Safety Questions – Ductless Fume Hoods

The STAO Safety Committee frequently responds to questions that Ontario teachers pose regarding safety in their science facilities.  Here is a sample:


Are ductless fume hoods a good alternative to traditional fume hoods?

Since the summer, we’ve been taking an inventory of the fume hoods in our secondary schools and sending in an inspector to look at them.  We’ve found a few schools with no working hoods.  Given the infrastructure and energy demands of the built-in ones, we were thinking of purchasing ductless ones, like those outlined in the attached brochure.

Our Health and Safety department is concerned that these may be in violation of some of our codes (see below).  Has STAO come across any of these ductless hoods in Ontario schools and/or have any thoughts of their effectiveness?


I have read through a 2006 safety review regarding ductless fume hoods. The review was conducted by a committee at the National Institute of Health (NIH), and referenced a previous study by the Occupational Health and Safety Branch (DOHS) 1998. I have also consulted two other sources, Princeton University website and the reference textbook, Prudent Practices in the Laboratory (2011).


These fume hoods were self-standing enclosures that used carbon filters or HEPA filters. The filters worked by providing an absorbing surface, and the filtered air was then recirculated back  into the classroom. They assessed the ductless fume hoods from:  Misonix, Inc., Sentry Air, Captair, NuAire, Air Clean Systems, and Air Science USA.

Evaluation Criteria: Face velocity,  Air flow Patterns, Spill Control, Retaining Ability of the Filters.

PLUS SIDE:  Quick installation

Cheap (no duct work)

Energy efficient (recirculate warm air)

MINUS SIDE:  Chemically contaminated filters are a hazardous waste

Requires tracking and maintenance in changing the filters

Filters have an affinity to a small number of chemicals

Filters nearing the saturation point will begin to leak

Low molecular weight chemicals can easily be stripped from the carbon filter

“Channelling” (improper filter installation) can allow chemicals to bypass the filter.

Exposure to new chemicals can cause previously absorbed chemicals to de-absorb

Hood sensors ability to detect contaminated air is questionable

Ownership of the fume hood maintenance is required (to choose the filter medium, determine date of saturation point and change filter, order and pay for filters)


Discouraged use of ductless fume hoods. Found they were unable to contain  spill. This was also stated in NIH review where they indicated that a 500 ml spill could not be contained by the fume hood. Other problems described included low face velocity, turbulence issues, and difficulty  knowing when to change filters.


States that there are serious limitations to ductless fume hoods. Applicable to removal of nuisance vapours and dusts. Not suitable for fire or toxicity hazards.



Princeton University Safety Manual.  Controlling Chemical Exposure

Prudent Practices in the Laboratory

I hope that this information will be useful to you in making the decision as to whether to install ductless fume hoods in your school board. If you have any new information regarding ductless fume hoods, please share this information with our committee. As always, please do not hesitate to contact us regarding your safety enquiries at

Dave Gervais

STAO Safety Chair