Environmental Geochemistry and Health 1994 16(3/4) page 97
Trace elements in the human environment:
problems and risks* Brian E. Davies Department of Environmental Science, University of Bradford, Bradford, West Yorkshire, BD7 1DP, England
Abstract
Early studies on the possible influences of environmental geochemistry and health were concerned with major diseases such as cancer or multiple sclerosis. They were generally confined to simple comparisons of geochemical and epidemiological maps and corresponding rural areas. Subsequently, studies of dental health produced evidence for the roles of environmental fluoride, selenium, molybdenum and lead in the aetiology of caries. In recent decades lead has been closely studied. Most people in the western world live in urban areas, rather than rural areas, and research on lead has yielded much information on the distribution of the element in streets, parks and gardens of cities and in the home environment. The role of lead in human health has thereby been better understood. Cadmium, selenium and aluminium are good candidates for future study but success will depend on applying appropriate methods of investigation and here the experiences from research on lead are a valuable model to follow.
Introduction For virtually all human evolutionary history death has most likely been the consequence of fatal injury, starvation, acute food poisoning or infectious diseases. The developmental history of mankind can be considered as a continuous and increasingly successful endeavour to become isolated from and unaffected by environmental influences. Our P a l a e o l i t h i c a n c e s t o r s i n v e n t e d clothes and discovered fire so as not to freeze to death as they roamed the tundra wastes of peri-glacial Europe. Now our houses are central heated and sometimes air-conditioned, our windows double or trebled glazed, our doors draught proofed; all with the intention of ensuring we pass our days both dry and warm and isolated from the natural environment. Yet humans cannot escape from the ecosystem of which they are part as is demonstrated by recent concerns that harmful radon may be accumulating in the air of our home environment now we have draughtless houses, i.e., no interchange of outside and inside air. The discovery that infectious diseases are caused by pathogens led to the provision of clean, piped water to houses, municipal sewerage instead of private cesspits, vaccines and antibiotics. Consequently, the tremendous biological onslaughts of European history, such as plague or cholera, have *This paper is based on a keynote lecture at the start of the SEGH conference.
now been c o n t r o l l e d and m i n i m i s e d . The contemporary western world is more concerned with an important groups of diseases, e.g., cancers, diseases of the central nervous s y s t e m and cardiovascular disease, the causes of which are by and large u n k n o w n and cure and control is uncertain. When the incidence or prevalence of these diseases is mapped within any one region variations are seen and these patterns may persist over many years (Howe, 1979). Environmental influences appear to be influencing the disease pattern and a role for environmental geochemistry has been suggested by many authors. The health of animals and man is known to be affected by the amounts and properties of the chemical elements present in food, beverages and inhaled dust or air. Recent years have seen more elements being recognised as essential, e.g., chromium or selenium, and the role of essential e l e m e n t s in m e t a b o l i c p r o c e s s e s is b e t t e r u n d e r s t o o d . M e r t z (1981) has w r i t t e n a comprehensive survey of those elements now regarded essential to man.
