Number of fibers or particles in a volume of air was used earlier because it was more accurate to sample a volume of air by passing it through a membrane filter and then counting the particles present on the filter by using a microscope. With electronic balance available today it is much easier to weigh the filter before and after air sampling and get the difference in mg. Knowing the mass deposited on the filter and the volume of air sampled through the filter gives the answer in mg/m³. This is called “gravimetric” analysis. The other way is to perform a chemical analysis for the aerosol collected on the filter. However, there are still exceptions, i.e., asbestos, requiring microscopic evaluation.