Arsenic in plants and food

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According to the US Environmental Protection Agency (EPA) the daily intake of arsenic into the human body that can be tolerated with no observable effects is 14 ^g/kg body weight, which for the average 70 kg adult amounts to around 1000 ^g. In fact the normal human diet contains much less than this, between 12 and 50 ^g/day, and the amount excreted is within this range. However, in Japan the average intake and excretion is in excess of

* Formula C6H5AsO,H

140 ^g/day due to the eating of fish and shellfish, which are known to be rich in arsenic. Even that is well within the EPA limit.

Seawater contains only 0.024 ppm of arsenic but, despite this, some marine organisms can concentrate quite high levels of arsenic in their bodies. In aquatic environments there are alga and cyanobacteria that can methylate arsenic and this begins a journey up the food chain as these are eaten by other creatures like shrimp, which are then eaten by fish, which may finally be eaten by humans, but as the food chain is ascended the amount of arsenic that is retained decreases. Nevertheless some edible species contain surprising amounts; oysters have around 4 ppm arsenic, mussels 120 ppm, and prawns 175 ppm. Even some varieties of fish that feed upon these creatures can have high levels of arsenic such as plaice with 4 ppm. Most fish, however, absorb only minute amounts. Seaweed is also rich in arsenic and on the remote Scottish island of North Ronaldsey there is a breed of sheep which feeds exclusively on seaweed and they appear to thrive on it.

Some plants thrive on arsenic. The Chinese Ladder fern Pteris Vittata, also known as the brake fern, grows rapidly and absorbs arsenic. It can often be seen growing out of brickwork, such is its hardiness. Its fondness for arsenic is an asset that may well come into play one day in cleaning up sites that have been heavily contaminated with this toxin, because of its ability to absorb arsenic, in quantities ranging up to 2% of its weight. This remarkable ability was discovered by a group led by Lena Ma, a soil chemist at the University of Florida, who found that it could extract arsenic from soil even where the level was low, for example 6 ppm, which is normal for many soils. When it was grown on soil with 100 ppm not only did it absorb more arsenic, but it also grew 40% larger than normal.

A typical area that might benefit from brake fern harvesting is Cornwall in England which has been described as one of the world's major arsenic geochemical provinces, and for good reason as we will discover in the next chapter. In that hapless county there are streams with sediments containing 900 ppm arsenic and similar levels are to be found in garden soils and on farmland. (The UK Government's recommended limit is 40 ppm.) Arsenic also turns up in household dust in homes and in some Cornish villages this can contribute 35 ^g of arsenic to the daily intake, but even in villages where the arsenic content of the soil exceeds 1%, the vegetables grown on this absorb very little and contain amounts well below the safety limits for foods. Another region to benefit would be the Repora district of New Zealand, south of the thermal hot springs of Waiotapu, where there is sufficient arsenic in the soil and grass to cause poisoning of cattle which graze there. The Champagne Pool at Waiotapu is also rich in arsenic.

Mushrooms can absorb a lot of arsenic and one species in particular Sarcosphaera coronaria was discovered to contain 2000 ppm which is 0.2% (dry weight), but ordinary mushrooms have an arsenic content a thousand times less than this. Some arsenic used to enter the body via the lungs, or at least the lungs of smokers. Lead arsenate was widely sprayed on the tobacco crop to protect it against leaf-eating insects and the average American cigarette contained around 40 mg of arsenic, a good deal of which was volatilized when the cigarette was smoked. Copper arsenite was also used as an insecticide to control the Colorado beetle in Mississippi and indeed by 1900 this was being so widely applied that state legislation was introduced to restrict its use. In 1912 calcium arsenate was introduced as an agricultural insecticide and this was found to be particularly effective against the boll weevil that infested cotton crops. Vineyards in France used arsenical pesticides sometimes to the extent that they poisoned the grapes and the wine. This was the explanation of the outbreak of arsenic poisoning on French merchant ships in 1932, which affected 300 sailors.

What is curious about arsenic, and what makes it so unlike mercury, is that its organic forms present less of a threat than the inorganic form. In the United Kingdom the Food Standards Agency carried out its own survey of arsenic in food and it differentiated organic and inorganic arsenic. A total diet study was completed in 1999 and this assessed the actual intake of arsenic by multiplying the arsenic content of individual foods with the amounts of that food in the British diet, taken from the Government's National Food Survey. The conclusions of the Food Standards Agency report were that dietary exposure to arsenic in food was not increasing, as some had suggested, and that in any case people need not worry about the amount or type of arsenic they were getting in their food. Fish was the main source, providing 3 mg/kg on average, and only 0.03 mg was inorganic arsenic, the kind thought to pose the most risk of cancer. The main arsenic compound present in fish was arsenobetaine, an organic molecule derived from glycine, the simplest of all amino acids.*

* The arsenobetaine molecule has the formula (CH3)3As+CH2CO2-.

