Arsenic is a ubiquitous naturally occurring chemical element, and the 20th most common element on Earth. Arsenic levels in the groundwater vary from around 0.5 parts per billion to 5000 parts per billion, depending on an area's geologic features, and possible presence on industrial waste. The highest groundwater arsenic levels have been recorded in Brazil, Cambodia, Afghanistan, Australia, and Bangladesh.
Arsenic is a ubiquitous element present in American drinking water.  In the US, the U.S. Geological Survey estimates that the median groundwater concentration is 1 μg/L or less, although some groundwater aquifers, particularly in the western United States, can contain much higher levels. For example, median levels in Nevada were about 8 μg/L but levels of naturally occurring arsenic as high as 1000 μg/L have been measured in the United States in drinking water. Groundwater associated with volcanics in California contain As at concentrations ranging up to 48,000 μg/L, with As-bearing sulfide minerals as the main source. Geothermal waters on Dominica in the Lesser Antilles also contain concentrations of As >50 μg/L. In Wisconsin, As concentrations of water in sandstone and dolomite aquifers were as high as 100 μg/L. Oxidation of pyrite hosted by these formations was the likely source of the As. In the Piedmont of Pennsylvania and New Jersey, groundwater in Mesozoic age aquifers contains elevated levels of As—domestic well waters from Pennsylvania contained up to 65 μg/L, whereas in New Jersey the highest concentration measured recently was 215 μg/L.
Organic arsenic is less harmful than inorganic arsenic. Seafood is a common source of the less toxic organic arsenic in the form of arsenobetaine. Because of its high toxicity, arsenic is seldom used in the Western world, although in Asia it is still a popular pesticide. Arsenic is mainly encountered occupationally in the smelting of zinc and copper ores.
In the United States, Schoof et al. estimated an average adult intake of 3.2 μg/day, with a range of 1–20 μg/day. Estimates for children were similar. Food also contains many organic arsenic compounds. The key organic arsenic compounds that can be routinely found in food (depending on food type) include monomethylarsonic acid (MMAsV), dimethylarsinic acid (DMAsV), arsenobetaine, arsenocholine, arsenosugars, and arsenolipids. DMAsV or MMAsV can be found in various types of fin fish, crabs, and mollusks, but often at very low levels.
Arsenobetaine is the major form of arsenic in marine animals, and is considered nontoxic. Arsenocholine, which is mainly found in shrimp, is chemically similar to arsenobetaine, and is considered to be "essentially nontoxic". Although arsenobetaine is little studied, available information indicates it is not mutagenic, immunotoxic, or embryotoxic. Arsenosugars and arsenolipids have recently been identified. Exposure to these compounds and toxicological implications are currently being studied. Arsenosugars are detected mainly in seaweed but are also found to a lesser extent in marine mollusks. Studies addressing arsenosugar toxicity, however, have largely been limited to in vitro studies, which show that arsenosugars are significantly less toxic than both inorganic arsenic and trivalent methylated arsenic metabolites.
It has been found that rice is particularly susceptible to accumulation of arsenic from soil.  Rice grown in the United States has an average 260 ppb of arsenic, according to a study; but U.S. arsenic intake remains far below World Health Organization-recommended limits.  China has set a standard for arsenic limits in food (150 ppb), as levels in rice exceed those in water.
The European Commission (2000) reports that levels of arsenic in air range 0–1 ng/m in remote areas, 0.2–1.5 ng/m in rural areas, 0.5–3 ng/m in urban areas, and up to about 50 ng/m in the vicinity of industrial sites. Based on these data, the European Commission (2000) estimated that in relation to food, cigarette smoking, water, and soil, air contributes less than 1% of total arsenic exposure.