How Does a Ductless Fume Hood Work?

Ductless Fume Hoods, often referred to as “Ductless Containment Hoods”, are used in a wide variety of industries including manufacturing, medical, chemical, laboratory, and pharmaceutical. These negative-pressure systems are designed to protect the operator, as opposed to positive-pressure systems (e.g. Portable Clean Room Hoods) that are designed to protect the application or product.

There a few key characteristics that distinguish Ductless Fume Hoods from Conventional Ducted Fume Hoods (that typically vent outdoors):

  • A “recirculating” air pattern and no external ducting or venting.
  • The use of high-quality filtration media.
  • Benefits including environmentally-friendly operation, energy efficiency, cost efficiency, minimal maintenance, and mobility.

Recirculating Air Pattern & Filtration Media

The most significant difference between a Ductless Fume Hood and Conventional Ducted Fume Hood is the exhaust method. While the exhausting air on Ducted (External Venting) Hoods typically empties outdoors (with or without filtration), Ductless Fume Hoods are designed to recirculate filtered air back into the immediate environment.

Click picture to view a larger image.

The diagram above outlines the major steps:

1. Operators perform their process in the work area of the hood. The access area at the front of the hood is often referred to as the “sash “. Sentry Air’s standard sash opening is 8” H. While performing in the work area, fume and particulate emissions from the application are quickly pulled into the filtration system by a powerful fan situated on top of the hood.

Click picture to view a larger image.
Click picture to view a larger image.

2. Air is processed through a pre-filter, which captures some of the particles.

3. Air is passed through the main filter and the majority of contaminants are captures. Main filter options include:

  • HEPA Filter [up to 99.97% efficient on particles 0.3 microns and larger]
  • ASHRAE Filter [up to 95% efficient on particles 0.5 microns and larger]
  • ULPA Filter [up to 99.9995% efficient on particles 0.12 microns and larger]
  • Activated Carbon Filter [efficiency rate varies with the chemical being extracted]
  • Specialty-blended Filters [e.g. Acid Gas, Aldehydes, Ammonia, Mercury]

4. Once filtered, air is distributed back into the immediate environment.

Other Features/Benefits

Environmentally-Friendly: In many cases, ducted exhaust hoods empty extracted fume and particulate directly outdoors without sending it through a filtration system first. Ductless Fume Hoods trap hazardous particulate in filters so they can be disposed of properly.

Energy Efficiency/Cost Efficiency: Ductless Fume Hoods recirculate existing room air, while externally ducting systems remove heated, cooled, or dehumidified air outdoors, which requires that air to be replaced. This has the potential to waste energy and increase cost.

Recirculating vs. Exhausting Systems
Shown with a wall-mounted fume extractor, but the
same concept applies to Ductless Fume Hoods.
Click picture to view a larger image.

Minimal Maintenance & Installation Requirements/Easy Mobility: While externally ducting exhaust hoods require complicated installation because of ducting construction, Ductless Fume Hoods can be up and running with minimal installation. They are essentially “plug and play” solutions. Ductless Containment Hoods do not require any ducting or external venting and can be moved from location to location with relative ease. The only maintenance requirements are replacing saturated filters and wiping the hood down with appropriate cleaning supplies from time to time.

Sentry Air Systems offers an extensive line of standard and custom Ductless Fume Hoods. Standard sizes include 12″, 18″, 24″, 30″, 40″, 50″, 60″, 70″ widths, and custom modifications include, but are not limited to, size, material, cutouts, air volume, and multiple points of entry.

Take a look at some custom examples below:

For more information:  visit our website, call us at 1.800.799.4609, email us at, or fill out this form to receive more information from a Sentry Air Systems Applications Specialist.