Poultry had an average of 0.070 ppm, then came cereals with 0.013, meat with 0.005, root vegetables 0.005 (but not potatoes, which had 0.002), and bread 0.004. Many foods had less than 0.001 ppm and almost all of it was organic arsenic. Things like eggs, greens, fruit, milk, cheese, etc. had amounts around the 0.001 level. The upshot of all this was that the daily intake of arsenic by the average person was 50 ^g of which only 1 ^g might be inorganic arsenic. In terms of dietary regimes there was no difference in arsenic intake between vegetarians and non-vegetarians.

Arsenic has poisoned thousands of people by being added to their food and drink, mistakenly, unknowingly, or deliberately, or by being present without anyone suspecting it was there. Curiously it is the last of these threats, when it has been present in only parts per million and causing no immediately observable effects, that has caused the most distress, damaging millions of lives even today, as we shall see in the next chapter. At the other extreme, excessive exposure to arsenic is realized within hours.

An example of this latter type of mass poisoning involved arsenic trioxide and it occurred in November 1858 at Bradford, West Yorkshire, when over 220 people were taken seriously ill after eating cheap peppermint lozenges that they had bought from a local market. The proper recipe for this quantity of the sweets called for 52 pounds of sugar, 4 pounds of edible gum, and ounces of peppermint essence, but a certain Mr Neale was a crafty manufacturer and found he could replace 12 pounds of the sugar with 12 pounds of much cheaper powdered calcium sulphate, and this is what he used. Sugar cost 6Vid per pound whereas the adulterant cost less than per pound.

In the week leading up to the Saturday market Mr Neale sent his errand boy to a pharmacist in nearby Shipley to get the necessary calcium sulphate, but the pharmacist was ill in bed and so his assistant, 18-year-old William Goddard went to the store room to get it. He was told it was kept in a cask in the corner, and indeed it was from such a cask that the young Goddard took 12 pounds of white powder, but it was arsenic tri oxide. Both it and the calcium sulfate were stored in similar casks, both without visible labels - in fact they were labelled but only on the ends on which they were standing. From that point on it was only a matter of time before the disaster occurred. Neale manufactured the sweets and the boy delivered them to the market stall of a Mr Hardaker whose special offer of 2 ounces for meant they were almost all sold that fateful Saturday.

On Sunday morning the Bradford police were informed that two young boys, 9-year-old Elijah Wright and 14-year-old Joseph Scott had suddenly died the previous evening under suspicious circumstances, and that the peppermint lozenges were to blame. The Chief Constable of Bradford took rapid action and sent members of his force around the city ringing bells and alerting people about the poisoned sweets, asking them to hand in all those which had not been eaten, with the result that 36 pounds were reclaimed, some of which were later shown to contain as much as 1000 mg of arsenic trioxide. But many had been eaten and in the week that followed another 20 people died and more than 200 needed hospital treatment.

A mass poisoning of 6000 people occurred in Manchester in 1900 when 70 people died. In this incident the agent was a local beer which was eventually discovered to contain 15 ppm of arsenic, so that five pints of the brew would provide a dangerously high dose of -40 mg, and many men drank this amount of beer every working day. The arsenic originated in the glucose used for brewing the beer. This glucose had levels of several hundred parts per million because it had been manufactured using sulphuric acid containing 1.4% of arsenious acid.* The sulphuric acid had been produced from iron pyrites (FeS2) which had a high arsenic content. When this ore is roasted, i.e. heated strongly in air, the sulphur comes off as sulphur dioxide which can be used to make sulphuric acid. If arsenic is present it too comes off as arsenic trioxide (As2O3), which then forms arsenious acid.

A Royal Commission was set up to investigate the Manchester incident, and it reported its findings in 1902. These led to stringent controls over the amount of arsenic in glycerine, glucose, malt, treacle, and beer, all of which might come into contact with sulphuric acid at some stage during their manufacture. The legal limit set for arsenic in such products was 0.01 grain per pound or gallon, equivalent to 0.14 ppm. What puzzled the Commission at one stage was the finding of arsenic in samples of malt that had never been in contact with sulphuric acid or glucose. An investigation finally discovered that it was being contaminated by dust from the walls and ceiling of the loft in which the barley that produced the malt was dried; dust had come from the coke fuel used in the heating system and this contained arsenic.

Even as late as 1952 large-scale accidents with arsenic continued to happen. In that year a French chemist Jacques Cazenive, 59, was convicted of killing

* Formula H2AsO

73 babies and injuring 270 others through a talcum powder called Baumol that he manufactured. This should have contained zinc oxide, which has known skin benefits, but it contained arsenic trioxide instead. Baumol was eventually traced as the cause of the children's illnesses and deaths because there was an outbreak of sores and damaged skin among those who had bought the powder, and when this was realized and the powder analysed, its high arsenic content was discovered.

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