Environmental lead exposure: a public health problem of global dimensions

Lead is the most abundant of the heavy metals in the Earth's crust. It has been used since prehistoric times, and has become widely distributed and mobilized in the environment. Exposure to and uptake of this non-essential element have consequently increased. Both occupational and environmental exposures to lead remain a serious problem in many developing and industrializing counties, as well as in some developed countries. In most developed countries, however, introduction of lead into the human environment has decreased in recent years, largely due to public health campaigns and a decline in its commercial usage, particularly in petrol. Acute lead poisoning has become rare in such countries, but chronic exposure to low levels of the metal is still a public health issue, especially among some minorities and socioeconomically disadvantaged groups. In developing countries, awareness of the public health impact of exposure to lead is growing but relatively few of these countries have introduced policies and regulations for significantly combating the problem. This article reviews the nature and importance of environmental exposure to lead in developing and developed countries, outlining past actions, and indicating requirements for future policy responses and interventions.

Keywords: lead, adverse effects; lead toxicity; lead poisoning; environmental exposure; occupational diseases; epidemiologic studies.

Lead, a ubiquitous and versatile metal, has been used since prehistoric times. It has become widely distributed and mobilized in the environment, and human exposure to and uptake of this non-essential element have consequently increased. At high levels of human exposure there is damage to almost all organs and organ systems, most importantly the central nervous system, kidneys and blood, culminating in death at excessive levels. At low levels, haeme synthesis and other biochemical processes are affected, psychological and neurobehavioural functions are impaired, and there is a range of other effects.

There is a long history of public exposure to lead in food and drink. Lead poisoning was common in Roman times because of the use of lead in water pipes and earthenware containers, and in wine storage. Lead poisoning associated with occupational exposure was first reported in 370 BC. It became common among industrial workers in the 19^th and early 20^th centuries, when workers were exposed to lead in smelting, painting, plumbing, printing and many other industrial activities. In 1767, Franklin obtained a list of patients in La Charite Hospital in Paris who had been admitted because of symptoms, which, although not recognized then, were evidently those of lead poisoning. All the patients were engaged in occupations that exposed them to lead.

In 1839, Tanqueral des Planches described the symptoms of acute lead poisoning on the basis of 1213 admissions to La Charite Hospital between 1830 and 1838. His study was so thorough that little has subsequently been added to the clinical picture of the symptoms and signs of acute lead poisoning in adults. In the mid-19^th century, occupational lead poisoning was a common disorder in the United Kingdom, and in 1882, following the deaths of several employees in the lead industry, a parliamentary enquiry was initiated into working conditions in lead factories. This resulted in the 1883 Factory and Workshop Act (Prevention of Lead Poisoning), which required lead factories to conform to certain minimum standards, e.g. the provision of ventilation and protective clothing.

Various adverse effects of lead exposure on human health have been recognized. The working environment in the lead industry, especially, in developed countries, has been much improved. Acute occupational lead poisoning has largely been controlled in developed countries through improved working conditions. However, concern has grown over the possible adverse effects of exposure to low levels of environmental lead. In particular, lead poisoning in children experiencing non-occupational exposure has attracted much attention.

In Australia, lead poisoning in children was first reported in 1892, although it was not until 12 years later that the source, peeling lead-based paint, was identified in a series of ten children with lead colic. In 1943 a follow-up study of 20 schoolchildren in the USA who had experienced acute lead poisoning in infancy or early childhood found that exposure to environmental lead at levels insufficient to produce clinical encephalopathy was associated with long-term, deficits in neuropsychological development. Case-control studies on mental retardation and hyperactivity in relation to environmental lead exposure showed that children who survived acute lead intoxication were often left with severe deficits in neurobehavioural function. It was subsequently recognized that longer-term sequelae were not limited to people affected by excessive exposure but also occurred in children who experienced relatively low-level exposure.

Much research over the last 30 years has demonstrated adverse health effects of moderately elevated blood lead levels, i.e. below 25 µg/dl. The permissible exposure level in the ambient (air, water, soil, etc.) environment, as well as in the working environment, has therefore been progressively lowered. Although the problems of overt lead poisoning have largely receded in developed countries, chronic exposure to low levels of lead is still a significant public health issue, particularly among some minorities and disadvantaged groups. Furthermore, both occupational and environmental exposures have remained a serious problem in many developing and industrializing countries.

Exposure of human populations to environmental lead was relatively low before the industrial revolution but has increased with industrialization and large-scale mining. Lead contamination of the environment is high relative to that of other non-essential elements. Globally, the extensive processing of lead ores is estimated to have released about 300 million tonnes of lead into the environment over the past five millennia, mostly within the past 500 years.

