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HUMAN BONE LEAD CONCENTRATIONS by Tony Waldron · PDF file 2017-08-25 · HUMAN BONE LEAD CONCENTRATIONS by Tony Waldron The uses to which the Romans put lead exposed them to a considerable

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  • HUMAN BONE LEAD CONCENTRATIONS by

    Tony Waldron

    The uses to which the Romans put lead exposed them to a considerable risk of developing lead poisoning and there is a good deal of documentary evidence to suggest that the disease was common amongst them, and that on occasions, it assumed endemic proportions. There is not much in the way of scientific evidence to support this view, however, although it is a relatively simple matter to assess the degree to which ancient populations were exposed to lead by an analysis of the concentration of lead in bone. During life at least 90% of the lead which is absorbed into the body is fixed in the skeletal tissues where it tends to accumulate with age, at least until the fifth or sixth decades. A high bone lead concentration may thus be taken as an indication that the individual (or the population of individuals) was heavily exposed during his lifetime.

    Some studies have already been undertaken to examine the lead content of bones from a variety of Romano-British sites, but the largest series so far has come from the excavations at Cirenccstcr.

    A total of 333 bones from this site, principally rib, vertebra or ulna, have been analysed from a total of 161 skeletons. Samples of different bone were taken from the same skeleton in order that the degree of variation in lead content from bone to bone could be determined. Each bone sample was cleaned, dried and weighed, and then wet ashed in a mixture of perchloric and nitric acids. The lead content was determined using atomic absorption spectrophotometry and all lead concentrations expressed as micrograms lead/gram (f,Lg/g) dry weight of bone.

    The reliability of the analytical method was tested by comparing the results obtained with those of a physical method employing neutron activation and with those of another laboratory using chemical analysis. In both cases, the results from the secondary analyses confirmed that the original method was sound. As a further check of the method, 27 duplicate samples from the same bone were sent for analysis without the prior knowledge of the analyst and the differences between the results were not statistically significant.

    From the results of the analyses it seems clear that the inhabitants of the site - or at least those who came to be buried there - were heavily exposed to lead (see Table 96 and fig. 85 mf. 4/5). In the modern population, the mean concentration of lead in bone is variously quoted, but the maximum concentration seems to be between 40-50 f,Lg/g. The concentrations in the Cirencester bones are markedly abnormal by present day standards and indeed, as can be seen from fig. 85, none is less than the contemporary maximum quoted above. The distribution of lead concentrations is very wide (fig. 85) and, especially in the rib and vertebra, noticeably skewed to the right. There is a substantial proportion of values (approximately 5% in all) in excess of 500 f,Lg/g, indicating exposure on a massive scale even for this population.

    There are no consistent sex differences (Table 96), nor is there any correlation between lead concentration and age (Table 97). In contemporary populations, bone lead concentrations are higher in men than in women and there is an increase in concentration with age, and the fact that this was not the case in the Cirencester bones was unexpected. It is possible that the similarity in lead concentration in the male and female bones is an indication that degree of exposure was the same, but since the differences noted in modern bones are thought to be due to an underlying difference in rates of absorption from the gut this seems, at first sight unlikely. However, it is known that the absorption of lead from the gut is markedly affected by the constitution of the diet; it increases particularly if the levels of calcium and iron are low. Now if the people at Cirencester had a diet deficient in calcium and iron but at the same time were presented with a great deal oflead, this combination of factors might be sufficient to off-set the rather small differences in absorption seen in men and women on well balanced diets containing relatively small quantities oflead. Since we have no information on the composition of the diet

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  • 204 CIRENCESTER EXCAVATIONS II

    of the Cirenccster people, this has to remain a matter for speculation. Failure to note an increase in lead concentration with increasing age may be due to a

    combination of circumstances, the most significant of which being the small numbers in some of the cells, the wide spread of the results within each cell, and the difficulties inherent in assigning a precise age to the bones. In most cases, especially with adult bones, the age can only be quoted as lying within a five, ten, or even fifteen year range. This necessarily makes the chances of demonstrating trends with age extremely difficult.

