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

Maintain Performance with Proper Operation
Commonly referred to as the OSHA Lab Standard, the OSHA standard for Occupational Exposure to Hazardous Chemicals in Laboratories, 29CFR1910.1450, does not specify procedures for safe hood operation, exhaust volumes, or face velocities.⁴ Basically, it requires that a chemical hygiene plan be prepared for every covered laboratory and provides the items that a complete CHP must contain. Regarding fume hoods it states, “fume hoods and other protective equipment are functioning properly and specific measures shall be taken to ensure proper and adequate performance of such equipment.” In addition, the non-mandatory Appendix A contains this statement: “airflow into and within the hood should not be excessively turbulent; hood face velocity should be adequate (typically 60-100 lfm).”³

As the operator, the lab worker must know how to adjust flows for his or her particular need. Where is capture needed for the particular experiment or task being conducted? Are you working with vapors that are lighter than air or heavier? If they are heavier than air then the dampers should be adjusted to capture at the bottom of the hood (e.g. open the bottom slot and close down the upper one). Second, check to see if storage is blocking the lower slot that may hinder flow and thus prevent proper capture. Although we do not recommend storing chemicals in the hood, one quick fix is to install a shelf above the lower baffle so that reagents and chemicals stored on the shelf do not block the lower slot. A final check for dead spots in the face velocity and hood flow is highly recommended.We recommend face velocity be checked using a grid pattern and that readings not differ by more than 10% between points. Alternately, air current or smoke tubes could be used to detect dead or low flow zones.

These chemical fume hood basics should get you started. 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.

References

1. Stanford Laboratory Standard and Design Guide, Stanford University, Environmental Health and Safety. 2006 http://www.stanford.edu/dept/EHS/prod/mainrencon/Labdesign.html
2. Laboratory Ventilation Z9.5-2003, American National Standards Institute. New York. 2003 http://webstore.ansi.org/RecordDetail.aspx?sku=ANSI/AIHA%20Z9.5-2003
3. National Research Council Recommendations Concerning Chemical Hygiene in Laboratories, CFR 1910.1450 Appendix A, Occupational Safety and Health Administration. Washington D.C. 2003 sha.gov/pls/oshaweb/ owadisp.show_document?p_table=STANDARDS&p_id=10107
4. Occupational Exposure to Hazardous Chemicals in Laboratories, Occupational Safety and Health Administration, US Department of Labor. Washington, D.C. 2006 http://www.osha.gov/pls/oshaweb/owadisp.show_document? p_table=STANDARDS&p_id=10106

Vince McLeod is an American Board of Industrial Hygiene Certified Industrial Hygienist and the senior IH with the University of Florida’s Environmental Health and Safety Division. He has 22 years of experience in all facets of occupational health and safety and specializes in conducting exposure assessments and health hazard evaluations.