Frequently Asked Questions

Fume Hoods

When should I use a fume hood?


Use a chemical fume hood anytime your work involves:

  • Chemicals with a National Fire Protection Association (NFPA) Health rating of 3 or 4

  • Toxic volatile materials such as chloroform or formaldehyde

  • Flammable chemicals

  • Carcinogens or particularly hazardous substances

  • A procedure that may create an aerosol of a toxic substance

  • Reactive or explosive materials or chemicals that may spatter

  • Toxic gases such as NH3, CO, F2, CL2, H2S, NO2

  • Odorous materials, both hazardous and non-hazardous




When should I use a Polypropylene fume hood?


A polypropylene fume hood may be used in any of the applications listed above, however, the primary purpose of polypropylene is to avoid deterioration of metal in highly corrosive environments.




What can I store in the hood?


Fume hoods are designed specifically for procedural work not for storage of any kind within the work zone. Equipment, chemicals and other large objects may disrupt airflow, eliminating the intended functionality of the fume hood.




What should I do if I must keep large pieces of equipment in the hood?


If the equipment is large enough to potentially block the rear baffle, raise it off the surface of the hood to allow air flow to the back of the hood.




Why must I keep work back from the front of the hood?


Chemical vapors near the front of the hood are not captured efficiently and can escape into the room. Chemicals should be kept at least 6 inches back from the front of the work zone.




What is face velocity?


The face of the hood is the opening where air capture takes place. Face velocity is the speed of the air as it is drawn into the hood from the room - across the face. A measurement of face velocity is done during performance testing at the factory and after installation. It is used as an indicator of the hood’s performance.




Why is 100 fpm used as the ideal face velocity?


As you work at the hood, air is drawn in a laminar flow through the face of the hood. The air velocity is one of the factors determining how well vapors and particles are captured and exhausted from the fume hood work zone. Vapor capture is also affected by air foils, hood shape, configuration, equipment inside the hood, and cross drafts from foot traffic, proximity to doors and room vents. Optimum face velocity has been found to be 80-120 linear feet per minute. At lower velocities, the fume hood may not capture and remove vapors at a sufficient rate. At higher velocities, air flow may become turbulent, producing eddies and backflows that retard the rate of removal or cause vapors to escape. Probably the best measurement of hood effectiveness is visualization, which can be done with dry ice or other visible fumes released in the hood.




Is it safe to use flammable chemicals in the hood if there are lights inside?


The lights installed inside fume hoods are sealed to keep flammable vapors away, these lights are called “intrinsically safe.” All electrical components in new fume hoods are intrinsically safe so flammable vapors should not be a fire hazard. However, if you use other electrical equipment or heating apparatus in the hood, they may not be safe from fire risk. Check to see if they are intrinsically safe or use them outside of the hood. Care should be taken with all heating devices and flammable solvents. Do not use open flames, and carefully monitor the temperature of other heating elements.




Why shouldn’t I use a standalone or “ductless” fume hood that doesn’t vent to the outside?


Fume hoods that do not vent solvents outside of the building rely on filters inside the cabinet to remove vapors and toxins from the air inside the hood before discharging into the air in the lab. Some vapors are not completely captured by the filters and they end up in your breathing air in the lab. Also, contaminants build up on the filters which can become a hazard. A maintenance schedule for removing filters must be set up; this is usually done by a contractor who will dispose of the filters as hazardous waste.