The continuous occlusive application of 25% crude coal tar to the backs of young adult males for three weeks regularly induced an acneiform eruption. In blacks, this was a monomorphous crop of comedones that were of the open variety from the very start. In whites, the first lesions were papulopustules resulting from toxic disorganization of the follicular epithelium. This inflammatory phase was followed by comedones.
Kaidbey KH, Kligman AM. A Human Model of Coal Tar Acne. Arch Dermatol. 1974;109(2):212-215. doi:10.1001/archderm.1974.01630020028006