Arsenic

• 1. Introduction

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  Arsenic carries the atomic number 33 and the chemical symbol As, but those tidy facts barely hint at the element's long, strange relationship with humanity. The United States Agency for Toxic Substances and Disease Registry ranked it number one on its 2001 prioritized list of hazardous substances at Superfund sites. Roughly 57 million people in the Bengal basin alone drink groundwater with arsenic concentrations above the World Health Organization's standard of 10 parts per billion. How did a metalloid sitting quietly in the Earth's crust come to touch so many lives? The story moves from ancient pigments and royal murders to modern semiconductors and cancer wards, and at every turn arsenic refuses to behave as a single, simple thing.

• 2. The Poison of Kings

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  Because the symptoms of arsenic poisoning are not specific, rulers and ambitious heirs found it almost impossible to detect until the 1830s, when the Marsh test gave chemists a reliable way to spot its presence. Before that breakthrough, arsenic earned the nickname "the poison of kings" and "the king of poisons." In the Renaissance era it was also called "the inheritance powder," a grim reference to its use in killing family members for a fortune.

  The compound's human history stretches far earlier than those courtly intrigues. The alchemist Jabir ibn Hayyan described isolating arsenic before 815 AD. Then Albertus Magnus isolated the element from a compound in 1250 by heating soap together with arsenic trisulfide. In 1649, Johann Schröder published two separate methods for preparing it.

  In the Victorian era, women would eat arsenic trioxide mixed with vinegar and chalk to make their skin paler, a sign in that period that they did not work outdoors. The practice was not confined to cosmetics. In small doses, soluble arsenic compounds were once considered stimulants and were popular as medicine for people from the mid-18th to the 19th century. This use was especially common with sport animals: race horses and work dogs were regularly dosed with Fowler's Solution, an arsenic preparation. Sydney veterinarian Percy Sykes, speaking of that era, recalled that arsenic "was quite a common tonic" and estimated that 90 per cent of horses had arsenic in their systems.

  A 2006 study of the remains of the Australian racehorse Phar Lap confirmed that its 1932 death was caused by a massive overdose of arsenic, a finding that closed one of Australian sporting history's most persistent mysteries.

• 3. Color, Wallpaper, and Slow Poisoning

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  Paris Green was discovered in 1814 and Scheele's Green in 1775, and both arsenic-based pigments were widely used before the broader public understood their dangers. Once the toxicity became widely known, these chemicals shifted from pigments to insecticides. In the 1860s, London Purple, a solid mixture of arsenic trioxide, aniline, lime, and ferrous oxide, was widely used as an arsenic byproduct of dye production before Paris Green displaced it.

  From the late 18th century onward, wallpaper manufacturers began using arsenic-based dyes because the compounds increased the brightness of pigments. One account of Napoleon's illness and death in 1821 implicates arsenic poisoning from wallpaper in the room where he was held. The mechanism proposed is bacterial decomposition of the green arsenic pigment in a humid environment, releasing highly toxic arsine gas.

  The accidental dangers of arsenic were not always subtle. In 1858, the adulteration of foodstuffs with arsenic caused the Bradford sweet poisoning, which resulted in 21 deaths. The incident marked a turning point in British awareness of arsenic contamination in everyday goods, and it accelerated pressure for food safety reform.

• 4. Metalloid With Many Faces

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  Grey arsenic is the form that matters most to industry. It adopts a double-layered structure of interlocked, ruffled six-membered rings, which gives it a Mohs hardness of only 3.5 and a relatively high density of 5.73 g/cm3. It is a semimetal in its normal state but becomes a semiconductor with a bandgap of 1.2-1.4 eV if amorphized.

  Yellow arsenic is the most toxic of the three common allotropes, the most volatile, and the least dense, at 1.97 g/cm3. It is produced by rapid cooling of arsenic vapor and rapidly converts to grey arsenic when exposed to light. Black arsenic, similar in structure to black phosphorus, forms when vapor is cooled in the presence of mercury vapors; it is glassy, brittle, and a poor electrical conductor.

  Arsenic sublimes when heated at atmospheric pressure, converting directly from solid to gas at 887 K without passing through a liquid state. However, at 817 degrees Celsius and 2.84 MPa, it does melt. When heated in air, it oxidizes to arsenic trioxide, and the fumes carry a characteristic garlic-like odor, a smell detectable even by striking arsenide minerals such as arsenopyrite with a hammer.

  Arsenic is the 53rd most abundant element in the Earth's crust, comprising about 1.5 parts per million. It ranks 41st in abundance in the universe and 22nd in abundance in seawater.

• 5. Industrial and Military Lives

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  The primary industrial use of arsenic is in lead alloys. Car battery components are strengthened by a very small percentage of arsenic, and as much as 2% of all produced arsenic goes into lead shot and bullets.

  Gallium arsenide is a semiconductor material used in integrated circuits. Circuits built from gallium arsenide are much faster than those made from silicon, though also much more expensive. Unlike silicon, gallium arsenide has a direct bandgap, which allows it to convert electrical energy directly into light in laser diodes and LEDs.

  The compound's capacity for harm extended into modern warfare. After World War I, the United States built a stockpile of 20,000 tons of lewisite, an organoarsenic blister agent and lung irritant. That stockpile was later neutralized with bleach and dumped into the Gulf of Mexico in the 1950s. Studies assessing the environmental impact of that disposal are lacking. During the Vietnam War, the United States used Agent Blue, a mixture of sodium cacodylate and its acid form, as one of the rainbow herbicides to deprive North Vietnamese soldiers of foliage cover and rice.

