Air is a mixture of gases composed of approximately 21% oxygen, 78% nitrogen, 1% argon and carbon dioxide, and traces of other gases. The air we breathe also includes particulate material and gases generated by nature, by man, and by industrial processes. We are concerned with the particulate matter and gasses that influence our health, our comfort, that damage spaces we occupy, or that affect the products or components we are manufacturing.

The concentration of different size particles in the atmosphere is usually measured by weight or count. Methods for testing filters are based on these two techniques.

Weight consideration can be determined by drawing a measured amount of air through a pre-weighted filter paper target. The paper can then be reweighed to determine its weight increase. This increase represents the weight of the dust in the quantity of air, which was drawn through the paper filter. Knowing the total weight of dust and the volume of air sampled, one can calculate the weight concentration of dust for a unit volume of air.

Counting and sizing of dust particles in the atmosphere were once tedious tasks. Initially, air samples were drawn though impinges in which the airborne dust was captured in a measured amount of water or some other liquid. Representative samples of the liquid containing the dust in suspension were then examined under a microscope and the different size particles counted.