Non-infectious Diseases and Environmental Geochemistry: Conventional Approaches Two groups of diseases have attracted much attention by medical geographers and geochemists namely: cancers and diseases of the central nervous system, especially multiple sclerosis. The strong
98
Trace elements in the human environment
link between fluoride and dental health has the frequency of stomach cancer in Britain. Soil stimulated much work on the influence of other organic matter, zinc and cobalt were related trace elements. positively with stomach cancer incidence but not Chemotherapy, radiotherapy and screening with intestinal cancer. Chromium was connected programmes are reducing the mortality of several with the incidence of both. Vanadium and iron cancers. In addition much progress is being made at showed inconclusive relations and nickel, titanium a molecular level in understanding the causes of and lead showed no connection. Subsequently, they cancers. But, at a higher, whole body level, the reported (Stocks and Davies, 1964) that the average causes are still not understood. Ultraviolet and logarithm of the ratio of zinc/copper in garden soils ionising radiation, many organic substances and was always higher where a person had just died of some inorganic substances are known to be stomach cancer after 10 or more years of residence than it was at houses where a person had died c a r c i n o g e n i c . T h e r e is e v i d e n c e for s o m e environmental factor(s) in the aetiology of cancers, similarly of a non-malignant cause. The effect was especially gastric cancer. Several authors have more pronounced and consistent in soils taken from speculated that this variability may be influenced by vegetable gardens and it was not found where the soil or water quality. Matthews (1990) has reviewed duration of residence was less than ten years. the e p i d e m i o l o g y of gastric c a n c e r and has The g e o g r a p h i c a l v a r i a b i l i t y o f c a n c e r discussed the possible role of environmental factors. incidence continues to attract the interest of Early reports relied upon very generalised geographers and geologists. Yet we have made little cartographical or similar associations between real progress in attempting to involve trace elements disease patterns and geographical or geological in the aetiology of cancer. Even some earlier factors (Legon, 1951, 1952; Tromp and Diehl, 1955; promising associations have been found to be Millar, 1961). Later publications tended to propose non-existent in later studies. For example, Piispanen (1989) compared cancer and geochemical maps of unspecified influences of one or more trace elements. These early reports were weakened by Finland but found only weak associations. When Armstrong (1990) revisited an earlier study area in their use of crude, unstandardised death rates. A l l e n - P r i c e ( 1 9 6 0 ) , in a p a p e r m u c h I c e l a n d he c o n c l u d e d that the p r o n o u n c e d commented on at the time, reported that mortality geographical variation in Icelandic stomach cancer (crude figures, unstandardised) from cancer was incidence that was widely cited from 1961 to 1965 unusually low in certain villages in Devon and is now believed to be spurious. Multiple sclerosis is a relatively uncommon unusually high in others. Within the village of Horrabridge mortality was linked to the origin of disease which is very difficult to diagnose in its different water supplies: lowest mortality was early stages. The numbers of patients in any area are associated with reservoir water from Dartmoor small which makes it difficult to transform the data whereas highest mortality was associated with well to produce standardised mortality ratios and it is or spring water derived from mineralised rock strata. usual to quote simple prevalence rates per 100,000. Subsequently, a more rigorous study by the Royal Acheson (1977) has suggested that rates in excess College of General Practitioners (RCGP, 1966) of 40/100,000 should be regarded as high and from 0 to 19/100,00 as low. Spillane (1972) commented d e m o n s t r a t e d that s e v e r a l D e v o n p r a c t i c e s coml~ared unfavourably with Birmingham practices that rates in excess of 40/100,000 are not known with respect to malignant neoplasms. In a sequel to between latitudes 40N and 40S and rates in western this study, Davies (1971) described widespread Europe are generally high. There is a general heavy metal contamination of agricultural land in consensus that the disease declines with latitude and is typically one of the cool, temperate climates. the Tamar valley. A major development in the epidemiology of Britain is a high risk area in general and there are cancer in Britain was the publication by Howe regional differences. Several authors have noted that (1963) of the National Atlas of Disease Mortality in north-east Scotland, especially Shetland and the the United Kingdom. The basic data comprised Orkneys have rates of the order 100 - 150/100,000 death certificates returned to the Registrar General. c o m p a r e d with the Faroes at a p p r o x i m a t e l y 50/100,000 (Allison, 1963; Shepherd and Downie, Howe used standardised mortality ratios, i . e . standardised for sex and age. The stomach cancer 1978; Sutherland, 1956). The disease may well be viral in origin maps (males and females