What is a fume hood? A fume hood is an enclosure that safely contains and ventilates hazardous fumes, vapors, gases and dust generated by chemical processes performed in the fume hood. Sometimes called a chemical hood or a lab hood, a fume hood protects workers from inhalation of hazardous substances.
The clear sliding window on a fume hood, called the sash, also shields workers from spills and splashes that may occur in the chemical fume hood.
Fume hoods are the workhorse of laboratory exhaust systems and are the most widely used approach for local ventilation.
How does a fume hood work?
A fume hood works by pulling air away from the user into the enclosure with a blower. The fume hood then filters and vents the air to the outdoors through a facility exhaust system. Alternatively, a fume hood may filter the air to remove dangerous fumes and then return the air to the room. Most fume hoods are equipped with gauges or alarms that warn the user of low airflow and potential exposure to hazardous fumes.
What is a fume hood used for?
Fume hoods allow lab employees to work with potentially dangerous chemicals while minimizing the risk of exposure to toxic fumes. The sash (window) on a chemical hood allows the worker to view and manipulate objects within the enclosure while keeping fumes from toxic or volatile chemicals away from the worker’s face.
Industries that use fume hoods include:
- Semiconductor manufacturing
- Medical device surface finishing such as electropolishing
- Aerospace surface finishing such as passivation with nitric acid
- Research departments at colleges and universities.
What types of hazards can a fume hood protect the user against?
When do you use a fume hood? In general, use of a laboratory fume hood is advised whenever working with hazardous chemicals. If the Safety Data Sheet (SDS) for the chemical you plan to use has warnings like “Toxic by inhalation” or “Do not breathe dust, fumes or vapors,” then a fume hood or other ventilation system is needed.
Fume hood use is also recommended when working with compounds that have a low boiling point, or chemicals that emit noxious odors.
Types of materials that should be used inside a chemistry fume hood include:
- Any volatile materials
- Corrosive acids and bases
- Irritating vapors and dust
- Asphyxiating gases
- Open sources of volatile radionuclides
Examples of chemicals used with a fume hood include nitric acid and hydrofluoric acid.
Fume hood vs. Laminar flow hood: What’s the difference?
Chemical fume hoods are often confused with laminar flow hoods, but they are not the same. Both use airflow as a means of protection, but the object of protection differs: Chemical fume hoods protect personnel, while laminar flow hoods protect the product.
In a fume hood, air is pulled away from the worker to protect the worker from hazardous fumes. By contrast, a laminar flow hood (also called a clean bench) blows filtered air outward. The smooth, non-turbulent airflow prevents contamination of the product such as a semiconductor wafer or biological sample from particulate matter.
Biosafety cabinet vs. Fume hood: What’s the difference?
Biosafety cabinets are another category of laboratory equipment that is frequently confused with fume hoods. Both use airflow to protect, but the focus of protection is on different hazards: Fume hoods protect against chemical fumes and vapors, while biosafety cabinets protect against pathogens and biological agents.
A biosafety cabinet (also called a biological safety cabinet or BSC) uses HEPA filters to remove infectious organisms from exhaust air. Depending on the class and type, a biosafety cabinet may also use HEPA filters on intake air to protect the product from contamination. By contrast, a fume hood does not usually use HEPA filtering on exhaust air vented outdoors.
Laboratory Enclosure Comparison
|Fume Hood||Laminar Flow Hood||Biosafety Cabinet|
|Primary Function||Protect the user from hazardous chemical fumes and vapors||Protect the product from contamination with particulate matter||Protect the user and the environment from pathogens|
|Used With||Chemicals that generate harmful fumes, volatile vapors and gases||Particulate-sensitive materials such as semiconductor wafers or biological samples||Infectious biological agents or hazardous particulates|
|Airflow||Away from the user||Non-turbulent, may be toward the user||Away from or around the user, varies by class|
|HEPA filters||Optional on exhaust||On intake air||Required on exhaust, optional on intake air|
Best Technology does not offer biosafety cabinets, as they are an entirely different product category from fume hoods, and outside our range of chemical process equipment.
Fume hood buying tips: What to look for in a fume hood
Size. A basic starting point when shopping for and specifying a fume hood is size. What size is the working area required for your application? What size is the available space on the factory floor or the laboratory for a fume hood? Best Technology builds custom fume hoods, so even if your size requirements are unusual, we can build a fume hood to fit your space.
