Occupational dermatoses may be caused by organic substances, such as formaldehyde; and by solvents or inorganic materials, such as acids and alkalis, and chromium and nickel compounds. Skin irritants are usually either liquids or dusts; they may have a primary toxic effect, as with solvents, acids and alkalis, or produce an allergic reaction after 3-4 weeks of exposure or longer (chromium and nickel compounds, formaldehyde). Dermatosis or eczema develops mainly on the skin areas exposed at work, such as the hands and forearms, but also on other parts of the body as a result of contact with contaminated clothes. Exposure to fine arsenical powder in the handling of arsenic compounds causes the development of warts on the skin; these may become malignant.