BACKGROUND: The death toll associated with inhaled ambient particulate matter (PM) is attributed mainly to cardio-vascular rather than pulmonary effects. However, it is unclear whether the key event for cardiovascular impairment is particle translocation from lung to circulation (direct effect) or indirect effects due to pulmonary particle-cell interactions. In this work, we addressed this issue by exposing healthy mice via inhalation and intra-arterial infusion (IAI) to carbon nanoparticles (CNP) as surrogate for soot, a major constituent of (ultrafine) urban PM. METHODS: Equivalent surface area CNP doses in the blood (30mm(2) per animal) were applied by IAI or inhalation (lung-deposited dose 10,000mm(2); accounting for 0.3% of lung-to-blood CNP translocation). Mice were analyzed for changes in hematology and molecular markers of endothelial/epithelial dysfunction, pro-inflammatory reactions, oxidative stress, and coagulation in lungs and extra-pulmonary organs after CNP inhalation (4 h and 24 h) and CNP infusion (4 h). For methodological reasons, we used two different CNP types (spark-discharge and Printex90), with very similar physicochemical properties [≥98 and ≥95% elemental carbon; 10 and 14 nm primary particle diameter; and 800 and 300 m(2)/g specific surface area] for inhalation and IAI respectively. RESULTS: Mild pulmonary inflammatory responses and significant systemic effects were observed following 4 h and 24 h CNP inhalation. Increased retention of activated leukocytes, secondary thrombocytosis, and pro-inflammatory responses in secondary organs were detected following 4 h and 24 h of CNP inhalation only. Interestingly, among the investigated extra-pulmonary tissues (i.e. aorta, heart, and liver); aorta revealed as the most susceptible extra-pulmonary target following inhalation exposure. Bypassing the lungs by IAI however did not induce any extra-pulmonary effects at 4 h as compared to inhalation. CONCLUSIONS: Our findings indicate that extra-pulmonary effects due to CNP inhalation are dominated by indirect effects (particle-cell interactions in the lung) rather than direct effects (translocated CNPs) within the first hours after exposure. Hence, CNP translocation may not be the key event inducing early cardiovascular impairment following air pollution episodes. The considerable response detected in the aorta after CNP inhalation warrants more emphasis on this tissue in future studies.