Ductless Fume Hoods

What are Ductless Fume Hoods?
Fume hoods are ventilation devices which exhaust chemical fumes, vapors, gasses, dust, mist and aerosols. Fume hoods also serve as physical barriers between reactions and the laboratory, offering a measure of protection against inhalation exposure, chemical spills, run-away reactions and fires.

However, unlike conventional fume hoods, ductless fume hoods filter out chemical fumes using activated carbon filters and recycle the air directly back to the working environment. Personnel protection is provided by drawing air at a controlled rate across a front opening into the hood, preventing toxic vapors generated during reactions from escaping into the general laboratory environment.

Although fume hoods remain the equipment of choice for ventilating hazardous airborne materials from the laboratory, there are situations that make ductless fume hoods a logical solution, such as:

  • Flexibility in placement of ducted fume hoods is limited. Laboratories located in the
    center or bottom of a several story building may not have a ducting option.
  • Some laboratories are “air-starved” and the make-up air available is insufficient to
    accommodate a ducted fume hood.
  • Loss of conditioned air results in a less energy efficient heating and cooling system
    and increased costs.
  • The initial expense for ductwork and installation is significant.
  • Portability is essential.

However, it should also be remembered that ductless fume hoods are not without limitations. Carbon based filters are processed or treated to adsorb specific types of chemicals at low evaporation rates, thus ductless fume hoods are not commonly used in any applications involving a broad array of chemicals or forced evaporation. Another limitation is carbon filter saturation monitoring, which can prove to be difficult.

Advantages of a Ductless Fume Hood

They protect the environment since toxic fumes are not released to the environment unlike in conventional fume hoods. Activated Carbon filters retain / neutralize pollutants eliminating harmful discharge to the environment.

  • Fully installed systems ready to operate are available at a lower cost than bulky conventional fume hoods. A ductless fume hood can be placed on a bench and connected to an electrical supply. No external ducting, building work or changes to heating and ventilation systems are necessary.
  • An expensive ducting and external blower system that is often difficult to maintain is not required.
  • They are mobile and can be relocated easily to meet your changing needs; perfect for schools and educational institutes. The hood may be easily re-positioned or filters changed to suit new requirements at anytime.
  • They allow energy savings since air which is costly to air-condition, or heat, is not removed from the laboratory. Recirculatory airflow eliminates the need for laboratory make-up air and integration into ventilation systems.

Activated Carbon Filtration

Activated carbon (activated charcoal, activated coal) is a form of carbon that has been processed to make it extremely porous, thus giving it a very large surface area for adsorption or chemical reactions. One gram of activated carbon has a surface area around 500 m² (5400 sq.ft.). The effectiveness of activated carbon as an adsorbent is attributed to its unique properties, including large surface area, a high degree of surface reactivity, universal adsorption effect, and favorable pore size. Other than filtration of chemical fumes, activated carbon is often used in purification, deodorization, decolorization and separation. Activated carbon is usually derived from wood, coal, coconut shell, or peat.

Common Applications for Activated Carbon:

  1. Environmental applications
    Carbon adsorption has numerous applications in removing pollutants from air or water streams both in the field and in industrial processes such as spill cleanup, groundwater remediation, drinking water filtration, air purification, and many other processes.
  2. Medical applications
    Activated carbon is used to treat poisonings and overdoses following oral ingestion. Tablets of activated charcoal are still used as a folk remedy and over-the-counter drug to treat diarrhea, indigestion, and flatulence. Activated charcoal is also used for bowel preparation by reducing intestinal gas content before abdominal radiography to visualize bile, pancreatic and renal stones.
  3. Fuel storage
    Gas storage in activated carbons is an appealing gas storage method because the gas can be stored in a low pressure, low mass, low volume environment that could be much more feasible than bulky on board compression tanks in vehicles.
  4. Gas purification
    Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapors, odors, and other hydrocarbons from the air. Activated carbon filters are also used to retain radioactive gases from a nuclear boiling water reactor turbine condenser.


Common Ductless Fume Hood Applications


  • Ductless fume hoods are useful for secondary and post secondary education, science classes and in laboratories where activated carbon filtration offers safety from selected aerosols and vapors.
  • Hoods are easily assembled, portable and configured for high visibility to improve classroom participation.
  • Installation costs are minimal; no ducting required.

