For ISO- 5 clean zones, USP 797-compliant testing includes determining if
set up with readings taken every 12 in. on center.
Compute the arithmetic mean of the velocities recorded.
Ex. average x = (y1 + y2 +…+ yn /n).
y= Average of each test position
n= Number of test positions
x = Average of filter face velocity
Compute the standard deviation (S)²=((y1 - x)² + (y2 - x)²
+ ….+ (yn - x)²) / (n-1)
Compute the relative standard deviation (RSD)= (standard
deviation (S) / average velocity (x)) 100
Most compounding pharmacy clean zones have I.V. bars
and bags which makes it important to get the relative standard
deviation as low as possible. IEST-RP-CC-002.3 Section 6.1.1
states, “The maximum relative standard deviation is typically
15% when using an electronic micro-manometer with multi-
It’s preferable to keep the RSD below 10% to create the
effect necessary to pass the smoke challenge without high
reflux, dead spots, or airflow that travels sideways as opposed
to straight down.
Selecting the filters for the clean zones
Careful consideration of filter manufacturers is recommended
as we have seen large discrepancies in uniformity of face
velocities among different manufacturers and even filter types,
like HEPA to ULPA designs. Uniformity or unidirectional flow
is the key to the functionality of these systems as well as a continuous low level return beneath the work zone.
Achieving a deviation of less than 14 degrees in an
ISO- 5 clean zone
There must be a continuous length of HEPA filter across a
wall away from the door with a 4 in. continuous wall return
cavity along the back of the area. The clean zone must have
100% coverage of the area, and use a polycarbonate directional flow shield from the ceiling grid to 5 ft. off the floor.
The work table or zone should be moved nearer or farther
away from the back wall as necessary to cause a waterfall
effect on the front and back edges of the work table. A solid
table produces an effect similar to a biological safety cabinet
centerline smoke split. (Note: Perforated tables are difficult
to clean and maintain.) Be sure the table is no more than