Fume hoods provide primary confinement in a chemical laboratory. They exhaust toxic, flammable, noxious, or hazardous fumes and vapors by capturing, diluting, and removing these materials. Fume hoods also provide physical protection against fire, spills, and explosion. Fume hoods provide the best protection when the fume hood sash is in the closed position. All chemical fume hoods must be ducted to the outside of the building.
Types of Fume Hoods
There are three basic types of fume hoods: (1) standard, (2) bypass, and (3) auxiliary air. In addition, there are three classes of fume hoods: A, B, and C.
- STANDARD FUME HOODS
- The face velocity of a standard fume hood is inversely related to the open face area, allowing a constant volume of air to be exhausted. If the sash is lowered, the inflow air velocity increases.
IMPORTANT: Face velocity that is too high may disturb sensitive apparatus, extinguish Bunsen burners, or create excessive turbulence.
- BYPASS FUME HOODS
- Bypass fume hoods are also called "balanced air" or "constant volume" fume hoods. As the sash is lowered, bypass fume hoods allow constant exhaust volumes that help keep the room ventilation system balanced. Constant exhaust volumes also eliminate the problem of high face velocity as the sash is lowered.
- AUXILIARY AIR FUME HOODS
- Auxiliary air fume hoods are also known as "supplied air" hoods. They use an outside air supply for 50% to 70% of the hood's exhaust requirements. This type of hood is designed to reduce utility costs and conserve energy. The face velocity of an auxiliary air fume hood may vary.
The following guidelines are recommendations for face velocities of three classes of chemical fume hoods.
- Class A Fume Hood
- Recommended average face velocity is 115 to 125 fpm with a minimum of 100 fpm at any point. This class of hood is suitable for use with highly toxic or volatile materials having a TLV of less than 1 ppm (e.g., tetraethyl lead, beryllium compounds, metal carbonyls, and carcinogens).
- Class B Fume Hood
- Recommended average face velocity is 95 to 110 fpm with a minimum of 80 fpm at any point. This class of hood is suitable for use with materials having a TLV of 1-100 ppm (e.g., acids, formaldehyde, chloroform, and phenol).
- Class C Fume Hood
- Recommended average face velocity is 75 to 95 fpm with a minimum of 60 fpm at any point. This class of fume hood is suitable for use with materials having a TLV greater than 100 ppm (e.g., ethanol, butanol, and acetone). It is also suitable for operations that create nuisance dust and fumes.
- Special Fume Hoods
- Special fume hoods are necessary when working with certain chemicals and operations. Examples of special fume hoods include the following:
- Perchloric acid fume hood
- These fume hoods have a water spray system to wash down the entire length of the exhaust duct, the baffle, and the wall. The water spray is used periodically or after each use to remove any perchloric acid or organic material that may have accumulated.
- Walk-in hoods
- These fume hoods have single vertical sashes or double vertical sashes and an opening that extends to the floor. These hoods are typically used to accommodate large pieces of equipment.
- Radioisotope hoods
- These hoods are labeled for use with radioactive materials. The interior of these hoods is resistant to decontamination chemicals. If special filtration is necessary with these fume hoods, contact the Environmental Health & Safety Department.
- Canopy hoods
- These hoods capture upward moving contaminants and are good for heat-producing operations. Workers may be exposed to contaminants if they work under the hood, however.
Fume Hood Safety Considerations
The potential for glass breakage, spills, fires, and explosions is great within a fume hood. Due to the chance for fires or explosions, fume hoods should be located towards the back of a laboratory, away from primary and secondary exits. Practice safe work habits when working with fume hoods, including the following:
- Air Flow and Ventilation
- Employee traffic in front of a fume hoods or opening/closing laboratory doors can interfere with hood performance. Ensure that there is sufficient aisle space in front of fume hoods.
- Fume Hood Type
- All fume hoods are not appropriate for all types of work. Ensure that hazardous chemicals are used in the proper type or class of hood. For example, use perchloric acid only in fume hoods specifically designed for perchloric acid.
Fume Hood Use and Care
To ensure safety and proper fume hood performance, follow these guidelines:
- Use a fume hood when working with chemicals or procedures that may produce hazardous fumes or vapors.
- Know how to properly operate a fume hood before beginning work. Inspect the fume hood before starting each operation if a digit display is present.
- Place equipment and chemicals at least six inches behind the fume hood sash. This practice reduces the chance of exposure to hazardous vapors.
- Do not allow paper or other debris to enter the exhaust duct of the hood.
- Do not store excess chemicals or equipment in fume hoods.
- Do not block the baffle area of the fume hood.
- Elevate any large equipment within the hood at least three inches to allow proper ventilation around the equipment.
- When working in a fume hood, set the sash at the height indicated by the arrow on the inspection sticker. The only time the sash should be completely open is while setting up equipment.
- Wear personal protective equipment, as appropriate.
- Do not alter/modify the fume hood or associated duct work.
- Clean up spills in the hood immediately
Fume Hood Inspections
Fume hoods should be tested at least annually. Fume hoods should also be tested in the following circumstances:
- When an employee requests an inspection
- When a procedural change requires a hood classification upgrade
- After major repair work
- After a fume hood is moved
Each HSC component is responsible to perform fume hood inspections and testing. The test includes an inspection of the hood system, airflow measurements, and an assessment of the use of the fume hood. If you suspect a problem with your fume hood, please contact the Environmental Health & Safety Department.