Ultrafine particles are airborne particulates of less than 100 nm in aerodynamic diameter. Examples of ultrafine particles are diesel exhaust particles, products of cooking, heating and wood burning in indoor environments, and more recently, products generated through the use of nanotechnology. Studies have shown that ambient ultrafine particles have detrimental effects on both the cardiovascular and respiratory systems, including a higher incidence of atherosclerosis and the exacerbation rate of asthma. Ultrafine particles have been found to alter in vitro and in vivo responses of the immune system to allergens and may also play a role in allergen sensitization. The inflammatory properties of ultrafine particles may be mediated by a number of different mechanisms, including the ability to produce reactive oxygen species, leading to the generation of pro-inflammatory cytokines and airway inflammation. In addition, because of their small size, ultrafine particles also have unique distribution characteristics in the respiratory tree and circulation and may be able to alter cellular function in ways that circumvent normal signaling pathways. Additionally, ultrafine particles can penetrate intracellularly and potentially cause DNA damage. The recent advances in nanotechnology, while opening up new opportunities for the advancement of technology and medicine, could also lead to unforeseen adverse health effects in exposed humans. Further research is needed to clarify the safety of nanoscale particles, as well as the elucidation of the possible beneficial use of these particulates to treat disease.