The particles that accumulate on and around the diffusers are predominantly from the room being supplied and not from the HVAC system. The
particles in this set of photographs were on tapelifts from the "black" deposit around the HVAC system diffuser supplying air to this office space.
These particles are not from the HVAC system but are typical of the dust found on other surfaces in this office. Particles from the HVAC
system would show accumulations of much smaller particles on their surfaces as a result of being exposed to the moving air stream. This
set of photographs also demonstrates the importance of using more than one form of illumination to characterized the particles in the field