The binder not only binds sand to sand, but also binds dirt to skin. Pitch warts represent a type of skin reaction to several chemicals that cause epidermal hyperplasia, which may be associated with tumor or cancer formation. Certain petroleum intermediates and products, retorted shale oil, coal liquification intermediates, a number of coal tar derivatives and arsenic can stimulate the epidermal cells to produce these effects.
Friction produces calluses, blisters, abrasions and lichenification.
Pressure causes bullae, skin atrophy and necrosis. Other trauma brings about the Koebner phenomenon in which a dermatosis localizes itself at the site of trauma. Here you see calluses and fissures produced by repetitive hand motion and by the use of certain types of tools. Commonly affected sites are flexural areas and where clothing is restrictive. In some individuals, especially those with dermographism, small erythematous papules may occur where the spicules have penetrated, as shown here.