Chemical Fume Hoods
A Chemical Laboratory Fume Hood is a type of protective barrier that is used in laboratories to minimize the risks of hazardous and toxic exposures to workers. Proper use of fume hoods, along with the use of Personal Protective Equipment (PPE), and departmental operational procedures to minmize exposure to hazardous vapors in the workplace.
This module is intended to enable you to:
1. Chemical Fume Hoods are ventilation devices that capture chemical vapors and ventilate them outside the building. This differs from a Biological Safety Cabinet that are designed to filter biological agents and other particulates out of the air circulating inside the cabinet. (Biological safety cabinets will be covered in a separate module.) It is important that all potentially harmful chemical work be conducted inside a properly functioning fume hood.
2. Self Contained Hoods (below, left) generally have a circulating fan mounted on the top of the hood, or beneath the workspace. Air is pulled through a filter in the front opening of the hood, before passing through the fan and pushed back into the contained workspace.
3. Perchloric Acid Fume Hoods Perchloric acid hoods are designed and constructed with corrosion resistant surfaces, ductwork, fan to withstand the caustic and corrosive effects of the acid. They are also constructed with a wash-down system. They have special regulations regarding their use and require specific fume hood use protocols. If you have questions or concerns about working with perchloric acid within a fume hood contact EHS for further guidance.
4. Floor Mount Hood is a chemical fume hood which sits directly on the floor and is characterized by a very tall and deep chamber that can accommodate large pieces of equipment. If you have a floor mount hood, contact EHS for operating protocol and inspection procedures.
5. Canopy Hoods are horizontal enclosures having an open central duct suspended above a work bench or other area. Canopy hoods are most often used to exhaust areas that are too large to be enclosed within a fume hood. The major disadvantage with the canopy hood is that it primarily ventilates and rather than contains. Meaning that contaminants are drawn away from the user's breathing zone but are not specifically contained and remain a risk factor.
6. Biological Safety Cabinets are designed to filter biological agents and other particulates from the air circulating inside the cabinet. This is different from a chemical fume hood that is designed to capture chemical vapors and ventilate them outside the building. If you are going to use or are using biological materials, please contact EHS for further information.
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1. A Sash is a movable glass panel that covers the face area of a fume hood. Sashes can be vertical, horizontal, or a combination of the two. Many hoods are installed with a sash stop, which stops the sash at approximately an 16-18 inch work level.
NOTE: **Sash stops should never be removed, overridden, or modified. It is recommended that all lab work in a properly functioning fume hood be performed at the sash stop level or lower whenever possible.**
2. Alarms, sensors, controls, and gauges are included to provide laboratory personnel with a constant reading of fume hood performance. If the face velocity falls below an acceptable work range, the hood sensors will trigger an alarm to notify personnel. Hoods usually go into alarm mode either because the sash has been raised above the normal working height of 18" and the hood can no longer exhaust a sufficient amount of air. When a hood alarm sounds, no chemical work should be performed, close the sash to a lower height or completely, until the exhaust volume is increased to a safe operating level. If the alarm situation does not remedy itself, contact KU Facilities Operations for adjustment of air handling system exhaust and any fume hood maintenance issues.
3. Air foil or sill, is located at the front of the hood beneath the sash. It creates a smooth air flow, minimizing turbulence of the air entering the hood.
REMINDER: **All work should be done at least six (6) inches into the recessed work area behind the sill.**
4. Air Jambs are vertical sills or side posts at the front of the hood. These are tapered to promote smooth air flow into the hood.
5. Baffles are movable panels located on the back wall of the hood that create slots in which air is exhausted. The pattern of the air moving into and through the hood is determined by the setting of the baffles. Once installed, they should not be readjusted by laboratory workers.
The location of the fume hood affects its efficiency and should be located in an area of minimal traffic. First, turbulence can be created just by someone walking past the face opening, causing contaminants to be drawn outside the hood. Next, if the air diffuser is located directly above the fume hood, air turbulence may be created causing contaminants to escape into the room. Finally, air flow into the room has an effect on the fume hood. All corridor entry doors should be kept closed to maintain the negative pressure of the lab. This ensures that any contaminants will be exhausted through the fume hood and not escape into the corridor.
Face velocity* is a measurement of the average velocity at which air is drawn through the face to the hood exhaust. The acceptable range of the average face velocity is generally 60-100 feet per minute (fpm). The ideal average face velocity is 80 fpm for most operations in newer hoods. Older design hoods are set at 100 fpm. Studies have demonstrated that flows greater than 125 fpm cause the creation of turbulence that causes contaminants to flow out of the hood and into the user's breathing zone.
Air flow indicators are small pieces of tinsel that are taped to the bottom corner of the sash. Inward movement of the tinsel indicates air is being drawn into the hood. Air flow indicators do not determine face velocity but only indicate that air is being exhausted through the fume hood.
Hood Monitor Alarm: is a more sophisticated indicator and control device that monitors the air flow exhausted by the hood. If the face velocity falls below an acceptable work range, the hood sensors will trigger an alarm to notify personnel. Hoods usually go into alarm mode either because the sash has been raised above the normal working height of 18" and/or no longer exhausting a sufficient amount of air.
*Face Velocity: air velocity at the plane of and perpendicular to the opening of an exhaust hood. (Brookhaven National Laboratory)
After chemical fume hoods are installed, they are checked by EHS personnel to ensure they meet ASHRAE 110 (American Society of Heating, Refrigerating and Air-Conditioning Engineers) standards for proper exhaust ventilation. The face velocity of the hood is measured in feet per minute (fpm) with an anemometer. As mentioned earlier, the fpm should be between 80-100 fpm for safe operation of the hood. It is the responsibility of Facilities Operations to arrange for testing and certifying the hood when the equipment is first installed. Supply and exhaust air flow should be in proper proportion to establish a negative pressure differential between the lab and the outside corridor. When operating, the hood's exhaust flow should be greater than the supply to create airflow movement from the hall into the lab to ensure airborne contaminants are contained.
Once the hood is certified, EHS performs fume hood inspections on an annual basis. All conventional hoods and specialty hoods are inspected individually. After initial post-installation checks, the annual inspection includes:
If at any time it is determined that the fume hood in your laboratory is not functioning properly, contact EHS for an immediate assessment.
The health and safety of laboratory personnel and building occupants must be the primary goal of principal investigators and/or laboratory supervisors. Properly functioning chemical fume hoods help achieve this goal with respect to the hazards of chemical vapors and other harmful airborne substances. It is important to remember that a fume hood is not a storage area. Keeping equipment and chemicals unnecessarily in the hood may cause airflow blockage.
Fume hoods and Enclosures
KU EHS Website
As you have learned, using protective barriers properly can reduce your risks of exposures to hazardous or toxic substances. Now that you have an understanding of different types of fume hoods and their uses, you must complete a quiz (with a score of 100%) and print out a certificate of completion for this module to receive credit. Good luck!
You have completed the Chemical Fume Hoods module. You should meet with your supervisor to go over information specific to your unit, your job, and the hazardous materials and/or processes in which your job will require. Using the skills that you have learned in this module, you should be able to identify and utilize the different types of fume hoods in your workplace. If you have any questions or concerns please contact the KU-EHS Department.
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