Other Laboratory Exhaust Systems

Other Laboratory Exhaust Systems

Many laboratories use equipment and apparatus that can generate airborne contaminants, but cannot be used within a fume hood. Examples include gas chromatographs, ovens, and vacuum pumps.

Other types of local exhaust ventilation systems may be required to control contaminants generated by these operations. Such systems must not be installed without explicit approval of the building facility manager, Facilities Engineering and/or maintenance personnel.

Elephant Trunks

An elephant trunk is a flexible duct or hose connected to an exhaust system. It can only capture contaminants that are very close to the inlet of the hose, typically less than a distance equal to one half of the diameter of the duct.

Elephant trunks can be effective for capturing discharges from gas chromatographs, pipe nipples or the end of tubing. However, the effectiveness of the elephant trunk should be carefully evaluated before they are used to control releases of hazardous substances.

Canopy Hoods

A canopy hood in a laboratory is constructed in a similar fashion to the overhead canopy hoods seen in kitchens. In order for the canopy hood to be able to capture contaminants, the hood requires a relatively large volume of air movement, making them somewhat costly to operate. The canopy hood works best when the thermal or buoyant forces exist to move the contaminant up to the hood capture zone.

One of the biggest problems with canopy hoods is that, in most cases, they are designed such that the contaminated air passes through the individual's breathing zone. The airflow is easily disrupted by cross currents of air.

For the most part, canopy hoods should only be used for exhaust of non-hazardous substances.

Slot Hoods

There are many types of slot hoods, each suited for different types of operations. In general, a slot hood requires less airflow than a canopy hood and is much more effective than an elephant trunk or canopy hood, when installed properly.

Slot hoods are best used for operations that require more working room than a fume hood and where a limited number of low toxicity chemicals are used. The placement of the opening(s) and the velocity of airflow are based on the application, particularly dependent upon the vapor density of the chemical(s).

Examples of good uses for slot hoods are darkrooms and acid dipping operations.

Downdraft Tables

Downdraft hoods or necropsy tables are specially designed work areas with ventilation slots on the sides of the work area. This type of system is useful for animal perfusions and other uses of chemicals with vapor densities heavier than air.

Toxic Gas Cabinets

Highly toxic or odorous gases should be used and stored in gas cabinets. In the event of a leak or rupture, a gas cabinet will prevent the gas from contaminating the laboratory.

Gas cabinets should be connected to laboratory exhaust ventilation using hard duct, rather than elephant tubing, since such tubing is more likely to develop leaks. Coaxial tubing should be used for delivering gas from the cylinder to the apparatus. Coaxial tubing consists of an internal tube containing the toxic gas, inside another tube. In between the two sets of tubing is nitrogen, which is maintained at a pressure higher than the delivery pressure of the toxic gas. This ensures that, in the event of a leak in the inner tubing, the gas will not leak into the room.

Biosafety Cabinets

Please see the Biosafety Cabinet page for more information.

Ductless Fume Hoods

Use of a "ductless fume hood" is strongly discouraged. These devices work by using a fan to draw air into a chamber, through one or more filters, and back into the laboratory. EHS and several professional safety and engineering organizations do not recommend the use of ductless fume hoods for several reasons. First, it is difficult to determine whether the filters are functioning adequately or need to be changed; thus, the potential for recirculating toxic materials into the laboratory is significant. In the event of a chemical spill, the hood is usually not able to contain the spilled material or the potentially high concentrations of chemical vapors.

Second, the face velocity of the hood is normally below 80 feet per minute. The hood is normally designed such that the air does not flow smoothly and evenly through the hood. Both of these characteristics make it likely for disruption of airflow or turbulence, causing unfiltered air to leak into the laboratory.

Clean Benches

Clean benches are similar to appearance as a fume hood however do not exhaust air from the laboratory. A clean bench is a device that draws air from the lab through a HEPA filter and vents the filtered air downwards onto a work surface to keep the materials within free from particulate contamination. These devices are not to be used with hazardous materials as they provide no personal protection. Do not store materials on top of this hood as this will block the filter, overload the motor, and provide poor product protection.




Stanley Howell
Sr. Program Manager
Chemical Safety

Steve Elwood
Director for Research Safety