What is the dirtest fossil fuel to burn?

The combustion of any fossil fuel creates pollution. Pollution can occur during the extraction, transportation, refining (for petroleum products) and burning. There isn't complete agreement on the dirtiest fossil fuel, but many would consider coal or oil as the worst. Without any scrubbers, coal probably takes number one position. Measures of the harmful effects are difficult, because it is the amount that is burned times the pollutants given offf. There are different forms of coal, with varying harmful effects. Also, there is a lot of ongoing efforts to make coal a clearer (not clean) fossil fuel. See related links.