Fume Hoods and Laboratory Ventilation
The fume hood is often the primary control device for protecting laboratory workers when working with flammable and/or toxic chemicals. OSHA’s Laboratory standard (29 CFR 1910.1450) requires that fume hoods be maintained and function properly when used.
Environmental Health and Safety (EHS) conducts routine performance testing on fume hoods annually to verify they are operating properly and have the appropriate face velocity to capture airborne hazards. An inspection label with the performance test date and results is posted on the front of the fume hood next to the sash. Performance tests expire the following year at the end of the inspection month indicated on the label. If you find that a fume hood performance test has expired, contact EHS at firstname.lastname@example.org or 785-532-5856.
If you find that your fume hood is not functioning properly or if an alarm will not reset, submit a service request through Facilities Customer Services (785-532-6389 or /facilities/request/) to have the hood repaired.
If your fume hood airflow alarm activates, close the sash and wait a moment and then reopen the sash to your working height. If the alarm continues, stop using the hood, close the sash and promptly contact Facilities Customer Services at 785-532-6389 to have the hood repaired.
Fume Hood Inspections and Pass/Fail Criteria
The fume hood inspection involves conducting a smoke test (containment/airflow visualization) and measuring the face velocity at multiple points. Face velocity measurements are taken with the sash open to the height indicated on the energy sticker located next to the sash or until it reaches the sash stop (typically 12-18”). Sash stops are recommended for all fume hoods.
The smoke test is used to visualize the hoods ability to contain vapors. A failed smoke test may indicate an obstruction to, or disturbance of airflow into the hood. Placement of items within and near the hood can affect this airflow. Equipment and chemicals improperly placed or stored in fume hoods can cause a failed smoke test. Fume hoods should not be used as storage cabinets.
To pass inspection, the fume hood must meet the following conditions:
- The fume hood must pass the flow visualization test. (i.e. no smoke can visibly flow out of the front of the hood).
2. The average face velocity of the fume hood with the sash positioned at the sash stop or height indicated on the hood (typically 12-18” from work surface) must be within the following ranges:
- 80-120 fpm for general use fume hoods (nuisance dust, noxious odors and low toxic materials).
- 100-140 fpm for fume hoods used for perchloric/hot acid, carcinogens and high toxic materials.
- 70-90 fpm for high performance (low flow) fume hoods.
A fume hood will fail inspection if any of the following criteria are met:
- Fume hood is inoperable.
- Fume hood is inaccessible (items are stored in front of hood).
- Reverse airflow (air flowing out of the hood enclosure).
- Smoke escapes outside of the hood enclosure during the smoke visualization test.
- Physical damage to the hood such as a broken sash, holes near the baffles or damage to the exterior or interior walls of the fume hood.
- Large equipment, chemicals or excessive storage of items in the hood enclosure obstructing proper airflow.
- Equipment placed inside the sash plane preventing the sash from closing.
- Two or more face velocity measurements are below 50 fpm or above 150 fpm.
- The average face velocity of the fume hood does not meet criteria listed above.
Fume Hood Mechanical Exhaust Operation Types
Constant Air Volume (CAV) Hoods
A constant air volume hood draws a constant volume of air into the fume hood, regardless of sash height. The face velocity of the hood is inversely proportional to sash position. For instance, the lower the sash, the higher the face velocity.
Variable Air Volume (VAV) Hoods
A variable air volume hood is equipped with a face velocity control to vary the volume of air exhausted from the hood in order to maintain a constant face velocity, regardless of sash height. This system allows for reduce operating costs, as less heated or cooled room air is exhausted by the hood.
Auxiliary Air Hoods
Auxiliary air hoods are constant air volume hoods that were designed to reduce the amount of room air and energy that is consumed. These hoods were intended for laboratories with insufficient air flow. Auxiliary air hoods have a portion of the total volume of exhausted air provided through a plenum. are equipped with a plenum located above and outside of the hood face that supplies a portion of the total volume of exhausted air unconditioned air to the face of the hood. Ideally, auxiliary air hoods were developed to reduce heating and cooling energy costs, but they tend to raise mechanical and operational costs due to additional ductwork and fans.
