As the famous Wimbledon tennis tournament approaches, your thoughts may be turning to Pimms and strawberries. I don’t want to put you off, but the strawberry has for several years running been named and shamed as the most pesticide-ridden fruit available. It could be worse, back in the 19th and 20th centuries strawberries and other fruit were liberally dosed with pesticide compounds containing arsenic and lead. In this article we will be looking at arsenic, why it is still used as a pesticide and in medical drugs and how we are all routinely exposed to it. My editor has ruled that I refrain from exploring the comedic potential of the names of arsenic compounds, but the interested reader might like to seek out the Molecule of the Week awarded by the American Chemical Society on April 18th 2011.
Most people know that arsenic is poisonous, either from the many deliberate historical poisonings or from accidental toxic effects of environmental arsenic. Many might assume that arsenic was some diabolical, synthetic creation by humans. It is actually a naturally occurring element, occupying position 33 on the periodic table. That said, predictably enough somebody did stick on a few atoms (two carbon, two hydrogen and three chlorine) to create a chemical weapon known as lewisite. Lewisite causes severe damage to the skin, eyes and respiratory tract. Its discoverer, a PhD student, was hospitalized for several days by the experience. Arsenic is a metalloid, along with others such as silicon and germanium. Metalloids are not metals but do have some metallic character, such as lustre and reasonable thermal conductivity.
Arsenic exists naturally in the Earth’s crust at an average abundance by mass of around 2 ppm (parts per million). 2 ppm means that in 1 kilogram of crust is found 2 mg of arsenic (two thousandths of a gram), or 0.0002 % by mass. In some places arsenic is locally enriched in sediments or soil. The highest local natural abundance of arsenic can be around 2 % by mass (20 000 ppm). Such hotspots can lead to toxicity issues with drinking water and food crops. In parts of Bangladesh, the arsenic burden in well water can be as high as 1500 µg (0.0015 g) per litre, higher than the World Health Organization guideline value of 10 µg per litre by a factor of 150. A programme of well‑digging in Bangladesh in the late 20th century may have reduced the occurrence of water‑borne disease such as cholera but has inadvertently exposed tens of millions of people to elevated arsenic intake. A massive mitigation programme is ongoing.
Arsenic forms both simple inorganic compounds and more complex carbon-based organic compounds having any of three oxidation states. The most toxic compounds tend to be those inorganic compounds with oxidation state III, where the arsenic forms bonds by donating three electrons. The metabolic pathways that make arsenic so much more toxic than the other metalloids are not fully understood, but its tendency to interact with sulfur-containing compounds such as proteins and enzymes seems to be important.
It would take a single dose of around 70 mg of arsenic to kill a human (or 0.07 g, roughly the weight of 16 grains of sand). This would involve drinking nearly 50 litres of the contaminated water described above. The affected people of Bangladesh tend to suffer from symptoms of chronic exposure rather than acute toxicity. These include skin lesions, diabetes, chronic obstructive pulmonary disease and various types of cancer. As well as naturally occurring arsenic, anthropogenic arsenic hotspots can be hangovers from the historical application of arsenic to fruit crops. In the south-west of England, such hotspots exacerbate a naturally elevated arsenic background. Much of the USA suffers from arsenic pesticide residues. A collection of Farmers’ Bulletins published in the USA in 1919 has several formulations with arsenic oxide or lead arsenate as the main ingredient. These are described as effective “stomach poisons” against a range of insect pests.
Knowledge of the toxicity of arsenic predated by centuries the enthusiastic application of arsenic compounds as pesticides, but the practice continued into the 1960s. Its use on food crops has now been phased out, but it is still used on cotton crops and, until recently, on tobacco. Chromated copper arsenate is used as a timber preservative, but not so much now in residential construction. There is enough arsenic knocking around that we all receive low level exposure. Potato skins contain arsenic at levels high enough that some sources advise against the eating or composting of potato skin. There is some evidence from animal studies that arsenic might be a biologically essential mineral. Studies in humans to investigate this are thought to be unfeasible as a group of humans with zero exposure to arsenic would be needed as an experimental control. Arsenic is so ubiquitous that it would be next to impossible to eliminate arsenic exposure to create such a group.
Unlikely though it seems, arsenic has medicinal uses for humans. In the early 1900s, an arsenical branded as Salvarsan was introduced as the first effective treatment against syphilis. It soon became the most prescribed drug in the world and has been described as the first “magic bullet”. Discovered in 1909 by Nobel laureate Paul Ehrlich, such was the demand that it was in mass production within one year. Syphilis is effectively treated by antibiotics now, but another arsenic compound, melarsoprol, is still sometimes used to treat sleeping sickness, a suite of diseases caused by protozoa (single-celled ‘animals’) and transmitted by tsetse fly bites. The mortality rate from the drug itself is a horrifying 5 – 6 %, but mortality in untreated sleeping sickness is close to 100 % due to the extent of the neurological damage caused by the parasite. An inorganic compound, arsenic trioxide, is used in chemotherapy against cancers such as leukaemia. Its mode of action is not well understood.
To recap, arsenic occurs naturally, and its toxicity has been known and exploited for centuries. This did not deter people from liberally applying it to food crops up until as recently as 60 years ago, and the residues still cause problems today. Arsenic is naturally elevated in sediments and water in some regions and causes a number of health problems including cancer. There are some positives. Arsenic is used in chemotherapy to treat cancer. It was the basis of the world’s first wonder drug, a treatment for syphilis, and is still used as a last-ditch treatment against sleeping sickness. We are all exposed to arsenic to some extent and this is not going to change any time soon. It might just turn out that arsenic is an essential element in the diet. Strawberries might be villain of the moment for pesticides generally, but we no longer douse them with arsenic and they do not accumulate arsenic from the soil to any great extent. And arsenic has given us some mildly amusing chemical names.