> I try to avoid use of ACH as a ventilation performance criterion because the same ACH involves different volumes of air for differing floor areas and ceiling heights. Since these must play into the engineering design and cost, I think it best to start from the actual room size and intended use, look at supplemental exhaust that may be present (fume hoods, etc.) and use a total cfm performance requirement, especially when dealing with design considerations and engineers.
This is an interesting point that we run into on a regular basis here at Cornell Ithaca. The ACH rates I mentioned earlier are those that we use to operate existing laboratories. This does create confusion when discussing ventilation with engineers who are designing new laboratories and laboratory renovations, who prefer Peter's approach.
The reason we in EHS prefer to think in terms of ACH is that this gives us a clue as to how quickly we can expect a specific chemical release to be cleared from the laboratory air, whereas design parameters for total CFM require the other information Peter mentions to give us that information. But it is an ongoing source of confusion in lab ventilation discussions with different stakeholders…
Ralph Stuart CIH
Chemical Hygiene Officer
Department of Environmental Health and Safety
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