separated) clearly show an excess of deaths for west and north Wales. Howe interfering with the body's auto-immune system. (1979) drew attention to the established link of Perhaps the virus lies dormant until triggered by an gastric cancer with blood group A and pointed out environmental influence leading to a breakdown in that this blood group is however more common in the immuno-suppressive system. Mims (1983) has eastern England where mortality from stomach argued that there may be no specific virus but cancer is lowest. He suggested that there was some multiple sclerosis results when antibodies to other special environmental, perhaps geochemical, factor viruses enter the brain. present in Wales which predisposed to gastric A specific role for lead in the aetiology of the cancer. disease has been proposed by some authors since Stocks and Davies (1960) studied the possible lead is a known neurotoxin. Campbell et al. (1950) relationships between garden soil composition and investigated patients who were long-term residents
B. E. Davies
of two villages in Berkshire and Gloucestershire, England. In both cases the garden soils were contaminated by lead and the lead content of the teeth of MS patients was significantly higher than control groups9 Warren and his colleagues have consistently argued a case for lead to be involved in the aetiology of MS (Warren, 1959, 1974; Warren et al., 1967)9 Nonetheless, as with stomach cancer, the apparently strong evidence for a geochemical link, especially with lead, has not always been confirmed. Ward et al. (1985) reported the e l e m e n t a l concentrations in blood and scalp hair from MS and control subjects from Oxford, England9 Their study did not support any association of MS with lead, zinc or m o l y b d e n u m 9 Their data for copper s u g g e s t e d a p o s s i b i l i t y that b l o o d c o p p e r concentrations were lower in the MS patients. Also, barium tended to be higher and vanadium lower in MS patients9 Some of the most convincing evidence that environmental geochemistry can affect the health of w h o l e c o m m u n i t i e s is p r o v i d e d by d e n t a l epidemiology. The relationship between environmental fluoride, especially in drinking water, and dental caries is so strong that the addition of 1 mg F L -1 to public water supplies has been undertaken regularly by some water utilities. Diagnosis of the disease is through a visual inspection. Where the survey population comprises children, typically 12 year olds, the time interval between cause and effect is short and it is possible to make direct associations between environmental quality and disease prevalence9 Moreover, a tooth is a precisely definable target organ in terms of a link between environment and health9 Dental caries is the localised destruction of tooth enamel. The process of this destruction is not fully understood but it is thought to be due to the presence of micro-organisms on the enamel surface9 They are responsible for the formation of plaque, in which the bacteria grow and proliferate 9 As s u b s t r a t e s , e s p e c i a l l y c a r b o h y d r a t e s , are metabolised within the plaque, organic acids are produced which are held, by the plaque, against the enamel until they diffuse away. The initial phase of tooth destruction is most probably due to the dissolution of the mineral phase of the enamel by these acids9 The p r i n c i p a l c o m p o n e n t of e n a m e l is hydroxyapatite microcrystals set in a protein matrix9 The protein represents about 1% of the enamel dry weight and appears to resemble keratin except it c o n t a i n s less sulphur. The apatite fraction approximates to the composition (Ca9.5 Mgo.2 Nao.1 Ho.5)(PO4)517(CO3)o.5(OH)2 But apatite, whether biogenic or mineral, contains traces of many other elements which occur in the crystal structure through isomorphous replacement. Many such substitutions are known and when they occur they alter the properties of the mineral especially its solubility. For example. tetravalent uranium has a radius which is very close
99
to that of divalent calcium and natural mineral apatites can contain up to 0.01% U. Other replacements for calcium which are common are by Mn, Pb, Ce, Th and Sr. An important series of replacements occurs between OH, F and C1 to p r o d u c e the a l m o s t p u r e end m e m b e r s hydroxyapatite, fluorapatite and chlorapatite. The structure of hydroxyapatite is a little expanded c o m p a r e d with that of f l u o r a p a t i t e and hydroxyapatite is relatively less stable9 This is reflected in their different solubilities, the solubility product for hydroxyapatite is 1.6 x 1058 and that for fluorapatite is 3 998 x 10-61. Phosphorus (PO4 3-) in the lattice can be replaced by SO42~ and MnO4-. Finally, in the protein matrix of enamel, selenium can readily replace sulphur. The composition of enamel is potentially very variable and it is interesting that many of the elements which geochemists recognise as common 'guests' in apatite have also been associated with differences in the prevalence of human dental caries. The mode of action of fluoride is not entirely clear. The most likely mechanism is a reduction in e n a m e l solubility t h r o u g h the f o r m a t i o n of fluorapatite. But Jenkins (1970) has reported that only one molecule in 20 converts to fluorapatite at the h i