Minimum effective velocity of a HEPA filter is:

• A. 80 ± 20 ft/min
• B. 90 ± 20 ft/min
• C. 100 ± 10 ft/min
• D. Two of the above

Answer: (D)

The minimum velocity through a HEPA filter is the lowest airspeed at which the filter can still meet its specified efficiency. In practice, that minimum is often given as a range because different testing standards and tolerances are used. The two ranges around 80 ± 20 ft/min (60–100 ft/min) and 90 ± 20 ft/min (70–110 ft/min) reflect commonly accepted lower bounds for effective performance under different standards. The option centered at 100 ± 10 ft/min (90–110 ft/min) sits higher and is not treated as the minimum threshold, so it isn’t counted as one of the minimum-velocity bands. Therefore, two of the options describe acceptable minimum effective velocity ranges, making “two of the above” the best choice.