Arsenic is a metalloid – a natural element that is not actually a metal but which has some of the properties of a metal. It is a natural component of the Earth’s crust, generally found in trace quantities in all rock, soil, water and air. However, concentrations may be higher in certain areas due to either natural conditions or human activities.
Arsenic can exist in many different chemical forms in combination with other elements. Some forms are inorganic, which do not contain carbon, and others are organic, which always contain carbon. Inorganic arsenic exists in four main chemical forms known as valency or oxidation states. Valency is a measure of the ability of a compound to combine with other elements, such as hydrogen. The dominant forms are arsenite, with a valency of 3, and arsenate, with a valency of 5.
The element arsenic itself is not soluble in water. Arsenic in combination with other elements (as salts) has a wide range of solubilities depending on the surrounding acidity and the presence of other chemicals.
Health impacts of arsenic exposure
Soluble inorganic arsenic is acutely toxic. Intake of inorganic arsenic over a long period can lead to chronic arsenic poisoning (arsenicosis). Effects, which can take years to develop depending on the level of exposure, include skin lesions, peripheral neuropathy, gastrointestinal symptoms, diabetes, renal system effects, cardiovascular disease and cancer. Organic arsenic compounds, which are abundant in seafood, are less harmful to health and are rapidly eliminated by the body.
Human exposure to elevated levels of inorganic arsenic occurs mainly through the consumption of groundwater containing naturally high levels of inorganic arsenic, food prepared with this water and food crops irrigated with high-arsenic water sources. Public health actions are needed to reduce human exposure to arsenic, particularly in areas with naturally high levels in groundwater.
The immediate symptoms of acute arsenic poisoning include vomiting, abdominal pain and diarrhoea. These are followed by numbness and tingling of the extremities, muscle cramping and death, in extreme cases.
Effects of long-term exposure
During long-term exposure to high levels of inorganic arsenic (e.g. through drinkingwater), the first changes are usually seen in the skin: pigmentation changes and then skin lesions and hard patches on the palms of the hands and soles of the feet.
Other effects of long-term exposure to high inorganic arsenic levels include peripheral neuropathy, gastrointestinal symptoms, conjunctivitis, diabetes, renal system effects, enlarged liver, bone marrow depression, destruction of erythrocytes, high blood pressure and cardiovascular disease. The latter has been found in Taiwan, China, to include “blackfoot” disease, a severe disease of blood vessels leading to gangrene. Malnutrition may contribute to its development.
Inorganic arsenic is one of the few substances that have been shown to cause cancer in humans through consumption of drinking-water. Cancer usually takes more than 10 years to develop. Arsenic can cause cancers of the skin, bladder and lungs, and there is limited evidence that it may also cause cancers of the kidney, liver and prostate.
The International Agency for Research on Cancer (IARC) has classified arsenic and arsenic compounds as carcinogenic to humans (Group 1), which means that there is sufficient evidence for their carcinogenicity in humans. The organic arsenic compounds monomethylarsonic acid and dimethylarsinic acid are the active ingredients of some herbicides and are metabolites of inorganic arsenic.
On the basis of sufficient evidence of cancer in experimental animals and because monomethylarsonic acid is extensively metabolized to dimethylarsinic acid, both compounds are classified as possibly carcinogenic to humans (Group 2B). Arsenobetaine and other organic compounds that are not metabolized in humans are not classifiable as to their carcinogenicity (Group 3). Furthermore, IARC has stated that arsenic in drinking-water is carcinogenic to humans (Group 1).
Arsenic can pass through the placenta. Pregnant women chronically exposed to arsenic-contaminated drinking-water are at increased risk for spontaneous abortion, stillbirth and preterm birth. In utero and early-life exposures to arsenic have been linked to the development of lung cancer and bronchiectasia later in life.
