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Know Your Flow!

The basic design principles and proper operation of your chemical fume hood.

Vince McLeod, CIH

Vince McLeod is an American Board of Industrial Hygiene-certified industrial hygienist and the senior industrial hygienist with Ascend Environmental + Health Hygiene LLC in Winter Garden, Florida. He has more...

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In a recent Lab Manager article (October, 2009), I wrote about the different types of chemical fume hoods, from constant volume to variable volume and everything in between. In research laboratories the fume hood is probably the single most-used piece of equipment. It is often shared by lab personnel and, if located in a common area, with many labs. This often leads to situations such as the one described in an e-mail I received not long ago.

A thoughtful reader wrote: Your article pointed out that hood function can be compromised by misuse. You specifically cite a condition that we constantly face here in my laboratory, which is the blockage of the back bottom slot by reagent bottles and overloading of the hood. The face velocity (at appropriate sash height) meets the required flow in spite of these conditions. That being the case, is it necessary to remove these items? We operate under the belief that as long as the face velocity meets specifications, we can use the hood without rearranging or removing the contents. Is there an OSHA standard that addresses this situation?

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I felt that the response to this question would be beneficial to all Lab Manager  readers. So I want to discuss here the basic design principles and proper operation of the chemical fume hood. One problem with hoods that we see a lot is that they are frequently used for storage and become a repository for longcompleted experiments and waste. Since the readers question dealt with chemical fume hood operation, lets take a look at fume hood design features and proper operation.

Chemical fume hoods are designed to capture and exhaust contaminants resulting from working with chemicals. Fume hoods are sometimes referred to as wet benches, since the chemicals used (solvents, corrosives, etc.) are usually liquid. Their design attempts to create a capture zone in front of the hood, to keep generated vapors and fumes within the hood, and to draw contaminants away from the worker and into the hood, where they are exhausted.

Fume hood design basics – Flow is key

One of the most important design features of a chemical fume hood is the entry. Basic principles of aerodynamics are used to promote a smooth flow of air into the hood. The sides and the sill (the lower lip across the front) are shaped similarly to the leading edge of an airplane wing, a foil, to guide the airstreams into the hood with a minimum of turbulence. The idea is to maintain a laminar, non-turbulent flow. The sill is also raised slightly off the bottom, or floor, of the hood to create an airstream across this surface. Take a look at your fume hood. Notice the gently sloping curved or angled edges on the top, bottom and sides of the entry. These edges help direct the airflow into the fume hood.

Since a smooth entry is vital, placement of the hood in the laboratory merits careful consideration. They should not be located near doors, busy walkways, or room air supply or return ducts, all of which can cause turbulence and disrupt laminar flow.

Another important design parameter is the velocity of the airflow entering the hood. The speed of the air needs to be just right. If it is too slow, it will not capture contaminants or push them out of the exhaust duct. If it is too fast, turbulence and eddies can lead to slipstreaming and dumping contaminants into the laboratory. The speed of the air across the hood opening is referred to as face velocity and is a function of the total exhausted volume and the area of the opening. The basic relationship is one in which velocity is equal to the flow volume divided by the area. So as the area increases, the velocity drops, and vice versa. Also, if the flow volume is reduced, the velocity goes down. The hood sash, the sliding door or window on the front of the hood, controls the area open for flow and thus controls the face velocity. Each hood design uses this basic principle differently. The main types of hoods are standard hoods, bypass hoods, auxiliary air hoods and VAV (variable air volume) hoods.

The third major design feature is the baffling, or guiding, of the flow in the hood. Chemical fume hoods are designed to handle a wide variety of operations and contaminants. Typically this is done with a series of baffles on the back wall and/or top of the hood. Baffles are slots with adjustable sliding covers, usually located near the bottom, center and top of the hood’s back panel. The opening and closing of the appropriate baffles allow more flow to be guided across the bottom, into the center slot or toward the top of the hood.

Final answer – Proper operation using the basics

The OSHA standard for Occupational Exposure to Hazardous Chemicals in Laboratories,1 29CFR1910.1450, commonly referred to as the OSHA Lab Standard, does not specify safe hood operation, flows or face velocities. It mandates that a chemical hygiene plan be prepared for every covered laboratory and lists the requirements of the CHP. One of those requirements states that “fume hoods and other protective equipment must be functioning properly and specific measures shall be taken to ensure proper and adequate performance of such equipment.” The nonmandatory Appendix A contains this statement: “[A]irflow into and within the hood should not be excessively turbulent; hood face velocity should be adequate (typically 60-100 lfm).”2

It is up to the operator to know how to adjust flows for his or her particular need. Some storage in the hood may not affect your use and could be left in place while performing other operations. Here are some things to check and keep in mind:

Are there dead spots in the face velocity or inside the hood, and are they located where capture is needed? We recommend that face velocity be checked using a grid pattern with a minimum of six readings and that readings not differ by more than 10 percent. Alternately, air current or smoke tubes could be used to detect dead or low-flow zones.

Where is capture needed? Are you working with vapors that are lighter or heavier than air? If they are heavier than air, the dampers should be adjusted to capture at the bottom of the hood (i.e., open the bottom slot and close down the upper one), and storage blocking the lower slot may hinder flow and thus hinder proper capture. One quick fix is to install a shelf above the lower baffle so reagents and chemicals stored on the shelf do not block the lower slot. If the vapors are lighter than air, you may be OK with some storage in the hood. Use smoke tests to confirm this.

These chemical fume hood basics should get you going in a safe direction. Pay attention to proper flow, and remember to adjust the baffles according to the work being done. Finally, routinely check the hood for adequate flow and velocity, and recheck if you suspect a problem. Comments or questions are always welcome. Contact thesafetyguys@


1. Occupational exposure to hazardous chemicals in laboratories, Occupational Safety and Health Administration, Washington, D.C. 2006. http:// owadisp.show_document?p_ table=STANDARDS&p_ id=10106.

2. National Research Council Recommendations Concerning Chemical Hygiene in Laboratories, OSHA Lab Standard, Appendix A (Non-Mandatory), Occupational Safety and Health Administration, Washington D.C. 2006. http:// owadisp.show_document?p_ table=STANDARDS&p_ id=10107.