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Arsenic is an element that occurs naturally in soil and rock, especially in minerals and ores that contain copper or lead. Arsenic may enter the environment during the mining and smelting of these ores. In the past, arsenic was used as a pesticide, primarily in cotton fields and orchards. The use of arsenic has declined, but it can still be found as a preservative for "pressure-treated" wood.

The concentration of arsenic in natural surface and groundwater is generally about 1 part in a billion parts of water (1 ppb) but may exceed 1,000 ppb in mining areas or where arsenic levels in soil are high. Groundwater is far more likely to contain high levels of arsenic than surface water. Soils near arsenic-rich geological deposits, mining and smelting sites, or agricultural areas with a history of arsenic pesticides may contain relatively high levels of arsenic.

Most arsenic is excreted in urine, but it can accumulate in the body and cause a variety of health problems, including cancer. Laboratory tests can measure arsenic in your blood, urine, hair, or fingernails.

If you work with arsenic-treated wood, you can limit exposure to sawdust with dust masks, gloves, and protective clothing. If you live in an area with a high level of arsenic in the water or soil, you can drink bottled water and limit contact with soil through use of a dense groundcover or thick lawn.

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