Let us compare the numbers of airborne particle or micro organism in cleanroom with our surrounding environment. The numbers of particles in general environment vary from time to time so that any fixed number cannot be determined, but roughly classified as shown in the diagram to the right. From this figure, you will see that such a clean condition in the highest class cleanroom cannot be found in the natural world, even in the upper area of stratosphere. Also, in the center of Pacific Ocean, the cleanliness level of the air is lower than that of middle class of cleanroom. In other words, cleanroom is an ultra clean space where airborne particles or micro organism are been eliminated, as we can never experience in our normal environment.
 

1.2.3 Dust generation from human body and its quantity

Human body is one of the biggest sources of dust generation. Its quantity are different from each other depending upon the person, kind of movement, kind of his garment and others and can hardly be determined. But the rough figures are shown below.