Proper Fume Hood Operation
Share this"We have a circumstance that we constantly face here in my laboratory, that 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 that we can use the hood without rearranging or removing the contents. Is there an OSHA standard that addresses this situation?"
The OSHA standard for Occupational Exposure to Hazardous Chemicals in Laboratories, 29CFR1910.1450, commonly referred to as the OSHA Lab Standard, does not specify safe hood operation, flows, or face velocities. It mandates a chemical hygiene plan be prepared for every covered laboratory and lists the requirements of the CHP, one of which is: “A requirement that fume hoods and other protective equipment are functioning properly and specific measures that shall be taken to ensure proper and adequate performance of such equipment.” 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).”
It is up to the operator to know how to adjust flows for their particular need.
Some storage in the hood may not affect your use and could be left in while performing other operations. Things to check and keep in mind are: First, are there dead spots in the face velocity and are they located where capture is needed? We recommend face velocity be checked using a grid pattern and that readings not differ by more than 10%.
Alternately, air current or smoke tubes could be used to detect dead or low flow zones. Second, where is capture needed? Are you working with vapors that are lighter than air or heavier? If they are heavier than air, 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) 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 okay with some storage in the hood. Use smoke tests to confirm this.
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 Fume Hood Basics by the Safety Guys