Arsenic in the human body

Chickens that are fed an arsenic-free diet do not grow properly and it appears this element may have a role in animal development, at least in chickens and maybe in humans. If so, we need very little of it and our requirement each day may be as little as 0.01 mg, although we take in and can tolerate much more than this. The average person, weighing 70 kg, contains around 7 mg of arsenic, which represents a level of 0.1 ppm. Our blood contains less than this, but our skeleton contains more, and the level in hair is generally about 1 ppm.

Even if arsenic is essential in tiny amounts, and unavoidable in our daily diet, we still know that a little too much is harmful - although we may not register symptoms - and that a lot too much is deadly. Thankfully our body recognizes excess arsenic as a toxin and excretes it quickly. But what sort of levels are we talking about? How much arsenic can the body tolerate without any symptoms appearing? How much arsenic does it take to make us ill? How much arsenic does it take to kill us? The answers to these three questions depend very much on the individual. Clearly it takes much less arsenic to affect a child than an adult, and to affect someone who is already in a weakened state than one who is healthy. A dose of 250 mg would certainly be fatal for most adults and doses of half this amount have also been known to kill. Yet for some people a dose of 500 mg can leave them unaffected, provided they have trained their body to tolerate it.

Arsenic is quickly removed by the liver, and at a rate that is more than enough to remove the daily surplus we consume. A sudden larger dose poses a problem and the body's first reaction is to empty the gut as quickly as possible by vomiting and diarrhoea, both of which can turn extremely violent. The person becomes severely dehydrated with their skin feeling cold and clammy, and soon they pass into a coma with heart failure ending their life within a day or two. Smaller doses of arsenic may produce some of these symptoms but the victim should survive and recover, being eventually none the worse for their experience. Even large doses can also be survived, provided an antidote is given - see Glossary.

Arsenic can enter the body via the skin, lungs, or stomach, although the latter is the only way that would-be poisoners could hope to kill their victim. From the stomach arsenic passes into the blood stream and so moves around the body, quickly ending up in the liver, kidneys, spleen, and lungs. Eventually it will permeate all body tissue and find its way into bone, hair, and nails. In the hair it becomes chemically bonded to the sulphur atoms in the keratin molecules of which hair is made, and indeed ordinary hair has a high level of arsenic because of this, and explains why the analysis of hair gives a good indication of the degree to which a person has been exposed.

The first symptom that results from a normal person taking a fatal dose is vomiting. This may start within 15 minutes or be delayed for many hours, depending on the amount of food in the stomach. Unfortunately, vomiting starts too late to remove the arsenic that has been absorbed into the system, although it may expel the remainder of a massive dose before it has time to be digested. The initial vomiting brings no relief and soon starts again. The victim may complain of thirst and a sore mouth and throat, and experience difficulty in swallowing. Drinking, however, does not assuage the thirst and may only serve to accentuate the vomiting. The stomach becomes very painful and sensitive to pressure.

The body next attempts to eject the poison by emptying the bowels, and diarrhoea begins after about 12 hours, eventually becoming watery, and will continue until a condition known as tenesmus is reached. This is a feeling of wanting to evacuate the bowel without anything emerging. The body also rids itself of arsenic through the kidneys and urine but this process of expulsion can be intermittent which is why, when arsenic poisoning is suspected, a negative result from a single urine sample is not enough to rule it out; two negative samples are necessary to be absolutely certain. The arsenic that has entered the bloodstream presents the greatest danger to the body: most of this has to be excreted via the urine, and while most of a small dose can be shed this way within two days of ingestion, larger doses tend to accumulate around the body. A notable effect is to produce muscle spasms, especially in the calves. It will take about 14 days for all the arsenic to be completely removed from the body, although most of it will have been expelled within a week.

In acute poisoning, the person deteriorates rapidly with typical shock symptoms: the pulse is weak and rapid, the skin cold, damp, and pale. Death usually occurs within 12-36 hours but some poor souls have lingered for four days. Rapid death can leave a lot of arsenic in the liver, up to 120 ppm, but survival beyond a day causes this to drop to much lower values of 10-50 ppm and in Chapter 8 we will see how a victim who lived for three days after being given a fatal dose was found to have relatively little arsenic left in his liver.

Less than lethal doses of arsenic produce no single symptom which can be said to be typical of arsenic poisoning. Its features are those of any irritant in the stomach such as occurs with ordinary food poisoning: vomiting, diarrhoea, stomach pains, thirst, and a furred tongue. For this reason it was most unlikely for a doctor to diagnose arsenic poisoning and why to some murderers it seemed the perfect poison - and it was, until chemistry came along and spoilt the fun.

Chronic poisoning by prolonged exposure to industrial arsenic fumes or dust, or by being given small doses medicinally, eventually leads to changes in the skin. In the short term this may be an improved complexion, but later a mottled brown pigmentation results. Thickening of the skin of the palms and soles occurs and this is a symptom unique to chronic arsenic poisoning. Weariness, irritability, a loss of appetite and weight, red and watery eyes, and other symptoms also come from prolonged arsenic intake, although to begin with just the opposite effects are observed, as if arsenic were behaving as a tonic. Small doses of arsenic act as a stimulant by speeding up the chemical process that supplies cells with energy; and it can even speed up horses. Unscrupulous racehorse trainers were able to improve an animal's chances of winning by giving it arsenic, but if arsenic is detected in the animal's urine, and exceeds 50 ppb, then it is a sure sign of its having been doped in this way.

Arsenic compounds have this tonic effect by stimulating the body's metabolism. For example the arsenite ion (AsO33-) boosts oxidative phosphorylation by forming an arsenic molecule that reacts more rapidly allowing oxidation to proceed at an increased rate. It was observed that those who were given arsenic tended to increase in weight, a fact that was used in the twentieth century to increase meat production in pigs and poultry by giving them Roxarsone (phenylarsonic acid)* in their feed. This additive was removed from their diet a week before slaughter so that any residual arsenic would have time to be excreted.

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Responses

  • michael zimmerman
    How arsenic can be used to kill a human?
    4 months ago
  • Christopher
    What does arsenic do to the human body in large doses?
    3 months ago

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