Question regarding classification of passthroughs and laminar flow hoods

The Question posed on LinkedIn:

Can we provide the grades of pass box and lafs in on critical area with respect to material movement ? For example if a pass box installed between grade c and grade d environment can be designated as grade b? Similarly laf installed in grade c area can be designated as grade c? If yes what woukd be the viable and non viable limits to be applied?

My reply was this:

If you have a pass through linking a grade D to a grade C area, you could classify it as Grade C with positive pressure.  Think of it as a step over in an airlock.  One side of the step over is Grade D, the other Grade C.   If you have a laminar flow hood in a Grade C area, then you could conceivably classify that as higher than Grade C.  You’d need to validate this of course.  Your limits are based on what the regs require at most and what your commissioning studies show at least..”