Metal ions occur naturally in rocks, soils, gases, and waters in both harmless and harmful forms and
concentrations. Metals are important in environmental health and on the study of human diseases
(pathology) because of their potential toxic effect(s) to one or more organs. Exposure to toxic metal ions
may occur via three principle routes: percutaneous absorption, ingestion, or inhalation. Dermal toxicity
results from local tissue responses through direct contact of the metal with skin, or alternatively, may
represent a manifestation of systemic toxicity following ingestion or inhalation. Allergic contact
dermatitis induced by nickel (Ni) is an example of a local tissue response. The adverse skin reactions
resulting from chronic ingestion or inhalation of arsenical compounds exemplify systemic toxicity. A
variety of toxic pathology responses in human tissues and organs (i.e., skin, liver, heart, kidney

Metal ions occur naturally in rocks, soils, gases, and waters in both harmless and harmful forms and concentrations. Metals are important in environmental health and on the study of human diseases because of their potentially toxic effect(s) to one or more organs.  Exposure to toxic metal ions may occur via three principle routes: (1) percutaneous absorption, or dermal contact, (2) ingestion, and (3) inhalation.  Dermal toxicity results from local tissue responses through direct contact of the metal with skin.  Allergic contact dermatitis induced by nickel (Ni) is an example.