  Wood preservation was once the single largest industrial application of arsenic in the United States. As of 2002, US-based industries consumed 19,600 metric tons of arsenic, and ninety percent of that went to treating wood with chromated copper arsenate. A voluntary phaseout of arsenic in consumer construction products began on the 31st of December 2003, and the European Union and the United States formally banned CCA in consumer products in 2004. The lethal human dose from burning treated CCA lumber is approximately 20 grams of ash, a hazard that persists wherever older pressure-treated wood is still in use.

• 6. Medicine and the Fine Line

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  Arsphenamine, developed by Paul Ehrlich, and arsenic trioxide, used by Thomas Fowler, were among the arsenic compounds prescribed as medicines during the 17th, 18th, and 19th centuries. Arsphenamine and its successor neosalvarsan were indicated for syphilis until modern antibiotics arrived. Arsenicals such as melarsoprol are still used to treat trypanosomiasis, despite severe toxicity, because the disease is nearly always fatal if left untreated.

  In 2000, the US Food and Drug Administration approved arsenic trioxide for treating patients with acute promyelocytic leukemia that is resistant to all-trans retinoic acid. A 2008 paper reported success in locating tumors using arsenic-74, a positron emitter, because the body does not concentrate arsenic in the thyroid the way it concentrates iodine-124, reducing image noise in PET scans.

  At the cellular level, arsenic's toxicity traces to its affinity for thiols, which sit at the active sites of many important enzymes. Arsenic disrupts ATP production by inhibiting lipoic acid in the citric acid cycle and uncoupling oxidative phosphorylation. The resulting multi-system organ failure is presumed to result from necrotic cell death rather than apoptosis, because energy reserves are too depleted for the more orderly form of cell death to proceed.

  Toxic levels of arsenic also cause significant DNA hypermethylation of tumor suppressor genes p16 and p53, increasing the risk of cancer through epigenetic changes rather than direct alteration of the DNA sequence. These effects have been studied in human kidney cells and in rat liver cells, and research into arsenic as an epigenetic factor is now being used to develop precise biomarkers of exposure.

• 7. A Global Water Crisis

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  Approximately 57 million people in the Bengal basin drink groundwater with arsenic above the WHO limit of 10 parts per billion. The arsenic there is of natural origin, released from sediment under the anoxic conditions of the subsurface. The contamination worsened after NGOs and the Bangladeshi government installed shallow tube wells in the late 20th century to prevent people from drinking bacteria-contaminated surface water; the program failed to test for arsenic.

  In Pakistan, a 2017 report in Science found that more than 60 million people are exposed to arsenic-polluted drinking water, with more than 66% of the more than 1,200 samples investigated exceeding the WHO limit of 10 micrograms per liter. A study by IIT Kharagpur found high arsenic levels in the groundwater of 20% of India's land, potentially exposing more than 250 million people. States including Punjab, Bihar, West Bengal, Assam, Haryana, Uttar Pradesh, and Gujarat have the highest exposed land areas.

  In the United States, arsenic is most commonly found in groundwater in the southwest, with significant concentrations also in parts of New England, Michigan, Wisconsin, Minnesota, and the Dakotas. Since 2006, the EPA's maximum allowed concentration in US drinking water is 10 parts per billion. Increased levels of skin cancer have been associated with arsenic exposure in Wisconsin even at levels below that standard.

  One documented case of human adaptation stands out: the people of San Pedro de Atacama in Chile have been drinking arsenic-contaminated water for several centuries, and genetic studies indicate that certain populations there have undergone natural selection for gene variants that enhance arsenic metabolism. This is considered one of the few documented cases of human evolution driven by chronic environmental exposure. Around the world, roughly 300 million people obtain drinking water from groundwater resources contaminated with unhealthy levels of arsenic or fluoride.

• 8. Life on the Edge of Poison

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  In 2010, researchers published a claim in Science that a strain of the bacterium Halomonas, designated GFAJ-1, could substitute arsenic for phosphorus in its biomolecules, including DNA, when grown in an arsenic-rich, phosphate-limited environment. The claim challenged long-standing assumptions about the chemical requirements for life. It was met with widespread skepticism. A 2011 follow-up study published in Science demonstrated that GFAJ-1 still requires phosphate to grow. A 2012 independent investigation found no detectable arsenate incorporated into the organism's DNA backbone, concluding that the original observations were likely due to contamination or insufficient purification. In 2025, the journal Science formally retracted the original paper for lack of sufficient experimental support, though the authors continued to stand by their data.

  Some bacteria do genuinely use arsenic in their metabolism, oxidizing arsenite to arsenate or reducing arsenate to arsenite as part of their energy cycles. A strain designated PHS-1, related to the gammaproteobacterium Ectothiorhodospira shaposhnikovii, was isolated as one of the bacteria that can perform a form of photosynthesis using arsenite as an electron donor in the absence of oxygen.

  The Chinese brake fern, Pteris vittata, hyperaccumulates arsenic from soil into its leaves and has a proposed use in phytoremediation of contaminated land, offering a plant-based approach to one of the world's most persistent environmental hazards.