Most health effects associated with biological particles are various respiratory conditions and skin reactions. Their mechanisms vary from infections, irritation symptoms, and allergic diseases to toxic or immunotoxic reactions and other conditions with less evident pathophysiology. It is often the case that epidemiological and clinical evidence of the association between biological agents and the adverse health effect is strong, but the causal link between the exposing agents and the disease are poorly known. Furthermore, the mechanistic pathways leading to a certain condition are often not well known.