards in their own right.
- The rating of the fume-hood may not be sufficient to cope with the materials used.
- The effectiveness of a fume-hood is much reduced if they are open too wide or cluttered with apparatus that interferes with the smooth flow of air.
- Fume-hood fans, especially those on the Standard models, are susceptible to failure. This means that the draft also fails leaving the hood effectively useless.
- The front sash of Standard fume-hoods is made up of a very heavy sheet of glass enclosed in a heavy wooden frame. Accidents have occurred when the sash cords have broken allowing the front to crash down.
- Chemicals stored in a fume-hood that is in active use for chemical work represent potential additional hazards in the event of an accident.
Fume-hoods are safety equipment but can be misused to the extent that they are less effective than expected. Injury from misuse can arise from two causes:
- From e.g. the fume-hood being poorly rated for the work required, from sashes being left open or from equipment being installed in the hood in such a way as to reduce containment of noxious material. Here the probability of injury is slight with slight to moderate severity but this may increase with the toxicity of any escaping material.
- From the apparatus itself i.e. a breaking of the Standard model sash cord:- the probability is very slight but the damage could range from moderate to severe.
Who is likely to be injured?
The most likely person to be injured is the user although the escape of noxious material into the laboratory can have an effect on all occupants.
- Maintenance of the fans and regular checks of the air-flow face velocities and of any sash cords and pulleys.
- Although the use of fume-hoods is part of the training of most Undergraduate Chemists, newcomers to the department should be instructed in the specific local rules.
- The rating performance of the fume hood must be adequate to contain the material in use effectively.
- The interiors of fume-hoods must be kept tidy and not cluttered with apparatus to disrupt the flow of air. Apparatus should not encroach within 10cm of the front edge of the hood.
- Any rear vents must be kept clear and free from a build-up of dust or blockage by solid material e.g. paper tissues.
- Unnecessary chemicals must not be stored in a fume-hood that is actively used for chemical work.
- The front sash must be kept closed as far as is comfortable while working and fully closed when not actively working.
- Heads must stay OUT of the fume-hood whilst working.
- To detect and be warned of any fan failure, a strip of tissue or some other visible indicator of air flow should be used.
- If airflow fails for any reason, work must stop and the sashes be closed.
Escape of toxic or noxious material.
- Leave the area immediately. Close all doors on exit. Warn people to avoid the affected area. Phone the PSSO immediately (Tel: 6516 2691) / Campus Security (Tel: 6516 1616, after office hours).
- Most Standard fume-hoods are part of a bank of hoods vented by a single fan. If the flow fails in one hood it will have failed in others. Most fume-hoods also have notices pinned to them indicating the fan number and the location of other hoods on the same fan. The instructions outlined on the notice must be followed and other users informed together with Campus Security as appropriate.
- If a Standard fume-hood has its warning notice turned over to red, it must not be used until declared safe.
- If ventilation fails to any fume-hood, work must stop and the sashes must be closed.