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.
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.
From: Keeping The Flow: Getting The Most From Your Chemical Fume Hood by Vince McLeod, CIH in The Safety Guys