This report presents the results of a systematic review of evidence of the health effects of black carbon in ambient air. Epidemiological studies provide sufficient evidence of the association of cardiopulmonary morbidity and mortality with exposure to black carbon. Toxicological studies suggest that black carbon may operate as a universal carrier of a wide variety of chemicals of varying toxicity to the human body. Although black carbon may not be a major, directly toxic component of fine particulate matter, reducing people’s exposure to particulate matter containing black carbon should reduce its effects on their health, as well as helping to mitigate climate change.