Arsenic is a naturally occurring, semi-metallic element distributed widely throughout the Earth’s crust. Because it is odorless and tasteless, its presence in the environment is often undetectable. Arsenic exists in several chemical forms, broadly categorized as either organic or inorganic. Inorganic compounds, which lack carbon, are the most highly toxic forms and are the primary focus of health concerns. Organic arsenic compounds, such as those found in seafood, are generally considered far less harmful to the human body.
Natural and Anthropogenic Sources of Contamination
The main source of widespread arsenic exposure is the natural geology of the planet, where the element is present in over 200 different minerals. As these mineral deposits undergo natural processes like weathering and erosion, arsenic leaches into groundwater. High arsenic concentrations are often found in well water that has been in contact with volcanic rock or specific sedimentary formations.
Human activities also contribute to inorganic arsenic contamination, primarily through industrial processes. Mining and metal smelting operations release substantial amounts of arsenic into the air, soil, and water as a byproduct. The burning of fossil fuels, particularly coal, also mobilizes arsenic into the atmosphere, which then settles onto land and water sources.
Historically, arsenic was used extensively in pesticides, herbicides, and industrial wood preservatives, resulting in widespread soil contamination in certain areas. For the general public, the most common route of exposure is the ingestion of contaminated drinking water and food. Rice, for example, accumulates inorganic arsenic from soil and irrigation water more readily than other crops.
How Arsenic Affects the Human Body
The toxicity of inorganic arsenic stems from its ability to interfere with fundamental cellular processes throughout the body. Once absorbed, primarily through the gastrointestinal tract, the element disrupts cellular respiration by targeting enzymes involved in the production of energy. Specifically, the trivalent form, arsenite, binds to sulfur-containing groups in proteins, suppressing the activity of hundreds of enzymes and ultimately blocking the cell’s ability to generate energy. The pentavalent form, arsenate, mimics phosphate and is mistakenly incorporated into metabolic pathways, decoupling the process of energy storage.
While acute, high-dose exposure causes severe gastrointestinal distress, cardiac dysfunction, and death, the more common public health issue is chronic, low-level exposure over many years. This long-term exposure causes a range of non-cancer health outcomes that manifest slowly.
A hallmark of chronic exposure is the development of dermatological issues, including patchy skin hyperpigmentation and hard lesions on the palms and soles (hyperkeratosis). Chronic exposure is also strongly linked to cardiovascular problems, such as hypertension and peripheral vascular disease, which impairs circulation. Arsenic is a known human carcinogen, with the strongest evidence linking it to cancers of the lung, bladder, and skin.
Prolonged exposure can also cause neurological damage, resulting in peripheral neuropathy characterized by numbness, tingling, and weakness in the limbs. Arsenic exposure has also been associated with an increased risk of developing Type 2 diabetes and may affect reproductive and developmental health. These systemic effects underscore the element’s interference with basic cellular function and DNA repair mechanisms.
Testing and Monitoring for Exposure
Individuals concerned about arsenic exposure, especially those relying on private water sources, should prioritize environmental testing of their drinking water. Private well owners are responsible for their own testing and must use a state-certified laboratory to ensure accurate results, rather than relying on less sensitive field test kits.
Certified laboratories use sensitive analytical techniques like Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to measure arsenic concentration. Results are typically reported in micrograms per liter ($\mu$g/L) or parts per billion (ppb). Testing should be conducted at least once to establish a baseline and then periodically, as groundwater arsenic levels can fluctuate seasonally.
Biological monitoring provides a direct measure of human exposure and is usually conducted by a physician or public health specialist. Urine analysis is the most common test for recent exposure, as the body excretes most arsenic within a few days. Accurate toxicity assessment requires differentiating between toxic inorganic arsenic and harmless organic arsenic found in seafood, which can cause false positives for total arsenic. For assessing long-term exposure, hair and nail samples can be analyzed, as arsenic accumulates in these tissues over time.
Strategies for Removal and Regulatory Limits
The United States Environmental Protection Agency (EPA) established a safety standard for public drinking water under the Safe Drinking Water Act (SDWA). The current Maximum Contaminant Level (MCL) for arsenic in public water systems is 10 micrograms per liter (10 $\mu$g/L), or 10 parts per billion. This regulation requires public water utilities to continuously monitor and treat their water to ensure compliance with this limit.
For private well owners seeking additional protection, several effective treatment technologies are available for home installation. Reverse Osmosis (RO) systems are highly effective, removing over 95% of arsenic by forcing water through a semi-permeable membrane. Distillation units, which boil water and condense the steam, also provide near-complete removal of the contaminant.
Specialized filtration methods are also highly reliable. These include adsorptive media filters, which use materials like activated alumina or iron oxides to chemically bind the arsenic. Ion exchange units function similarly to water softeners, using charged resins to swap arsenic ions for less harmful ions like chloride.
Most of these methods are most effective at removing the pentavalent form of arsenic, Arsenic (V). Water containing the more toxic Arsenic (III) may require a pre-oxidation step, such as adding chlorine, before filtration.