This chapter focuses on particulate air pollutants, specifically introducing the concepts for their formation, propagation and measurements. Presented alongside, where applicable, are the trends showing contribution from different natural and anthropogenic sources to the ambient air concentrations. The chapter discusses the general aspects of particles, such as definition of size and the way they behave in the atmosphere, and describes the sources of particles and their measurement methods. The finest metallurgical fumes are condensed volatiles that are so small that they overlap with the molecular size range, although these are often quickly aggregated into longer chains. Real atmospheric particle size distributions are the outcome of many competing processes – condensation, sedimentation, evaporation, agglomeration, impaction, gas-to-particle conversion. Reducing ambient Particulate matter levels is difficult due to the large contribution of secondary particles formed in the atmosphere from a range of gaseous pollutants.