Hazardous air pollutants or air toxics are pollutants that are known or suspected to cause serious health effects. This paper presents a methodology to quantify source contributions to air toxics health risks. First, a linear, no-threshold risk model was used to identify gas-phase organic air toxics that contribute significantly to cancer risks. Next, Positive Matrix Factorization (PMF) was performed on high time-resolved measurements of these air toxics, and the additive cancer risks associated with each factor was determined. Finally, the PMF factors were linked to sources and source classes (mobile, nonmobile, secondary/background) using a combination of meteorological data and comparisons with published source profiles. The analysis was performed using data from three sites in Pittsburgh, Pennsylvania: a downtown site near a heavily traveled bus route, a residential site adjacent to a heavily industrialized area, and an urban background site. At all three sites emissions from nonmobile sources were the dominant contributors to the cancer risks from air toxics included in the PMF model, including benzene and other air toxics often associated with mobile source emissions. Emissions from both large industrial sources, such as coke works and chemical facilities, and smaller point sources, such as dry cleaners, contributed significantly to the cancer risks at all sites. This method can provide insight for decision makers to prioritize sources for risk reduction.