    The variations in lead content in the different bones was expected, mean levels in rib and vertebra being higher than in the ulna by a factor of 1.6-1.7. The correlation between levels in rib and vertebra is highly significant (Table 98), but not between vertebra and ulna. There is a significant correlation between levels in rib and ulna in males but not in females; the reasons for this difference are obscure.

    The practical consequences of the variation in lead levels are that in any other studies of this kind, the type of bone used should be specified in order to allow proper comparisons to be made with the results of other investigations.

    The major source of lead to which the individuals on the site were exposed must have been the diet, but how the lead got into their food and drink remains unknown, and there are no dues from the excavation. This is an area in which there is an obvious need for further study both at this and other sites.

    Contamination of the bones by lead in the soil has always to be borne in mind although in general this is not a serious problem since lead is firmly bound to organic materials in the soil and mobile only under conditions of high acidity, conditions which do not favour good bone preservation. Soil samples were analysed from several parts of one of the trenches on the site (see figs. 38 and 86 mf. 4/5) and the lead concentrations were in no way exceptional (Table 99).

    The most important question which we have to consider is, did the high lead levels to which those folk living at Cirenccster were exposed have any adverse effects on their health? Is it possible that any of them died of lead poisoning?

    So far as the adults in the group are concerned, it is not possible to state categorically that any actually had lead poisoning since elevated bone lead concentrations may result if large (but sub-toxic) amounts of the metals are absorbed over a long period. Lead workers are an obvious example of those who may be subjected to prolonged heavy exposure and, as expected, their bones may contain several times the 'normal' concentration, and levels in excess of 200 J.l.g/g have been reported. On the other hand, an enhanced intake of lead may also be due to environ- mental factors. In Glasgow, for example; where the water is extremely plumbosolvcnt, mean bone lead concentrations are almost twice as high as those in a hard water (non-plumbosolvent) area and in one study of autopsy material, 6.5% of the levels were in excess of 200 J.l.g/g, the highest value being 540 J.l.g/g; none of the patients had had symptoms of lead intoxication during life. Chronic, heavy exposure to lead may not be an immediate threat to life, but it may well be accompanied by an increased morbidity. For example, some of the Cirencestcr inhabit- ants with the highest exposure would very likely have been anaemic, and this would have rendered them less able to cope with infections and other relatively trivial illnesses.

    If high bone levels in adults cannot be taken as necessarily implying a diagnosis of lead poisoning, then conversely, levels within the normal range do not exclude it. Most cases of severe clinical intoxication arise from the absorption of unusually large amounts of lead over a short period of time. This is expccially so in children. Now if previous exposure has been slight, bone lead concentrations before the onset of symptoms may be correspondingly low and although they will certainly rise following the intoxicating dose, the increase may not be suf- ficient to take the concentration outside the range of normal values.

    These difficulties in the interpretation of bone lead concentrations apply equally to children as to adults, as noted above, although high concentrations in the bones of very young children are much more suggestive of clinical intoxication since infants are much less able to tolerate an increased burden of lead than older children or adults.

    The results of the youngest children in the study are shown in Table 100 (mf. 4/5) and it seems highly likely that some of these children did actually die from lead poisoning.

  • THE ROMAN CEMETERIES OF CIRENCESTER

    In order to enhance the examination of the Bath Gate cemetery a study of the known burial pattern around the town was undertaken. The full gazetteer, mf. 5/5, brings together all the recorded information of burials within the immediate environs of the Roman town, culled from journals, manuscripts, museum records, newspaper cuttings, and excavations. An open-ended numbering system was adopted to permit additions in the future, with blocks of numbers being assigned to the areas outside the west (A-1999 including those burials excavated in CS and CT 69-76), south (2000-2999), east (3000-3999), and north gates (4000-4999). Finds of human bones recovered from within the town defences are numbered 5000-5999; with unprovenanced finds 6000-6999.

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