Common Misuses & Limitations
Used appropriately, a fume hood can be a very effective device for containment of hazardous materials, as well as providing some protection from splashes and minor over pressurizations. Even when used as designed, the average fume hood does have several limitations.
- Projectiles: A fume hood is not designed to contain high velocity releases of particulate contaminants unless the sash is fully closed.
- Pressurized systems: Gases or vapors escaping from pressurized systems may move at sufficient velocity to overcome the capture velocity and escape from the fume hood.
- Explosions: The hood is not capable of containing explosions, even when the sash is fully closed. If an explosion hazard exists, the user should provide anchored barriers, shields or enclosures of sufficient strength to deflect or contain it. Such barriers can significantly affect the airflow in the hood.
- Perchloric Acid: A conventional fume hood must not be used for perchloric acid. Perchloric acid vapors can settle on ductwork, resulting in the deposition of perchlorate crystals. Shock-sensitive perchlorates can accumulate on surfaces and have been known to detonate on contact, causing serious injury to researchers and maintenance personnel. Specialized perchloric acid hoods, made of stainless steel and equipped with a wash-down system must be used for such work.
- Air Foil Sills: Many fume hoods are equipped with flat or rounded sills or air foils which have been specifically designed to direct the flow of air smoothly across the work surface. Sills should not be removed or modified by the hood user. Objects should never be placed on these sills. Materials released from containers placed on the sills may not be adequately captured. In addition, an object on the sill may prevent the quick and complete closure of the sash in an emergency.
- Spill Containment Lips: Most modern fume hoods have recessed work surfaces or spill containment lips to help contain minor liquid spills. In many cases, these lips are several inches wide. Containers of liquids should not be placed on the hood lip.
- Tubing for Exhaust: Tubing is frequently used to channel exhaust to the hood from equipment located some distance away. This is not an effective control method.
- Connections to the Exhaust System: Occasionally, a researcher may need local exhaust ventilation other than that provided by an existing fume hood. A new device may not be connected to an existing fume hood without the explicit approval of the department's facilities manager or Special Facilities supervisor. Adding devices to even the simplest exhaust system without adequate evaluation and adjustment will usually result in decreased performance of the existing hood and/or inadequate performance of the additional device.
- Microorganisms: Work involving harmful microorganisms should be done in a biosafety cabinet, rather than a chemical fume hood. See the Biosafety Manual for more information.
- Highly Hazardous Substances: A well designed fume hood will contain 99.999 – 99.9999% of the contaminants released within it when used properly. When working with highly dangerous substances needing more containment than a fume hood offers, consider using a glove box.
- Pollution Control: An unfiltered fume hood is not a pollution control device. All contaminants that are removed by the ventilating system are released directly into the atmosphere. Apparatus used in hoods should be fitted with condensers, traps or scrubbers to contain and collect waste solvents or toxic vapors or dusts as appropriate.
- Waste Disposal: A fume hood should not be used for waste disposal. It is a violation of environmental regulations to intentionally allow volatile waste to evaporate and escape up the hood stack. As described above, the hood is not a pollution control device.
The Fume Hood as a Storage Device
Fume hoods are designed specifically to provide ventilation for the protection of lab occupants during chemical manipulations. The airflow they provide is greatly in excess of that needed for storage of closed containers of even the most toxic of volatile materials. Storing materials in this way is, therefore, a misuse of an expensive piece of equipment.
In general, the storage of chemicals in fume hoods is strongly discouraged. The realities of available space and equipment in some laboratories may make it difficult or impossible to completely prohibit the use of hood workspaces for storage. In such a case, the following general policy is recommended:
Hoods Actively in Use for Experimentation
Storage of materials should be minimized or eliminated altogether. Materials stored in the hood can adversely affect containment. In addition, the hood is frequently the focus of the most hazardous activities conducted in the laboratory. The presence of stored flammable or volatile, highly toxic materials can only exacerbate the problems resulting from an explosion or fire in the hood. Even if they are not directly involved in such an event, attempts to control or extinguish a fire may result in the spilling of stored materials.
Hoods Not in Active Use
Materials requiring ventilated storage (e.g., volatile and highly toxic, or odorous substances) may be stored in a hood if they are properly segregated and the hood is posted to prohibit its use for experimental work.
Alterations to Fume Hood
Changes or additions to an existing fume hood without the explicit approval of the department's facilities manager or Special Facilities supervisor is prohibited. Adding devices to even the simplest exhaust system without adequate evaluation and adjustment will usually result in decreased performance of the existing hood and/or inadequate performance of the additional device. Additionally, some components of older hoods may contain asbestos and therefore should not be damaged.