"Still bottoms," or the residues after coal tar distillation, find useful application as road construction materials, and are popular designated "road tars." The chemical composition of road tar is so complex as to constitute a "hell's field" for the industrial toxicologist, who may observe pathologic states of the greatest dissimilarity among workmen exposed to this substance. One patient may exhibit a skin carcinoma, or "tar cancer"; another a fulminating pulmonary edema; another extensive hemorrhage from the mucous membranes, lungs or stomach, or into the skin, and another convulsive seizures. This diversity of response to the toxic action of road tars becomes rational and probable in view of the variability of the chemicals that may be present.
Theoretically, road tar formation is preceded by the orderly extrusion of about 210 known substances directly derivable from coal through distillation. The principal fractions or "cuts" are shown in the accompanying table.