Or suck air down the stack if the building is under negative . Likely if other hoods are left on. and their exhaust could be entrained to the building. If those hoods are used or have chemical storage could have an issue. So whatever is done balancing the systems should be done.
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On Mar 30, 2020, at 11:31 AM, "Stuart, Ralph" >> In the (unlikely) event you have 4 ACH with the hoods off then the ducts will act as stacks and suck in air from the outside. ---
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> There are many class labs that are likely to have ACHs that are much higher than 4, if there is more than one hood per 500 square feet of floor space. I agree that turning hoods off entirely may not be appropriate, but if the chemicals are not being used and appropriately stored, there are likely to be opportunities to reduce air flow in many hood settings. We have reduced our air flow in our hoods significantly in our two class labs.
>
> - Ralph
>
> Ralph Stuart, CIH, CCHO
> Environmental Safety Manager
> Keene State College
> 603 358-2859
>
> ralph.stuart**At_Symbol_Here**keene.edu
>
> ---
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