There are two main types of fume hoods, ductless and ducted, intended to minimize operator exposure to toxic vapors and gases. A ductless hood is a standalone, self-contained workstation. Air is drawn from the front of the hood and passes through carbon and/or HEPA filters to remove fumes, vapors, and particulates in the air before it is recirculated back into the lab. The flow of air into the hood and the face velocity protect the user from exposure to fumes and vapor. A ducted hood is attached directly to a building exhaust system and draws room air into it. Air streams that enter the hood prevent chemicals in the air from escaping. Air in the hood is then exhausted, drawing vapors, gases, and particulates out.
Whether ductless or ducted, a fume hood functions by maintaining a relatively negative pressure in its interior to prevent any contaminant from escaping while drawing air in through the fume hood opening at a consistent rate. A suitable hood face velocity (the speed at which air is drawn into the hood) is important for the safe and effective operation of a fume hood, with excessive face velocities often resulting in turbulence and reduction of containment, and insufficient velocities that may compromise hood performance.
The components of a fume hood include a hood body that contains the toxic gases and vapors; baffles, which are moveable partitions used to create slotted openings along the back of the hood body and keep the airflow uniform across the hood opening to eliminate dead spots and efficiently capture gases and vapors; and a moveable sash window that serves to maintain a relatively negative pressure in the hood interior to stop the discharge of contaminants. An airfoil located along the bottom and side edges of the fume hood streamlines the flow of air into the hood and prevents the creation of turbulent eddies that can carry vapors out of the hood. The space below the bottom airfoil provides a source of room air for the hood to exhaust when the sash is fully closed, and removing the airfoil can cause turbulence and loss of containment. Another component of a fume hood is the exhaust plenum, which helps distribute the airflow evenly across the hood face. A ductless fume hood is generally equipped with bonded carbon or HEPA filters.
To minimize exposure to toxic vapors and gases and ensure their maximum containment, a fume hood should be situated in the laboratory so as to avoid cross-currents at the hood face that may result from pedestrian traffic, opening and closing of doors, and supply air diffusers. Sufficient laboratory supply air must be available to allow the fume hood to operate at the specified face velocity.
No fume hood, however well situated or designed, can provide adequate containment unless the operator follows a set of guidelines for proper usage.
Guidelines for using a fume hood
- Before using the fume hood:
- Develop a process to identify any toxic vapors and gases that may be emitted and look for opportunities to reduce them
- Become familiar with the standard operating procedures (SOPs) of the fume hood and design experiments so that the SOPs can be maintained whenever hazardous materials might be released
- Ensure that the sash, baffles, airfoil, and fume hood exhaust system operate properly, i.e., check the baffles for obstructions, and if the hood is equipped with an airflow monitor, check its status.
- While using the fume hood:
- Keep the operator face outside the pane of the fume hood while using the sash for partial protection
- Work at least 6 inches from the face of the hood
- Stay alert to changes in airflow
- Use the sash for partial protection during hazardous work
- Wear appropriate protective equipment such as safety glasses, gloves, and a laboratory coat
- When using large equipment inside the hood, place the equipment on blocks to allow air to flow beneath the hood
- Do not make quick motions inside or out of the hood, use fans, or walk quickly past the hood opening as these can cause airflow disturbances
- Keep the hood closed, except during setup or when it is necessary to work within the hood
- Keep the sash closed as much as possible to maximize hood performance and to conserve energy
- Walk away from the hood and close the sash completely if you suspect the hood is not working properly
- Remove contaminated items from the hood after decontamination to avoid releasing them into the laboratory air.
- At all times:
- Maintain laboratory good housekeeping practices:
- Do not use the hood as a waste disposal location
- Use the appropriate glassware to contain and collect waste solvents, toxic vapors, or dust
- Store hazardous chemicals such as flammable liquids in a safety cabinet rather than in the fume hood
- Cap all chemical reagent bottles tightly and check the fittings on laboratory glassware to minimize vapor loss.
- Maintain and follow a testing and maintenance program for the fume hood:
- Evaluate the fume hood before each use to ensure adequate face velocities and the absence of excessive turbulence
- Measure the fume hood face velocity, conducting smoke tests and trace gas containment to certify on an annual basis that the fume hood is operating safely.
- Avoid improper use of the fume hood, which can potentially maximize operator exposure to toxic vapors and gases and cause injury. Improper uses may stem from:
- Inadequate flow rates due to the sash being left open, or excess clutter, which reduces the containment of noxious substances
- A malfunctioning fume hood due to slipping fan belts, blocked exhaust duct, or restricted damper
- During power outages, failure to lower the hood sash to maintain a chimney effect to keep some air flowing into the hood
- Restricted exhaust airflow from the use of paper products, aluminum foil, and other lightweight materials within a hood.
Conclusion
A wide range of fume hoods are available to protect the operator from toxic vapors and gases. For more information, please visit https://www.aircleansystems.com/.
Lina Genovesi, Ph.D., JD, is a technical, regulatory, and business writer based in Princeton, NJ, U.S.A.; e-mail: [email protected]; www.linagenovesi.com