Fume Hood Design Types
A walk-in, or floor-mounted, fume hood is designed to enable the use of large laboratory equipment in a contained space. Typically, these hoods allow for access from the floor to the top of the hood’s interior. Contrary to its name, users should never walk into the hood, as these hoods are solely for the storage and usage of large apparatuses.
Perchloric Acid Hoods
Perchloric acid hoods are designed for the use of perchloric acid (HClO4), which is capable of violent explosions under various conditions. These hoods must be watertight, explosion-proof, and constructed of Type 316 stainless steel. Perchloric acid fume hoods must also be equipped with a wash down system and must be ducted separately, rather than integrated into the building’s exhaust system.
A ductless fume hood is an enclosure in which a fan draws contaminated air out of the hood, through an activated carbon filter, and back into the laboratory. These fume hoods should only be used for the manipulation of very low risk hazards.
Other Laboratory Ventilation and Local Exhaust Devices
Laminar Flow Hoods
A laminar flow hood is designed to provide a contaminant-free working area for sterile procedures and biological materials. Air is drawn in near the top of the hood, filtered to remove particulates, and exhausted across the work surface, toward the user, in a constant, unidirectional manner. Laminar flow hoods do not provide protection for the worker or the environment and should not be used when working with hazardous materials.
Biological Safety Cabinets
Biological safety cabinets are classified as Class I, Class II, or Class III cabinets.
Class I biological safety cabinets provide protection for the worker and the environment, but not the product. Unfiltered air is drawn across the work surface, but is HEPA (high efficiency particulate air) filtered prior to being exhausted. Class I cabinets should be used for the manipulation low to moderate risk agents of biosafety levels 2 and 3.
When correctly installed and operated, a Class II biological safety cabinet aims to capture microbial and infectious contaminants, providing protection for the worker, the product within the working area, and the environment. Air is drawn in near the front of the work space, HEPA filtered to remove contaminants, and recycled back into the work area. Unlike a laminar flow hood, contaminated air is contained within the hood or exhausted out of the building, rather than expelled toward the worker. Class II cabinets should be used to manipulate most microorganisms and agents of low to moderate risk of biosafety levels 2 and 3.
Class III biological safety cabinets provide maximum protection for the worker, the product, and the environment by the use of HEPA filtered supply and exhaust air. It functions similarly to a Class II biosafety cabinet, with the addition of a non-opening enclosure, viewing window, glove ports and decontamination capabilities for the entry and exit of sensitive material. Class III cabinets should be used for the manipulation of infections agents of biosafety levels 3 and 4.
A glovebox is a sealed, controlled environment used to manipulate hazardous or sensitive materials. Laboratory operations are performed inside the enclosure through gloved openings to protect the worker, environment, and the products that are sensitive to water or air vapor.
Local Exhaust Ventilation (LEV)
A local exhaust ventilation system is designed to reduce worker exposure to airborne contaminants. This is accomplished by locally capturing and transporting the waste product through a filter, then to a safe point of exhaust. This engineering control consists of a hood, ductwork, air filter, air mover, and discharge that enables hazardous materials to be expelled from the work environment before laboratory personnel are exposed.
Fume Exhaust Connections: “Snorkels”
A fume exhaust connection, or snorkel, is a mobile extractor that can be placed over an area in need of ventilation. This apparatus is connected to the building’s ductwork to safety exhaust hazardous fumes out of the building. For maximum effectiveness, this funnel-shaped connection must be placed within six inches of the work space.
A canopy hood is designed to vent materials such as heat, steam, and odors from bulky apparatuses that do not require physical enclosures, such as ovens and autoclaves. These hoods can be mounted on a wall or suspended from the ceiling to effectively exhaust non-toxic materials.
Designed to draw contaminants away from the user's breathing zone and into slot vents. Slots must be at the proper height and positioned less than 4 inches away from the contaminant in order to capture. Often used in photography developing areas or for mixing dry ceramic glazes.
