Particle fallout in cleanrooms is a primary concern for industries manufacturing critical products with surfaces highly susceptible to particulate contamination. Limited research has been conducted to measure the deposited particles in cleanroom. In the frame of the ISO 14644-3, this paper compares the traditional technique for particle fallout measurements using witness plates to a real-time and in situ measurement done by an innovative device: CLEAPART-100. The CLEAPART-100 detects the same contamination event as optical counters detect. The comparison with the settle plates observed under a microscope shows the loss of particles using that indirect measurement which is unacceptable for an accurate risk analysis assessment.