Sources of exposure to arsenic
Drinking-water poses the greatest threat to public health from arsenic. Inorganic arsenic is naturally present at high levels in the groundwater of a number of countries, such as Argentina, Chile, China, India (West Bengal), Mexico, the United States of America, and particularly Bangladesh where approximately half of the total population is at risk of drinking arsenic-contaminated water from tube wells. In one estimate, consumption of arseniccontaminated drinking-water in Bangladesh resulted in about 9100 deaths and 125 000 disability-adjusted life years (DALYs*) in 2001
Most arsenic in industrial processes is used to produce antifungal wood preservatives, which can lead to soil contamination. It is also used in the pharmaceutical and glass industries, in the manufacture of alloys, sheep dips, leather preservatives, arsenic-containing pigments, antifouling paints and poison baits and, to a diminishing extent, in the production of agrochemicals (especially for use in orchards and vineyards). Arsenic compounds are also employed in limited amounts in the microelectronics and optical industries. High arsenic levels in air can be found in the working environment as well as the general environment around non-ferrous metal smelters, where arsenic trioxide may be formed, and some coalfired power plants (especially those using low-grade brown coal).
In areas where arsenic is not naturally present at high levels, food usually contributes most to the daily intake of arsenic. Fish, shellfish, meat, poultry, dairy products and cereals are the main sources of dietary intake. However, the arsenic content of fish and shellfish usually involves organic compounds (e.g. arsenobetaine) that are of low toxicity.1 In areas where arsenic is naturally present at high levels, food (e.g. rice) prepared with high-arsenic water and food crops irrigated with contaminated water also contribute to total daily intake.
Exposure of smokers to arsenic arises from the natural inorganic arsenic content of tobacco. This content is increased where tobacco plants have been treated with lead arsenate insecticide. Smelter workers, who have an elevated risk of developing lung cancer due to arsenic exposure, further increase their risk by smoking.
What are the indicators of arsenic exposure?
The amounts of arsenic or its metabolites in blood, hair, nails and urine are used as indicators – biomarkers – of arsenic exposure. Blood arsenic is only useful for indicating either acute poisoning or repeated high-level exposures occurring over a long period. This is because arsenic rapidly disappears from blood.
Arsenic persists longer in hair and nails, which can, therefore, be used as indicators of past exposure. The concentration of arsenic, along a hair may be used to estimate the timing of an exposure.
The best estimate of recent exposure to inorganic arsenic is to measure it and its specific chemical metabolites in urine. However, consumption of certain seafood high in organic arsenic, such as seaweed or mollusks, produces one of the same metabolites as inorganic arsenic and may therefore exaggerate estimates of inorganic arsenic exposure in some people at certain times. Such foods should be avoided for 2–3 days before urine sampling.
What have WHO and IARC established about arsenic?
International bodies have previously evaluated arsenic.
The World Health Organization (WHO) has set a provisional guideline value of 10 µg/litre for arsenic in drinking-water as the practical quantification limit. That is, it is acknowledged that even this limit may not be entirely free of health risks but there are practical problems in many areas of the world in reducing levels in drinking water below this limit.
Arsenic and arsenic compounds were evaluated by the International Agency for Research on Cancer (IARC). There was sufficient evidence for carcinogenicity to humans and limited evidence for carcinogenicity to animals, and the overall evaluation was that “arsenic and arsenic compounds” are carcinogenic to humans (Group 1) . This evaluation applies to the group of chemicals (i.e. arsenic and arsenic compounds) as a whole and not necessarily to all individual chemicals within the group.
What is the treatment for arsenic exposures?
The main treatment is to remove the source of the arsenic and prevent re-exposure. This may involve use of proper protective equipment on the job or with hobbies. It may involve discontinuation of folk remedies and supplements. Some patients, depending on test results, arsenic levels, and symptoms, may require medications called chelators (kee-laytors), to help remove some of the arsenic from their bodies.
What are chelators and when would they be used?
Chelators attach to some metals and remove them from the body, but they also remove good minerals. Chelators can cause damage to other organs and can cause life threatening allergic reactions. Because of these dangers it is important that patients be assessed by a medical doctor and that appropriate tests are performed at an appropriate laboratory. This decision to chelate should be made by a medical doctor after considering the history of exposure, the test results, and the patient’s symptoms.