In a downdraft table, the work surface doubles as the collection area for toxins. Contaminated air is drawn downward into the table, away from the worker’s breathing zone, then filtered, and recirculated into the room upon decontamination.
Toxic and flammable gases such as arsine, sulfur dioxide, silane, hydrogen chloride, and ammonia, must be used in a gas cabinet. Gas cabinets are vented by a constant volume mechanical system to the roof, similar to a chemical fume hood. Some gas cabinets are equipped with monitoring devices and alarm systems that sense hazardous conditions, warn employees of a gas release, and automatically shutoff the gas flow.
Fume Hood Safe Work Practices
Operate the hood at a proper sash height. The optimum sash height for proper performance is 12 to 18 inches. Do not operate the hood at a height above 18 inches. The sash should be lowered to a position that can provide additional protection from potential splashes, sprays, or fires. EHS recommends the installation of sash stops.
Minimize pedestrian traffic in front of the hood to reduce the release of contaminants. Disturbances, such as people walking by or quick motions, in front of the hood can create undesired airflow. It is also important to minimize other disturbances, such as doors opening and closing, for maximum fume hood effectiveness.
Do not positions fans, air conditioners, or heatersdirectly in front of the hood. If airflow from these apparatuses crosses the face of the hood, it could interfere with the containment of hazardous materials.
Do not block the airfoil. If paper absorbent is used for the interior of the hood, it should not cover the airfoil. All electric cords should be run under the airfoil.
Place all bulky equipment towards the rear of the hood and raise it at least 2 inches off the surface with blocks, racks, or shelves. Raising large objects allows airflow around and under the equipment. These items should be placed near the back of the hood, but not against the rear baffles. Equipment placed near the face of the hood, as well as near the sidewalls, can cause a variation in airflow.
Work as far inside of the hood as possible. All items should be stored and all operations should be performed at a minimum of 6 inches from the sash to ensure that lab personnel are not exposed to contaminants.
Keep the sash face clear and clean. Remove any obstructions on the face of the hood that limits visibility into the hood at any sash height.
Do not use the fume hood as storage cabinet for equipment or chemicals. Unneeded objects inside of the fume hood should be kept to a minimum and stored in a manner than does not interfere with airflow.
Do not use a hood for a function it was not designed for. Some fume hoods are designed specifically for the use of perchloric acid or radioisotopes. Perchloric acid fume hoods are equipped with a wash-down system to properly eliminate the explosive crystals generated by perchloric acid. Failure to use a hood designated for the use of perchloric acid can lead to the buildup of explosive crystals in the building’s ductwork.
Clean chemical residues from hood surfaces after each use. To avoid unwanted and dangerous reactions or accidents, properly clean up any residues from the work surface.
Keep chemical containers closed in the hood unless actively working with them. To minimize unwanted spills and accidents, all containers within the hood should remain closed unless they are in use.
Keep the sash completely closed when the fume hood is unattended. In the case of fires and explosions in an unattended hood, it is important to close the sash when leaving experiments or chemicals unattended.
There must be a minimum 3.5 feet from the face of the fume hood to the nearest item of furniture, and a minimum of 5 feet to the nearest opposite wall or other obstruction that is taller than the work surface height. Large objects, such as refrigerators or bulky laboratory equipment, can impede the hood’s airflow. For this reason, large furniture should be placed in an appropriate location for maximum fume hood effectiveness.
Do not use the hood if it failed the EHS inspection until the cause of the failure has been addressed. Contact EHS if you have any follow-up questions or concerns.
Do not use the hood if the flow rate monitor is alarming or the monitor readings differ significantly (±20%) from the EHS inspection sticker. Using a malfunctioning fume hood can be of great danger to the worker.
Always wear personal protective equipment. Fume hoods do not prevent spills and splashes, nor is the use of a hood a substitute for PPE. It is important to wear protective equipment that is appropriate for the type of work being conducted, such as safety glasses, gloves, and/or lab coats.