g h e s t , n o r m a l f l u o r i d e intake. Other mechanisms proposed include a bactericidal effect and an enhanced repair mechanism in which small areas of decalcification become refilled with apatite rather than other, more soluble calcium compounds. I n e v i t a b l y , t h e r e c o g n i t i o n that water containing fluoride was protective against caries led to investigations of the possibility that other trace constituents of food or water might affect dental health (Adler and Straub, 1953; Hadjimarkos, 1966, 1967; Losee and Adkins, 1969; Adkins and Losee, 1970; Glass et al., 1973). Similarly, associations with soil composition have been reported (Ludwig et al., 1960; Cadell, 1964; Barmes, 1969; Barmes et al., 1970. Anderson (1966, 1969) studied child caries in Somerset, England. The area has been described by Lewis (1943) as approximately 75,000 acres of flat, low lying country where cattle are liable to scouring when turned out on to certain pastures. Both the pastures and the condition are locally known as "teart" and the disorder arises because of very high molybdenum contents in both soil and grass. In a pilot study 270 children were examined from the teart district compared with 163 children from other parts of southern England. The mean D.M.E score (i.e., the sum of the number of teeth which were observed to have Decay, or had experienced decay and had been extracted (Missing) or were Filled) was 4.3 in the teart area compared with 6.2 elsewhere (p 10 ~tg Pb g-l (geometric mean = 266 ~tg Pb g-l) in 4,126 garden soils from locations excluding London and areas affected by mining and smelting. In the 579 gardens sampled in London Boroughs the mean value was 654/.tg P b g-l. Agricultural soils in Britain usually contain from 10 to 150 gg Pb g-l. Lower lead values in soils from households of less than 30 years of age compared with those of I00 or more years of age were. considered by the authors to reflect (a) voluntary reduction in lead content of household paints and (b) a shorter history of soil contamination from bonfires and fossil fuel residues from open coal fires and soot commonly used as soil improvers and ameliorants. Other investigations have revealed yet finer detail about the geochemical nature of cities. Getz and c o w o r k e r s (1977) f o u n d that soil lead concentrations declined in the first 5 m away from a house and then rose towards the adjacent street They concluded that the accumulation of lead in dusts and soils around buildings appeared to be directly related to the volume of traffic on neighbourhood streets and roads. Other sources of lead included paint and wash off from gutters containing wind borne dust. Preer et al. (1984) also observed that soil lead concentrations were higher nearer a house than away from it. From the late eighteenth century until the late nineteenth century
102
Trace elements in the human environment
the only white pigments used in paint were lead compounds. Since the end of the nineteenth century paint formulations have included TiO2, BaSO4, CaSO4 and CaCO3. White paints have generally relied on three carbonates of lead, namely, normal lead carbonate, PbCO3, and the basic lead carbonates, 2PbC O3.Pb(OH)2 and 4PBCO3.2Pb(OH)2.PbO. O t h e r w h i t e lead compounds in use include PbSO4 and the silicate 3PbO.2SiO2.H20. Red colours can be based on red lead (Pb304) and the chromates (PbCrO4 and PbCrO4.PbO) provide the yellow colours for street markings. Since 1974 paint for interior use in the USA should not exceed 0.06% Pb. (Dunn, 1975). The inhabitants of urban areas are exposed not only to soil (the in situ biologically altered rock) but also to dust, the fine mineral material that settles on to solid surfaces. The analysis of street, school playground and house dust samples for lead has received considerable attention over the past decade. A general consensus of results points to the presence of elevated lead concentrations in dusts both within and without the home. Guidance on acceptable levels of lead within urban locations has not generally been formulated, though the former Greater London Council adopted a guideline lead concentration of 500 gg Pb gq in the
Trace elements in the human environment:
problems and risks* Brian E. Davies Department of Environmental Science, University of Bradford, Bradford, West Yorkshire, BD7 1DP, England
Abstract
Early studies on the possible influences of environmental geochemistry and health were concerned with major diseases such as cancer or multiple sclerosis. They were generally confined to simple comparisons of geochemical and epidemiological maps and corresponding rural areas. Subsequently, studies of dental health produced evidence for the roles of environmental fluoride, selenium, molybdenum and lead in the aetiology of caries. In recent decades lead has been closely studied. Most people in the western world live in urban areas, rather than rural areas, and research on lead has yielded much information on the distribution of the element in streets, parks and gardens of cities and in the home environment. The role of lead in human health has thereby been better understood. Cadmium, selenium and aluminium are good candidates for future study but success will depend on applying appropriate methods of investigation and here the experiences from research on lead are a valuable model to follow.
