Coal forms from the accumulation of plant debris, usually in a swamp environment. When plant debris dies and falls into the swamp the standing water of the swamp protects it from decay. Swamp waters are usually deficient in oxygen, which would react with the plant debris and cause it to decay.
Porokeratosis is generally considered to be a benign condition though malignant transformation has been seen in all but the punctate variety, necessitating regular followup of patients with porokeratosis (Maubec et al., 2005; Sasson & Krain, 1996).
The differential diagnosis of lung nodules includes many benign diseases. Benign etiologies of pulmonary nodules include healed or nonspecific granulomas and active granulomatous infections, which account for 25% and 15% of all benign causes, respectively. Active granulomatous infections include tuberculosis, coccidioidomycosis, histoplasmosis, cryptococcus, and aspergillosis.