Airborne particulate matter (PM) is linked to a number of adverse health effects in humans. Previous studies have shown that the chemical composition, size distribution and surface reactivity of PM differs greatly between various occupational settings for military personnel and first responders. These factors also affect the toxicity of PM. The purpose of this report is to identify and clarify the mechanisms and particle characteristics that are thought to underlie the observed clinical health effects. One of the most important mechanisms for which PM acts, is perhaps through its induction of an increased production of reactive oxygen species and inflammatory responses. Chemical compounds associated with PM, including oxidized polycyclic aromatic hydrocarbons (oxy-PAHs) and transition elements, can trigger the production of reactive oxygen species. The pro-inflammatory effects of inhaled PM are characterized by cytokine/chemokine production and an increased influx of white blood cells from the blood circulation. Reactive oxygen species are also produced naturally by cells as part of the inflammatory process. Overall, this creates an oxidative stress in the cells that underlie many of the effects observed after exposure to particles, such as increased risk of blood coagulation, chronic lung diseases such as asthma and COPD, and acute cardiovascular diseases, mutations and long-lasting impact of gene expression (epigenetics). Also slower processes, such as particle induced increased risk of carcinogenicity and atherosclerosis, are believed to work by the same basic mechanisms. In conclusion, the results of mechanistic studies are in many cases limited by the ambiguous results which are primarily attributed to the complex composition of the PM studied, and by the fact that the PM activates multiple parallel systems in the body. The clearest link with increased levels of reactive oxygen species and inflammation is associated to PM having aerodynamic diameters of less than 2,5 µm and/or having a high content of transition elements and organic compounds such as PAH and oxy- PAH.