Introduction For virtually all human evolutionary history death has most likely been the consequence of fatal injury, starvation, acute food poisoning or infectious diseases. The developmental history of mankind can be considered as a continuous and increasingly successful endeavour to become isolated from and unaffected by environmental influences. Our P a l a e o l i t h i c a n c e s t o r s i n v e n t e d clothes and discovered fire so as not to freeze to death as they roamed the tundra wastes of peri-glacial Europe. Now our houses are central heated and sometimes air-conditioned, our windows double or trebled glazed, our doors draught proofed; all with the intention of ensuring we pass our days both dry and warm and isolated from the natural environment. Yet humans cannot escape from the ecosystem of which they are part as is demonstrated by recent concerns that harmful radon may be accumulating in the air of our home environment now we have draughtless houses, i.e., no interchange of outside and inside air. The discovery that infectious diseases are caused by pathogens led to the provision of clean, piped water to houses, municipal sewerage instead of private cesspits, vaccines and antibiotics. Consequently, the tremendous biological onslaughts of European history, such as plague or cholera, have *This paper is based on a keynote lecture at the start of the SEGH conference.
now been c o n t r o l l e d and m i n i m i s e d . The contemporary western world is more concerned with an important groups of diseases, e.g., cancers, diseases of the central nervous s y s t e m and cardiovascular disease, the causes of which are by and large u n k n o w n and cure and control is uncertain. When the incidence or prevalence of these diseases is mapped within any one region variations are seen and these patterns may persist over many years (Howe, 1979). Environmental influences appear to be influencing the disease pattern and a role for environmental geochemistry has been suggested by many authors. The health of animals and man is known to be affected by the amounts and properties of the chemical elements present in food, beverages and inhaled dust or air. Recent years have seen more elements being recognised as essential, e.g., chromium or selenium, and the role of essential e l e m e n t s in m e t a b o l i c p r o c e s s e s is b e t t e r u n d e r s t o o d . M e r t z (1981) has w r i t t e n a comprehensive survey of those elements now regarded essential to man.
