Arsenic
Arsenic is widely distributed in the environment, particularly in rocks, sediments and some water supplies. Organic forms of arsenic (e.g. Arsen betaine, Arsencholine) occur naturally in seaweed, fish and shellfish, but are mostly nontoxic. Arsenic is metabolised in the liver to mono- and dimethyl lated species as a means of detoxification. Different forms of arsenic have widely differing human toxicity. These forms are also handled by the body in different ways, which causes problems in their determination and in the interpret ation of laboratory findings, particularly when the source of arsenic is unknown. Arsenic has three common valence or oxidation states: 0 (metalloid), 3+ (arsenite) and 5+ (arsenate). The trivalent inorganic salts of arsenic (e.g. sodium arsenite, NaAsO2) are the most toxic and may cause serious toxicity or death after acute ingestion of relatively small doses (<200 mg). Inhalation of arsine gas (AsH3) may cause massive haemolysis, renal failure and rapid death, as in industrial accidents. The signs and symptoms of chronic arsenic poisoning (arsenic oxide) are quite complex and include weight loss, malaise, hyperpigmentation of the skin, transverse white lines on the nails, liver damage, changes in the blood, peripheral neuropathy and increased risk of skin and liver cancer. Acute poisoning is characterised by bloody diarrhoea, vomiting, excruciating abdominal pain, circula tory collapse and coma. Skin and respiratory cancer is another major feature of chronic arsenic exposure; huge populations are affected in parts of India, Bangladesh and Vietnam because of the consumption of contaminated well-water supplies (Piamphongsant 1999; Ahsan et al. 2000; Chowdhury et al. 2000; Smith et al. 2000; Berg et al. 2001). The main sources of the problem are wells sunk into land that contains rocks with a high content of arsenic salts, with subsequent contamination of the underground aquifers. Ingestion of seafood can give rise to elevated levels of arsenic and consideration should be given to this possibility when interpreting analytical results. Measure ment of urine arsenic helps in the assessment of acute or chronic exposure (Apostoli et al. 1999). Hair analysis has also been used in the diagnosis and evaluation of chronic arsenic poisoning, particularly suspected homicides. However, there can be problems distinguishing external contamination from ingested arsenic (Hindmarsh 2002).
Normal values for arsenic are 10 lg/L in blood and urine but elevated values can be seen after ingestion of seafood and where there is occupational exposure, and elevated levels of arsenic potentially arising from these sources should be taken into account when assessing measured arsenic concentrations. In cases of acute inorganic arsenic poisoning, concentrations above 500 lg/L may be seen in blood and urine.
