The total amount of lead in an average adult today is probably around 100 mg. In the next generation it might be as little as 50, and in subsequent generations even as low as 10 mg. Most lead in the body is to be found in the skeleton, which can have up to 30 ppm. The level to which people in the past were exposed to lead is preserved in their bones and it has been possible to mark the degree of lead exposure by analysing bones from former ages. If the date of death is accurately known then this can be a useful guide to conditions prevailing at the time. By the late twentieth century the level of lead in recently deceased children's bones was down to 2 ppm although for most adults it was still around 5 ppm. The analysis of bones taken from the crypts of Polish churches has proved particularly important. These have been stored in dry conditions and thereby prevented from absorbing lead from their surroundings. Some of these showed levels of lead in excess of 100 ppm. In the Middle Ages levels were around 30 ppm, but began to rise thereafter, so that skeletons from people living in the 1700s had on average around 50 ppm, and this rose to 60 ppm for those who died in the 1800s. Clearly their environment was heavily contaminated.
Where does the lead in our body come from today? Most now comes from our diet, but some we breathe in. The average daily diet probably contains more than 200 ^g of lead, of which about 10 ^g gets into the blood, where it is joined by about 5 ^g of lead from our lungs (depending on where we live) so that our daily intake probably comes to about 15 ^g and the body can easily rid itself of such an amount.
Thankfully, only a few per cent of the lead in our food and drink gets into our bloodstream, but about half of the lead in the dust we breathe in gets so absorbed. This was shown by research carried out in 1995 by a group at Columbia University, New York, headed by Joseph Graziano. Six volunteers were given a capsule of soil collected from a site which had been an old lead mine and which was contaminated with 3000 ppm of lead. The isotopic ratio of lead-206/lead-207 (see Glossary) was lower than normal and so the amount of lead entering the bloodstream could be assessed by seeing how this ratio changed. If the capsule was given on an empty stomach the results showed that as much as 25% of the lead was absorbed. Volunteers who were given the capsule after they had eaten a good breakfast absorbed only 3%.
Of the lead that enters the bloodstream, either from the lungs or the gut, 5 ^g are extracted by the kidneys and excreted in the urine, and 10 ^g are deposited as insoluble lead phosphate in the skeleton. The bone takes up lead in the same way as it does calcium, by forming an insoluble phosphate. Day by day our skeletal lead burden increases, until by the time we are 40 it could well exceed 100 mg, but then as we get older it begins slowly to be leached out again, just as calcium is leached out, and our bones become weaker. Earlier generations whose skeleton had accumulated much more lead than this would then be at risk of suffering a mild but continuous form of lead poisoning later in life.
Lead gets into the food chain because all plants contain some lead, although not very much. For example, sweet corn has 0.02 ppm (fresh weight), while fruits contain hardly any at all, with tomatoes having only 0.002 ppm and apples a mere 0.001 ppm. The amount of lead that plants absorb depends on the lead content of the soil: lettuces grown near a lead processing plant were found to have up to 3 ppm, although even eating lots of these lettuces would be unlikely to result in symptoms of lead poisoning. Drinking water, unless it comes through lead pipes, is not likely to contain much lead, and even that which comes through lead pipes will not dissolve much lead if the water is hard, which means that it contains dissolved calcium and magnesium salts. The World Health Organization reduced its recommended upper limit for lead in drinking water from 0.05 to 0.01 ppm in 1995, saying that all countries should aim to achieve this by 2010. The US Environmental Protection Agency lays down a safe limit of 0.015 ppm, but in Washington, DC it discovered that more than 4000 of the 6000 homes whose water it tested exceeded that limit. The water taken from one house had as much as 48 ppm.
Lead in water is present as lead ions, Pb2+, whereas that in food is likely to be in an insoluble form although this may dissolve and release its lead if it comes into contact with solubilizing molecules, such as fruit acids. Lead ions carry the same charge as calcium ions, but they are much larger in size and in that respect they will not easily squeeze through the intercellular junctions of the gut wall to penetrate the channels of the gut membrane, and thereby gain access to the bloodstream. Nevertheless, some does get through.
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