Non-infectious Diseases and Environmental Geochemistry: Conventional Approaches Two groups of diseases have attracted much attention by medical geographers and geochemists namely: cancers and diseases of the central nervous system, especially multiple sclerosis. The strong
98
Trace elements in the human environment
link between fluoride and dental health has the frequency of stomach cancer in Britain. Soil stimulated much work on the influence of other organic matter, zinc and cobalt were related trace elements. positively with stomach cancer incidence but not Chemotherapy, radiotherapy and screening with intestinal cancer. Chromium was connected programmes are reducing the mortality of several with the incidence of both. Vanadium and iron cancers. In addition much progress is being made at showed inconclusive relations and nickel, titanium a molecular level in understanding the causes of and lead showed no connection. Subsequently, they cancers. But, at a higher, whole body level, the reported (Stocks and Davies, 1964) that the average causes are still not understood. Ultraviolet and logarithm of the ratio of zinc/copper in garden soils ionising radiation, many organic substances and was always higher where a person had just died of some inorganic substances are known to be stomach cancer after 10 or more years of residence than it was at houses where a person had died c a r c i n o g e n i c . T h e r e is e v i d e n c e for s o m e environmental factor(s) in the aetiology of cancers, similarly of a non-malignant cause. The effect was especially gastric cancer. Several authors have more pronounced and consistent in soils taken from speculated that this variability may be influenced by vegetable gardens and it was not found where the soil or water quality. Matthews (1990) has reviewed duration of residence was less than ten years. the e p i d e m i o l o g y of gastric c a n c e r and has The g e o g r a p h i c a l v a r i a b i l i t y o f c a n c e r discussed the possible role of environmental factors. incidence continues to attract the interest of Early reports relied upon very generalised geographers and geologists. Yet we have made little cartographical or similar associations between real progress in attempting to involve trace elements disease patterns and geographical or geological in the aetiology of cancer. Even some earlier factors (Legon, 1951, 1952; Tromp and Diehl, 1955; promising associations have been found to be Millar, 1961). Later publications tended to propose non-existent in later studies. For example, Piispanen (1989) compared cancer and geochemical maps of unspecified influences of one or more trace elements. These early reports were weakened by Finland but found only weak associations. When Armstrong (1990) revisited an earlier study area in their use of crude, unstandardised death rates. A l l e n - P r i c e ( 1 9 6 0 ) , in a p a p e r m u c h I c e l a n d he c o n c l u d e d that the p r o n o u n c e d commented on at the time, reported that mortality geographical variation in Icelandic stomach cancer (crude figures, unstandardised) from cancer was incidence that was widely cited from 1961 to 1965 unusually low in certain villages in Devon and is now believed to be spurious. Multiple sclerosis is a relatively uncommon unusually high in others. Within the village of Horrabridge mortality was linked to the origin of disease which is very difficult to diagnose in its different water supplies: lowest mortality was early stages. The numbers of patients in any area are associated with reservoir water from Dartmoor small which makes it difficult to transform the data whereas highest mortality was associated with well to produce standardised mortality ratios and it is or spring water derived from mineralised rock strata. usual to quote simple prevalence rates per 100,000. Subsequently, a more rigorous study by the Royal Acheson (1977) has suggested that rates in excess College of General Practitioners (RCGP, 1966) of 40/100,000 should be regarded as high and from 0 to 19/100,00 as low. Spillane (1972) commented d e m o n s t r a t e d that s e v e r a l D e v o n p r a c t i c e s coml~ared unfavourably with Birmingham practices that rates in excess of 40/100,000 are not known with respect to malignant neoplasms. In a sequel to between latitudes 40N and 40S and rates in western this study, Davies (1971) described widespread Europe are generally high. There is a general heavy metal contamination of agricultural land in consensus that the disease declines with latitude and is typically one of the cool, temperate climates. the Tamar valley. A major development in the epidemiology of Britain is a high risk area in general and there are cancer in Britain was the publication by Howe regional differences. Several authors have noted that (1963) of the National Atlas of Disease Mortality in north-east Scotland, especially Shetland and the the United Kingdom. The basic data comprised Orkneys