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Toxic heavy metal
A toxic heavy metal is any relatively dense metal or metalloid that is noted for its potential toxicity, especially in environmental contexts.〔Srivastava & Goyal 2010, p. 2〕 The term has particular application to cadmium, mercury, lead and arsenic,〔Brathwaite & Rabone 1985, p. 363〕 all of which appear in the World Health Organisation's list of 10 chemicals of major public concern.〔World Health Organisation 2015〕
Toxic heavy metals are found naturally in the earth, and become concentrated as a result of human caused activities. They enter plant, animal and human tissues via inhalation, diet and manual handling, and can bind to, and interfere with the functioning of vital cellular components. The toxic effects of arsenic, mercury and lead were known to the ancients but methodical studies of the toxicity of some heavy metals appear to date from only 1868. In humans, heavy metal poisoning is generally treated by the administration of chelating agents. Some elements regarded as ''heavy metals'' are essential, in small quantities, for human health.
== Contamination sources ==
Toxic heavy metals are found naturally in the earth, and become concentrated as a result of human caused activities. Common sources are from mining and industrial wastes; vehicle emissions; lead-acid batteries; fertilisers; paints; treated woods; and aging water supply infrastructure.〔Harvey, Handley & Taylor 2015〕 Arsenic, cadmium and lead may be present in children's toys at levels that exceed regulatory standards. Lead can be used in toys as a stabilizer, color enhancer, or anti-corrosive agent. Cadmium is sometimes employed as a stabilizer, or to increase the mass and luster of toy jewelry. Arsenic is thought to be used in connection with coloring dyes.〔Finch, Hillyer & Leopold 2015, pp. 849–850〕
Lead is the most prevalent toxic heavy metal contaminant.〔Di Maio 2001, p. 527〕 As a component of tetraethyl lead, , it was used extensively in gasoline during the 1930s–1970s.〔Lovei 1998, p. 15〕 Lead levels in the aquatic environments of industrialised societies have been estimated to be two to three times those of pre-industrial levels.〔Perry & Vanderklein 1996, p. 336〕 Although the use of leaded gasoline was largely phased out in North America by 1996, soils next to roads built before this time retain high lead concentrations. Lead (from lead azide or lead styphnate used in firearms) gradually accumulates at firearms training grounds, contaminating the local environment and exposing range employees to a risk of lead poisoning.〔Houlton 2014, p. 50〕
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