Editing Toxic heavy metal (section)

== Contamination sources ==
[[File:Tetraethyllead-3D-balls.png|thumb|[[Tetraethyl lead]] is one of the most significant heavy metal contaminants in recent use.{{sfn|Wright|Welbourn|2002|p=288}}]]

Toxic metals are found naturally in the earth, and become concentrated as a result of human activities, or, in some cases geochemical processes, such as accumulation in peat soils that are then released when drained for agriculture.{{sfn|Qureshi|Richards|McBrideBaveye|2003}} Common sources include [[fertilisers]];{{sfn|Wilson|1997}} aging water supply infrastructure;{{sfn|Harvey| Handley|Taylor|2015}} and [[microplastics]] floating in the world's oceans.{{sfn|Howell|Lavers|Paterson|Garrett|2012}}{{sfn|Cole|Lindeque|Halsband|Galloway|2011|p=2589‒2590}} Arsenic is thought to be used in connection with coloring dyes.{{sfn|Finch|Hillyer|Leopold|2015}} [[Rat poison]] used in grain and [[Mash ingredients|mash]] stores may be another source of the arsenic.{{sfn|Aggrawal|2014|p=680}}

The geographical extent of sources may be very large. For example, up to one-sixth of China's arable land might be affected by heavy metal contamination.{{sfn|Hillman|Abbot|Yu|Bain|2015|p=3349}} 

Lead is the most prevalent heavy metal contaminant.{{sfn|DiMaio|DiMaio|2001|p=527}} As a component of [[tetraethyl lead]], {{chem|(CH|3|CH|2|)|4|Pb}}, it was used extensively in [[gasoline]] during the 1930s–1970s.{{sfn|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.{{sfn|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(II) 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]].{{sfn|Houlton|2014|p=50}}