have rates of the order 100 - 150/100,000 death certificates returned to the Registrar General. c o m p a r e d with the Faroes at a p p r o x i m a t e l y 50/100,000 (Allison, 1963; Shepherd and Downie, Howe used standardised mortality ratios, i . e . standardised for sex and age. The stomach cancer 1978; Sutherland, 1956). The disease may well be viral in origin maps (males and females separated) clearly show an excess of deaths for west and north Wales. Howe interfering with the body's auto-immune system. (1979) drew attention to the established link of Perhaps the virus lies dormant until triggered by an gastric cancer with blood group A and pointed out environmental influence leading to a breakdown in that this blood group is however more common in the immuno-suppressive system. Mims (1983) has eastern England where mortality from stomach argued that there may be no specific virus but cancer is lowest. He suggested that there was some multiple sclerosis results when antibodies to other special environmental, perhaps geochemical, factor viruses enter the brain. present in Wales which predisposed to gastric A specific role for lead in the aetiology of the cancer. disease has been proposed by some authors since Stocks and Davies (1960) studied the possible lead is a known neurotoxin. Campbell et al. (1950) relationships between garden soil composition and investigated patients who were long-term residents
B. E. Davies
of two villages in Berkshire and Gloucestershire, England. In both cases the garden soils were contaminated by lead and the lead content of the teeth of MS patients was significantly higher than control groups9 Warren and his colleagues have consistently argued a case for lead to be involved in the aetiology of MS (Warren, 1959, 1974; Warren et al., 1967)9 Nonetheless, as with stomach cancer, the apparently strong evidence for a geochemical link, especially with lead, has not always been confirmed. Ward et al. (1985) reported the e l e m e n t a l concentrations in blood and scalp hair from MS and control subjects from Oxford, England9 Their study did not support any association of MS with lead, zinc or m o l y b d e n u m 9 Their data for copper s u g g e s t e d a p o s s i b i l i t y that b l o o d c o p p e r concentrations were lower in the MS patients. Also, barium tended to be higher and vanadium lower in MS patients9 Some of the most convincing evidence that environmental geochemistry can affect the health of w h o l e c o m m u n i t i e s is p r o v i d e d by d e n t a l epidemiology. The relationship between environmental fluoride, especially in drinking water, and dental caries is so strong that the addition of 1 mg F L -1 to public water supplies has been undertaken regularly by some water utilities. Diagnosis of the disease is through a visual inspection. Where the survey population comprises children, typically 12 year olds, the time interval between cause and effect is short and it is possible to make direct associations between environmental quality and disease prevalence9 Moreover, a tooth is a precisely definable target organ in terms of a link between environment and health9 Dental caries is the localised destruction of tooth enamel. The process of this destruction is not fully understood but it is thought to be due to the presence of micro-organisms on the enamel surface9 They are responsible for the formation of plaque, in which the bacteria grow and proliferate 9 As s u b s t r a t e s , e s p e c i a l l y c a r b o h y d r a t e s , are metabolised within the plaque, organic acids are produced which are held, by the plaque, against the enamel until they diffuse away. The initial phase of tooth destruction is most probably due to the dissolution of the mineral phase of the enamel by these acids9 The p r i n c i p a l c o m p o n e n t of e n a m e l is hydroxyapatite microcrystals set in a protein matrix9 The protein represents about 1% of the enamel dry weight and appears to resemble keratin except it c o n t a i n s less sulphur. The apatite fraction approximates to the composition (Ca9.5 Mgo.2 Nao.1 Ho.5)(PO4)517(CO3)o.5(OH)2 But apatite, whether biogenic or mineral, contains traces of many other elements which occur in the crystal structure through isomorphous replacement. Many such substitutions are known and when they occur they alter the properties of the mineral especially its solubility. For example. tetravalent uranium has a radius which is very close
99
