This research aims two major objectives: first, the influence of respiration on the particles concentration is investigated numerically in a cleanroom with a specified geometry and second, the respiration and the manner of particles diffusion are simulated. Generally, in many hospital cleanrooms such as the open-heart surgery, organ transplantation and neurosurgery rooms, reduction of the pollutant particles is important as a factor that can lead to capillary clogging during operation. In addition, the significance of reducing the concentration of large particles reveals more according to the fact that dust particles may act as a means for various pathogenic bacteria and viruses. Every bacterium has a dimension between 0.5 and 5 micrometers and the dimension of each virus is between 20 and 300 nanometers. Thus, it is evident that at least 1 bacterium and 15 viruses can fit on a 5-micrometer particle.