Fume hoods are designed to provide personnel protection from toxic or volatile chemicals by continuously delivering airflow away from the user to the work area. Air is then filtered and/or treated by the building’s exhaust system before exiting the facility (ducted) or by filters contained in the fume hoods, which clean the contaminated air and recirculate it directly back into the laboratory (ductless).
What is a Fume Hood?
A fume hood is a piece of laboratory equipment that is used to protect scientists from the possible negative effects of an experiment. This is primarily done by keeping the air inside the fume hood away from the people doing the experiment. Fume hoods also offer other features and protections that are useful in a laboratory setting.
Fume hoods have distinct physical characteristics. The base of the fume hood usually looks like a typical lab bench, and may have storage built into it, above the floor. On top of and connected to the bench is a very large enclosure called a cabinet. The cabinet will have a movable door, or sash, on the front that can be raised or lowered to provide access to the worktop portion of the fume hood. Openings around the edges of the worktop and/or at the top of the cabinet allow for the removal of gases in the cabinet. Experiments are performed inside the cabinet with the sash as low as possible so as to protect the experimenters.
Fume hoods provide protection from more than just dangerous gases. The enclosed nature of the cabinet helps to protect experimenters from being hurt by debris if something explodes or splashes. Fume hoods are also usually made of flame-retardant materials, so unexpected fires tend not to be as damaging as they would be in an open area.
Most fume hoods have numerous features built into them to assist with experimentation. Vapor-sealed lights allow experimenters to see what they are doing. Electrical outlets both inside and outside the fume hood are common, and allow for electronic devices to be easily used during the experiment. Vacuum pumps are also common, and allow for low-pressure or vacuum experiments to be conducted. Gas outlets that can be connected to various tanks and flows are useful for specific experiments, as are water faucets and drains.
Different Types of Fume Hoods
Every lab is unique. That’s why fume hoods come in a wide variety of sizes and functionalities so that you can find one that fits your exact needs. When comparing fume hoods, here are some of the different types to consider:
- Ducted fume hoods: A ducted fume hood connects to your building’s HVAC system, which then carries the air outside. When choosing a ducted fume hood, it will be necessary to work with your building’s infrastructure team to ensure the ventilation system can provide the required volume of air to ventilate the hood properly. Because the ducted fume hood is connected to your duct system, keep in mind that you may have limited options as to where you can place the unit.
- Ductless fume hoods: A ductless fume hood is a freestanding unit that does not require your building’s HVAC system. Instead, the hood uses a carbon-based filter to absorb fumes and vapors. The filtered air then recirculates back into the workspace. A secondary particulate-trapping HEPA filter can be added after the carbon filter to provide exhaust air suitable for cleanroom applications. Unlike a ducted fume hood, a ductless fume hood can be placed anywhere and easily moved.
- Light-duty hoods: These smaller, basic hoods are used to vent non-toxic materials such as heat, steam, odors and smoke.
- Specialized hoods: There are several fume hoods available for specialty applications. Acid digestion hoods are built with chemical-resistant materials to avoid corrosion, while radioisotope hoods are built out of stainless steel to prevent the absorption of radioactive materials. For educational settings, full-view hoods have transparent backs and sides for enhanced visibility when conducting demonstrations or monitoring students, along with multiple openings so students can work from both sides of the hood.
How Does a Fume Hood Work?
The primary purpose of the fume hood is to contain gases, vapors and fumes and then exhaust them out of the area. The fume hood begins this process by pulling room air into and through the fume. The air enters the hood through the sash. Depending on its design, the sash may move vertically or horizontally, or a combination of the two. The sash also acts as a barrier between the worker and the inside of the fume hood and provides limited protection.
As the air flow works its way through the middle of the fume hood, it pushes the harmful gases, vapors and fumes toward the exhaust area. This area is managed by slots and baffles which act as exit doors for the fume hood before reaching either the connected ductwork (ducted fume hood) or the fume hood filters (ductless fume hood). In many fume hoods, the slots and baffles are adjusted to allow for even air flow.
If the baffles are closed (or blocked) the exhaust path will also be blocked. But when used properly, baffles eliminate dead spots or reverse air flows which would result in a loss of containment. After air exits the fume hood it is transported through ductwork to where it is released into the atmosphere; or on ductless fume hoods, the air is filtered and recirculated back into the room.