to that of divalent calcium and natural mineral apatites can contain up to 0.01% U. Other replacements for calcium which are common are by Mn, Pb, Ce, Th and Sr. An important series of replacements occurs between OH, F and C1 to p r o d u c e the a l m o s t p u r e end m e m b e r s hydroxyapatite, fluorapatite and chlorapatite. The structure of hydroxyapatite is a little expanded c o m p a r e d with that of f l u o r a p a t i t e and hydroxyapatite is relatively less stable9 This is reflected in their different solubilities, the solubility product for hydroxyapatite is 1.6 x 1058 and that for fluorapatite is 3 998 x 10-61. Phosphorus (PO4 3-) in the lattice can be replaced by SO42~ and MnO4-. Finally, in the protein matrix of enamel, selenium can readily replace sulphur. The composition of enamel is potentially very variable and it is interesting that many of the elements which geochemists recognise as common 'guests' in apatite have also been associated with differences in the prevalence of human dental caries. The mode of action of fluoride is not entirely clear. The most likely mechanism is a reduction in e n a m e l solubility t h r o u g h the f o r m a t i o n of fluorapatite. But Jenkins (1970) has reported that only one molecule in 20 converts to fluorapatite at the h i g h e s t , n o r m a l f l u o r i d e intake. Other mechanisms proposed include a bactericidal effect and an enhanced repair mechanism in which small areas of decalcification become refilled with apatite rather than other, more soluble calcium compounds. I n e v i t a b l y , t h e r e c o g n i t i o n that water containing fluoride was protective against caries led to investigations of the possibility that other trace constituents of food or water might affect dental health (Adler and Straub, 1953; Hadjimarkos, 1966, 1967; Losee and Adkins, 1969; Adkins and Losee, 1970; Glass et al., 1973). Similarly, associations with soil composition have been reported (Ludwig et al., 1960; Cadell, 1964; Barmes, 1969; Barmes et al., 1970. Anderson (1966, 1969) studied child caries in Somerset, England. The area has been described by Lewis (1943) as approximately 75,000 acres of flat, low lying country where cattle are liable to scouring when turned out on to certain pastures. Both the pastures and the condition are locally known as "teart" and the disorder arises because of very high molybdenum contents in both soil and grass. In a pilot study 270 children were examined from the teart district compared with 163 children from other parts of southern England. The mean D.M.E score (i.e., the sum of the number of teeth which were observed to have Decay, or had experienced decay and had been extracted (Missing) or were Filled) was 4.3 in the teart area compared with 6.2 elsewhere (p 10 ~tg Pb g-l (geometric mean = 266 ~tg Pb g-l) in 4,126 garden soils from locations excluding London and areas affected by mining and smelting. In the 579 gardens sampled in London Boroughs the mean value was 654/.tg P b g-l. Agricultural soils in Britain usually contain from 10 to 150 gg Pb g-l. Lower lead values in soils from households of less than 30 years of age compared with those of I00 or more years of age were. considered by the authors to reflect (a) voluntary reduction in lead content of household paints and (b) a shorter history of soil contamination from bonfires and fossil fuel residues from open coal fires and soot commonly used as soil improvers and ameliorants. Other investigations have revealed yet finer detail about the geochemical nature of cities. Getz and c o w o r k e r s (1977) f o u n d that soil lead concentrations declined in the first 5 m away from a house and then rose towards the adjacent street They concluded that the accumulation of lead in dusts and soils around buildings appeared to be directly related to the volume of traffic on neighbourhood streets and roads. Other sources of lead included paint and wash off from gutters containing wind borne dust. Preer et al. (1984) also observed that soil lead concentrations were higher nearer a house than away from it. From the late eighteenth century until the late nineteenth century
102
Trace elements in the human environment
the only white pigments used in paint were lead compounds. Since the end of the nineteenth century paint formulations have included TiO2, BaSO4, CaSO4 and CaCO3. White paints have generally relied on three carbonates of lead, namely, normal lead carbonate, PbCO3, and the basic lead carbonates, 2PbC O3.Pb(OH)2 and 4PBCO3.2Pb(OH)2.PbO. O t h e r w h i t e lead compounds in use include PbSO4 and the silicate 3PbO.2SiO2.H20. Red colours can be based on red lead (Pb304) and the chromates (PbCrO4 and PbCrO4.PbO) provide the yellow colours for street markings. Since 1974 paint for interior use in the USA should not exceed 0.06% Pb. (Dunn, 1975). The inhabitants of urban areas are exposed not only to soil (the in situ biologically altered rock) but also to dust, the fine mineral material that settles on to solid surfaces. The analysis of street, school playground and house dust samples for lead has received considerable attention over the past decade. A general consensus of results points to the presence of elevated lead concentrations in dusts both within and without the home. Guidance on acceptable levels of lead within urban locations has not generally been formulated, though the former Greater London Council adopted a guideline lead concentration of 500 gg Pb gq in the
