Rolling Coal is the practice of altering the fuel and exhaust system in a diesel vehicle so that there is a high volume of uncombusted fuel and soot in the exhaust stream. The resulting dark plume is toxic, haze-forming, and should be illegal.
The pollutant in the dark plume is referred to as "particulate matter," and is classified as a group 1 carcinogen. According to wikipedia, "particulates are the deadliest form of air pollution due to its ability to penetrate deep into the lungs and blood streams unfiltered, causing permanent DNA mutations, heart attacks and premature death." (http://en.wikipedia.org/wiki/Particulates) This same toxic plume also contributes to smog, which has numerous negative health effects in urban areas, and contributes heavily to the formation of acid rain.