Arsenic is a naturally-occurring metalloid - that is, a substance having both metallic and non-metallic properties. Arsenic in nature is virtually always combined with other elements, often in minerals that also contain copper or lead. Most arsenic compounds are solids with no odor or taste. Arsenic in combination with carbon and hydrogen is called organic arsenic, while arsenic in combination with with elements like oxygen, sulfur, and chlorine is called inorganic arsenic.
Some manufacturers use small amounts of arsenic in various products like lead-acid batteries, semiconductors, and light-emitting diodes. Arsenic is also an important component of copper chromated arsenic (CCA), a wood preservative, though in 2003 U.S. preserved wood manufacturers phased arsenic out of products intended for certain residential uses (for example, boardwalks, decks, fencing, picnic tables, and play sets).
In the past, some fruit growers and cotton farmers used inorganic arsenic compounds as pesticides. Residues from those uses often remain in areas that were sprayed. Certain organic arsenic compounds still see use as pesticides in cotton production.
Arsenic in the Environment
Arsenic does not biodegrade, though under appropriate conditions it can change from organic to inorganic forms (or vice-versa), react with other chemicals, or move between air, water, and soil.
Arsenic emissions from power plants or other combustion sources processes are usually in the form of very small particles that may stay airborne for days and travel great distances. Windblown dust, which may contain arsenic, usually consists of larger particles that typically settle to the ground more quickly.
Some arsenic compounds are water soluble, which means that arsenic can end up in waterways or groundwater. Most of this arsenic eventually becomes attached to soils or sediments.
Some plants and animals - particularly shellfish - can also absorb arsenic from their immediate environment. Most arsenic in fish and shellfish is organic, in a form called arsenobetaine, which is considered less toxic than many other forms of arsenic.
Exposure to Arsenic
Arsenic occurs in nature, so some exposure is inevitable.
Soil: Arsenic concentrations in soil vary with geology, soil type, and past land use. Soils that received applications of arsenic-containing pesticides, hazardous waste sites, and areas downwind of some mines and smelters may all contain higher than normal levels of arsenic. On the other hand, sometimes arsenic binds tightly to soils or minerals, so that plants and animals do not readily absorb it.
Water: Arsenic concentrations in groundwater are generally quite low - about 1 part in a billion parts of water (1 ppb). However, contaminated areas or areas with high naturally-occurring concentrations of arsenic in rocks and soil can result in high concentrations of arsenic in groundwater. Surface water is far less likely to have high concentrations of arsenic than is groundwater.
Air: Levels of arsenic in the air vary according to upwind industry, soil type, and weather. Sources of airborne arsenic include copper or lead smelters, coal fired power plants, volcanoes, and incinerators.
Food: Food is typically the largest source of exposure to arsenic. The largest sources are seafood, rice and some rice products, mushrooms, and chicken. Arsenic concentrations in food typically range from roughly 20 to 140 ppb, mostly less harmful organic arsenic.
Other sources of arsenic exposure include:
Working with arsenic in industries like smelting, wood treating, or pesticide application.
Sawing, sanding, or burning arsenic-treated wood.
Using very old household products (ant or rat poison, weed killer) that contain arsenic.
Arsenic in the Body
The body excretes most ingested arsenic in the urine within days. However, some arsenic can remain in the body for a much longer time. If you breathe dust or particles containing arsenic, they can end up in the lining of your lungs. From there, most of the arsenic passes into the blood and is excreted in urine. In most cases, comparatively little of any arsenic that you touch will ordinarily enter your body.
Health Effects of Arsenic
The ancients knew that arsenic is poisonous, and that large doses can kill. Ingesting lower doses can cause irritation of the digestive tract, decreased blood cell production, fatigue, abnormal heart rhythms, damage to blood vessels, and/or a pins-and-needles feeling in hands and feet. Ingesting inorganic arsenic over long periods affects the skin. Symptoms may include dark patches, corns or warts on the palms, soles of the feet, and torso, changes in surficial blood vessels, and cancer. Breathing high levels of inorganic arsenic may cause sore throat, lung irritation, and lung cancer.
U.S. EPA's IRIS database classifies inorganic arsenic as a human carcinogen. It notes increased lung cancer mortality from inhalation exposure, increased mortality from cancers of the liver, kidney, lung, and bladder due to ingestion of inorganic arsenic in drinking water, and an increased incidence of skin cancer due to consumption of drinking water containing inorganic arsenic.  The International Agency for Research on Cancer (as of 2012) has determined that arsenic and inorganic arsenic compounds are carcinogenic to humans.  The U.S. Agency for Toxic Substances and Disease Registry has determined that arsenic is known to be a human carcinogen. 
There is comparatively little information on the human health effects of exposure to organic arsenic, though animal studies suggest it is generally less toxic than inorganic arsenic. Effects noted during animal experiments include diarrhea, and damage to the kidneys and bladder.
United States Environmental Protection Agency, Integrated Risk Information System (IRIS), April 10, 1998 revision.
 NIOSH Pocket Guide to Chemical Hazards, published by the National Institute for Occupational Safety and Health