Arsenic is a naturally occurring metalloid found in soil, water, and food. Humans are exposed through contaminated drinking water (a significant issue in some parts of Asia, South America, and regional Australia), consumption of rice and seafood (which can contain organic arsenic), and occupational exposure in mining, smelting, and pesticide manufacturing. Two forms exist: inorganic arsenic (more toxic, found in water and soil) and organic arsenic (less toxic, found in seafood).

Blood arsenic testing measures total arsenic concentration in the bloodstream and reflects recent exposure rather than cumulative body burden. Chronic low-level exposure to inorganic arsenic is a known carcinogen associated with bladder, lung, and skin cancers, as well as cardiovascular disease, peripheral neuropathy, and skin changes (hyperkeratosis, Mees' lines on nails). Urine arsenic measurement provides a better assessment of ongoing exposure; blood arsenic reflects acute recent intake.