Fuel is a hydrocarbon (closely defined as Octane which has the chemical formula C8H18)
when it burns you thus get CO2 (Carbon Dioxide), H2O (Water), CO (Carbon Monoxide) and some C (Pure Carbon) (OK you'll get various oxides of Nitrogen due to the presence of Nitrogen in the air)
The pure carbon manifests itself as small black particulates which is what the filter helps to remove due the carcniogenic nature of this item