Materials. Some chemistries require specialized materials for containment. Highly corrosive acids at high concentrations, for example, often require a fume hood constructed from polypropylene rather than stainless steel for better corrosion resistance. For hydrofluoric acid, polycarbonate is the preferred material for the sash, to avoid etching of a glass window. Knowing the expected chemistry that will be put in the fume hood is crucial to ensuring years of reliable functioning.
Standards. Does your fume hood need to meet certain industry standards, such as FM 4910? Does your chemical process need to meet other specialized standards or requirements? Sometimes a “one-size-fits-all” fume hood just won’t work. In those cases, look to the applications engineers at Best Technology for a customized fume hood.
Window orientation. Some fume hoods use a horizontal sliding window, which limits the access of the operator to the working area inside the fume hood. Look for a vertically sliding window (sash), so that users have more available working area in the interior of the fume hood. Best Technology offers fume hoods with counterbalanced, vertically sliding windows.
Ducted fume hoods
A ducted fume hood relies on the facility’s ventilation system for venting exhaust air outdoors using ducts. The fan or blower is typically located on the roof of the building, allowing for quiet operation of the fume hood. To prevent recirculation of contaminated air, the ductwork for a fume hood should be separated from the rest of the facility’s ventilation ducts.
Avoid ductless fume hoods. A ductless fume hood, also called a recirculating fume hood, uses a blower on the fume hood to pull contaminated air through a HEPA filter, and then recirculates the air back into the room. The type of filter required varies depending on the chemistry, so the operator must ensure use of the proper filter for safety. Filters also must be changed regularly for safety.
Though ductless fume hoods avoid the upfront cost of ductwork installation, they add to noise, maintenance requirements, and a greater risk of chemical exposure to workers and other equipment. For these reasons, many research universities in the U.S. have banned the use of ductless fume hoods.
Best Technology offers only ducted fume hoods, and not ductless fume hoods.
Fume hood safety
Keep your head out of the fume hood. User behavior is critical to ensuring that the safety features of a fume hood are not defeated. Only hands and arms should be inserted into a fume hood, and never the head.
Sash down. To obtain the safety benefit of a fume hood, the user needs to keep the sash down. The lower the sash position, the safer the user is from chemical exposure. Many fume hoods specify a maximum fume hood sash height to maintain safe exposure levels.
Fume hood safety instruments
Most fume hoods are equipped with a means of verifying airflow within the enclosure, either a magnehelic gauge or an airflow meter. These instruments verify airflow in slightly different ways.
- A magnehelic gauge measures the differential in air pressure, inside vs. outside the fume hood.
- An airflow meter measures the actual flow rate of air within the fume hood.
These instruments allow the operator to confirm that the exhaust system is functioning. If the air isn’t exhausting as expected, many organizations have emergency procedures for containing potential hazardous vapors.
Fume hoods also use a measurement called face velocity to evaluate exhaust power. The face of a fume hood is the opening where air enters. Face velocity, then, is the velocity of air when it enters the fume hood, measured in feet per minute (fpm). A face velocity of 80 – 125 fpm is generally considered acceptable.
Fume hood operation tips
Keeping the sash as low as possible, and closed when not in operation, reduces energy costs. Since the fume hood exhausts conditioned air (heated or cooled, depending on local climate), reducing the amount of exhaust reduces the load on the facility’s HVAC system.
Be aware that sash position can affect air velocity within the enclosure such that a closed sash may have enough increased airflow to disturb delicate parts or instruments. Some fine-tuning of the blower speed may be required to find the balance that works for you.
Fume hood design options
Proper construction materials for a fume hood depend on the intended application. Options for construction materials for fume hoods include:
Fume hoods are a flexible engineering solution for a variety of chemical handling needs. Here are a few of the many possible options and features that can be included in a fume hood:
- Rinse sinks and tanks with connection to a DI water (deionized water) source
- Tank heaters
- Strainers for handling small parts
- Test tube and beaker holders
- Space for customer-supplied laboratory hot plates and magnetic stirrers
- Compressed, clean dry air (CDA) spray guns for drying parts
- Nitrogen spray guns (N2 spray guns) for ultra-clean drying of dust-sensitive materials such as semiconductors.
To learn more about fume hoods offered by Best Technology, visit our Fume Hoods page.
Learn from a fume hood expert
Ready to start your project? For expert help with your wet chemical process systems, look to Best Technology. Our applications engineers are available to answer your questions and get you the fume hood that fits your needs. Contact us for more information and a free proposal.