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Fume Hoods

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 Chemical Fume Hoods

There are three basic types of general purpose fume hoods: (1) standard, (2) bypass, and (3) auxiliary air.


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.

Face velocity that is too high may disturb sensitive apparatus, extinguish Bunsen burners, or create excessive turbulence.



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 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.

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 Hoods

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 EHSRM.

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.

Canopy Hood
Canopy Hood


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 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 to ensure it is working.
  • 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.

If a power failure or other emergency occurs (e.g., building fire or fire within the fume hood), close the fume hood sash and call for emergency assistance.

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

EHSRM performs fume hood inspections and testing monthly. The test includes an inspection of the hood system, airflow measurements, and an assessment of the use of the fume hood. The calibration procedure used at Texas State University is RMS-05.02 “Constant Flow Fume Hood Calibration”. If you suspect a problem with your fume hood, please contact EHSRM.

Exhaust Fan Maintenance

The exhaust fans that operate to pull air through the fume hoods receive maintenance twice a year. The program is described in detail in the Chemical Hygiene Plan.


June 2011
Revised November 2014