Life Sciences

  • Ductless fume hoods are ideal for protocols using small quantities of solvents.
  • Depending on the solvents used, numerous vapor-generating laboratory processes such as HPLC preparation and biochemistry protocols can be performed with improved safety and comfort.


  • Forensic laboratories frequently use sterilants and preservatives. Formaldehyde, often mixed with alcohols or phenols, is commonly used. OSHA has a specific standard for formaldehyde that stipulates initial and periodic monitoring, protective equipment and clothing, training and designating regulated areas among others.
  • Common chemicals used:
    – formaldehyde
    – methyl methacrylate
    – ammonia


  • Industrial and commercial applications range from quality control in cosmetic production to photo-chemicals, solvent extraction, purification and other processes where fumes or vapors are generated.


  • Ductless fume hoods will not remove tempered air from the cleanroom. A secondary HEPA filter can be installed to prevent particles generated during the work process from being recirculated to the cleanroom.

Other Examples of Applications:

  • Touch Up Painting
  • Microscopy
  • Slide Preparation
  • Histology
  • Fingerprinting
  • Dental Laboratory
  • Spray Adhesives
  • Solvent Cleaning

Ductless Fume Cabinets – Safety Precautions

  • Ductless fume hoods should not be used for laboratory work in which chemicals of different types are used repeatedly. For example, the hood should not be used for acid emitting processes where hydrocarbon type filters are installed. Ductless fume hoods should not be specified or used for unknown chemicals or to contain byproducts of reactions for which the characteristics are not known.
  • Ductless fume hoods should not be used for multiple chemical processes where two or more chemicals could combine in the filter and cause reactions with toxic, exothermic or explosive properties. The chemicals may react later when the second chemical is adsorbed even if the chemicals are not present in the ductless fume hood base at the same time.
  • Ductless fume hoods should not be used with certain types of chemicals, virus or bacterial emissions, high concentration acid emissions or processes with very high levels of chemical emissions such that the filter life would be very short. For such types of applications, standard fume hoods, glove boxes, biological safety cabinets should be used. Consult Esco for more information.
  • Extreme caution should be taken when working with ignition sources inside a ductless fume hood. Ignition sources such as electrical connections; controllers and open flame can be used inside a ductless fume hood as long as there are no operations involving flammable or explosive vapors. If possible, ignition sources should remain outside the hood at all times.
  • Ductless fume hoods are potential locations for fires and explosions due to the types of reactions conducted in these hoods. The location of the ductless fume cabinets should be within the laboratory so that in the event of a fire or explosion within the fume hood, exit from the laboratory would not be difficult. Depending on the types of reactions, plastic hoods, which may not physically withstand a fire, should not be used.
  • Ductless fume hoods should be located away from high traffic lanes within the laboratory because personnel walking past the hood may disrupt the flow of air into the hood and cause turbulence, drawing fumes into the laboratory.
  • Safety devices such as drench showers, eye wash stations, fire extinguishers, first aid kits and fire blankets should be located convenient to the hood. Proper instructions should be posted as to their use and function.

Proper working procedure

  • Ensure the hood is operating correctly before commencing work.
  • Use appropriate personal protective equipment including goggles and gloves.
  • Keep your head outside of the hood. The operator should work at least 15 centimeters (six inches) beyond the plane of the sash.
  • Work as far into the hood as possible and with slow, deliberate movements, to minimize airflow disturbances. The operator should attempt to slowly approach and withdraw from the ductless fume hood. The opening and closing of the sash should be done slowly.
  • Work with the sash as fully lowered as possible, utilizing the sash as a natural barrier to accidents which may occur in the hood. The operator should make sure that the head and upper body remains outside the plane of the hood opening at all times.
  • The contaminants and equipment above the work surface of the hood should be elevated so as to enable flow beneath and around the obstructions.
  • Do not use the hood as a storage area. Items can block airflow and interfere with containment.
  • Laboratory personnel traffic should be kept to a minimum while working in the ductless fume hood. Substantial cross drafts can be generated which may affect containment.
  • The sash should be shut when not working in the hood.
  • Filter saturation should be checked regularly, at least once every 60 hours of use.


Airflow Pattern in a Typical Ductless Fume Hood

Information required to to quote on a Ductless Fume Hood

  1. What size do you need?
  2. Will you be using heat?
  3. What chemicals will you be using?
  4. Do you require a stand?
  5. Do you need internal plug points?
  6. Do you need gas taps?
  7. Do you need any other services?

Further Reading