in the atmosphere. Humans are exposed to black carbon by inhalation of air in the immediate vicinity of local sources. Important indoor sources include candles and biomass burning whereas traffic and occasionally forest fires are the major outdoor sources of black carbon exposure. Concentrations of black carbon decrease sharply with increasing distance from (traffic) sources which makes it an atypical component of particulate matter. This makes it difficult to estimate exposure of populations. For particulate matter, epidemiological studies have traditionally relied on single fixed site measurements or inferred residential concentrations. Recent studies have shown that as much black carbon is inhaled in traffic and at other locations as at the home address. Despite the fact that a large portion of the exposure occurs as short peaks of high concentrations, it is unclear how to define peaks and determine their frequency and health impact. High peak concentrations are encountered during car driving. High in-vehicle concentrations of black carbon have been associated with driving during rush hours, on highways and in dense traffic. Even relatively low exposure concentrations of black carbon have a direct effect on the lung function of adults and an inflammatory effect on the respiratory system of children. A recent study found no effect of black carbon on blood pressure when combined with physical activity. The public health benefits of reduction in the amount of soot and other particulate matter has been recognized for years. However, high concentrations persist in industrializing areas in Asia and in urban areas in the West such as Chicago. The WHO estimates that air pollution causes nearly two million premature deaths per year. By reducing black carbon, a primary component of fine particulate matter, the health risks from air pollution will decline. In fact, public health concerns have given rise to leading to many efforts to reduce such emissions, for example, from diesel vehicles and cooking stoves. Climate impacts Direct effect Black carbon particles directly absorb sunlight and reduce the planetary albedo when suspended in the atmosphere. Semi-direct effect Black carbon absorb incoming solar radiation, perturb the temperature structure of the atmosphere, and influence cloud cover. They may either increase or decrease cloud cover under different conditions. Snow/ice albedo effect When deposited on high albedo surfaces like ice and snow, black carbon particles reduce the total surface albedo available to reflect solar energy back into space. Small initial snow albedo reduction may have a