Tracer Particles should remain visible from where they are diffused into the clean air system being tested all the way to the air return. This allows for the visualization of air flow directions and air mixing in both unidirectional and non-unidirectional air flow cleanrooms. Microrite Tracer Particles have an extremely low vapor pressure, creating Tracer Particles that take up to 20 minutes to evaporate leaving no residual Tracer Particles on HEPA filters.