Chemical Fume Hoods
Chemical fume hoods provide protection from vapors, splashes, and impacts caused by chemicals and their reactions. However, fume hoods do not completely eliminate the hazards, and they have to be used correctly to minimize the chance of exposure.
Notes: Biological safety cabinets are not equivalent to chemical fume hoods and do not provide protection from hazardous vapors. Procedures in which perchloric acid is heated above ambient temperatures must be performed in a specialized fume hood equipped with a wash-down system.
Chemical fume hoods are equipped with a fan in the duct work that generates air flow from the front to the back of the hood. Air escapes through slots between baffles at the bottom and top of the hood. The air flow captures vapors and prevents them from diffusing into the room. Hoods are designed so that the air velocity at the plane of the sash is optimal for capturing vapors when the sash is in the recommended position. For vertical sliding sashes, the optimal height of the sash is 18 inches; a larger opening limits the performance of the hood. The opening should be minimized on hoods with horizontal sliding sashes. On combination hoods that include both the vertical and horizontal sashes, it is best practice to put the vertical sash all the way down before opening and use the horizontal sashes. Opening the sash beyond the optimal position will dramatically lower the air velocity and allow vapors to escape into the room. Blocking the airflow with large equipment in the hood can also lead to insufficient vapor capture and potential exposure, as shown in the image below:
Fig 1: In the left image, the airflow to the bottom slot is blocked, allowing vapors to escape into the room. In the right image, air flows underneath the elevated equipment, improving efficiency of the hood.
A person standing in front of the hood also disrupts the airflow, generating eddy currents. Vapors within those currents may escape into the room. Eddy currents drop off at about six inches into the hood.
Fig 2: Top view of air currents created when someone is working in front of a hood.
A campus-wide Chemical Fume Hood Surveillance Program is managed by the Division of Safety and Compliance at no charge to the users. Fume hoods are inspected annually to measure the face velocity. If the velocity falls within the optimal range of 80 and 120 linear feet per minute (fpm), the hood will receive a green sticker bearing the date of the inspection and an arrow indicating the optimal sash position.
If the face velocity is between 60 and 79 fpm or above 120 fpm, the hood will receive an orange sticker meaning that its use is "Restricted," and it should not be used for protection from toxic or flammable chemicals. F & S should be contacted for flow adjustments.
Finally, if the face velocity of the fume hood is below 60 fpm, the hood will receive a red DANGER sign stating that the fume hood should not be used. F&S should be contacted for repair.
The hood can function properly only if its integrity is not compromised. A hood with missing side panels or cracked sashes will not provide adequate exposure control.
Because the status of the chemical fume hood can change anytime, continuous air flow indicators are recommended. New chemical fume hoods should be equipped with air flow monitoring devices which will alert the user if there is a problem with air flow. For older hoods without air flow monitoring devices, a simple visible test to ensure flow into chemical fume hoods and other ventilating devices is to tape a tissue to the hood and note its movement when the exhaust fan is turned on.
Safe Work Practices
To reduce the risk of exposure as much as possible, adhere to the following work practices:
- Open the sash or sashes only as much as is required to perform your work comfortably. Never open it beyond the indicated position when hazardous chemicals are present. Position the sash to maximize protective shielding.
- Verify that the chemical hood is drawing air. Check the flow monitor if present, or hold a Kimwipe in the hood and check its movement. Note: Do not let go of the wipe. Wipes sucked into the ductwork will block airflow and may cause damage requiring expensive repairs.
- Perform all work at least 6 inches into the hood and do not store items near the opening where they interrupt airflow and pose a spill hazard.
- NEVER put your head in the hood when hazardous chemicals are present.
- Do not position large equipment or containers of chemicals in the back of the hood where they block airflow. Elevate equipment by placing it on blocks, jack stands, or legs so that air can flow underneath to the bottom baffle slot. To store items in the back of the hood, install shelves to elevate containers (see Fig 3). Keep inside the hood only what needs to be there and remove everything that can be stored outside.
- Route service connections under the airfoil and secure all loose and dangling electrical cords, tubes and tubing with tie-raps, twist ties, or rubber bands.
- Discontinue work and close all containers with hazardous chemicals if the alarm sounds. If possible, mute the alarm, put an out-of-order sign on the hood, and contact your department business office to arrange for repair.
- When the hood is not in use, keep the sash at a six-inch opening. This will significantly reduce energy consumption and the sash will act as a shield in the event of an unexpected release. A small opening is required to serve as air exhaust for the laboratory.
Fig 3: A plank shelf allows chemical storage inside the hood while maintaining proper hood functioning.
Last Update: 9/8/2015