Following the advent of motor vehicles at the beginning of the 20^th century, there was a substantial increase in environmental lead contamination because of the use of lead in petrol. This resulted in an increase in community exposure to environmental lead throughout much of the century. World lead consumption rose steadily between 1965 and 1990, when it reached about 5.6 million tonnes. Between 1980 and 1990 the consumption of lead in developed countries increased only slightly, whereas between 1979 and 1990 in developing countries it increased from 315000 tonnes to 844000 tonnes per annum. Global lead contamination, attributable to the greatly increased circulation of lead in soil, water and air as a result of human activities, remains significant.

The pre-industrial or natural blood lead level in humans is estimated to have been about 0.016 µg/dl, 50-200 times lower than the lowest reported levels of people today people in remote regions of the southern and northern hemispheres (0.78 µg/dl and 3.20 µg/dl respectively). This level is about 625 times lower than the current level of concern for children (i.e. 10 µg/dl) proposed by the Centers for Disease Control and Prevention in the USA. Lead levels in human skeletal remains indicate that the body lead burden of today's populations is 500-1000 times greater than that of their pre-industrial counterparts.

Much research has been conducted recently on children with moderately raised blood lead levels associated with environmental exposure. The potential for adverse effects of lead exposure in children is heightened because:

Elevated lead levels continue to be a particular problem among socially and economically deprived children. Poor people are more likely to live in substandard housing and be near industry and heavy traffic, to be exposed to lead dust brought home by lead workers, and to be nutritionally deprived and therefore susceptible.

Debate continues over the nature, magnitude and persistence of the adverse effects on human health of low-level exposure to environmental lead. However, the accumulated epidemiological evidence indicates that such exposure in early childhood causes a discernible deficit in cognitive development during the immediately ensuing childhood years. It used to be thought that neuropsychological manifestations disappeared or declined if the ingestion of lead was stopped or reduced. Recent data, however, show that such effects are largely irreversible.

One of the reasons for past uncertainty about the health effects of low-level exposure to lead was the methodological pitfalls that beset many cross-sectional studies, in which exposures and outcomes are measured approximately simultaneously. Many of the earlier studies failed to control for the effects of confounding factors, e.g. parental intelligence, socioeconomic status, and quality of the home environment. A single measure of contemporary exposure also provided limited scope for answering questions on the natural history of the association between lead exposure and outcomes. Such questions included those related to the possibility of critical periods of exposure and to the persistence or reversibility of lead-associated deficits.

More recent prospective studies, e.g. the Cincinnati and Boston cohort studies in the USA and the Port Pirie cohort study in Australia, have used highly sophisticated longitudinal designs to examine the effects of antenatal and postnatal blood lead levels on childhood development. These studies employed common and more sensitive measures of exposure and developmental outcomes, considered a wide range of covariates and confounding variables, and used stringent quality control measures. Children have been followed up prospectively from birth, and the nature of the relationship between exposure and outcome has been more readily determinable than previously. Despite the methodological problems associated with the earlier cross-sectional studies, the associations they suggested have in many instances been confirmed by the subsequent prospective studies.

These epidemiological finding, considered in conjunction with animal data, stimulated discussion and debate among scientists, consumers, industrialists and decision-makers during the 1980s and 1990s, leading to a shift in public and expert opinion about the dangers of lead. It is now known that exposure to lead during the early stages of a child's development is linked, inter alia, to deficits in later neurobehavioural performance, For blood lead levels < 25 µg/dl it appears that the size of the effect on IQ, as assessed at 3 years of age and above, is probably 1-3 points for each 10-µg/dl increment in blood lead level, with no definitive evidence of a threshold. In addition to neurobehavioural effects, there have been reports of effects on haeme synthesis and on a number of enzymes and biochemical parameters, as well as of reduced gestational age. A spectrum of effects of lead has been clearly identified.

Unlike overt lead toxicity, where there is usually one identifiable source, low-level environmental exposure to lead is associated with multiple sources (petrol, industrial processes, paint, solder in canned foods, water pipes) and pathways (air, household dust, street dirt, soil, water, food). Evaluation of the relative contributions of sources is therefore complex and likely to differ between areas and population groups.

Where lead derived from petrol comprises the major part of atmospheric lead it is a significant contributor to the body lead burden and is the most widely distributed source of the metal in the environment. It is thus desirable to phase out the use of lead additives in fuels as quickly as possible on the global scale. Atmospheric lead that is deposited in soil and dust may then be ingested by children and may substantially raise their blood lead levels. For the population at large, which is not occupationally exposed, food and water are important sources of baseline exposure to lead, in addition to atmospheric lead that is inhaled. Table 1 shows the relationship between the median level of blood lead and the intake of lead for the general population.

Even in countries where considerable efforts have been made to control lead, vast reservoirs of the metal still exist in soil, dust and house paint, and these sources will continue to affect populations for many years. Lead exposure is clearly a global public health issue, but it is only just being recognized as a potential problem in many developing countries, with studies only now being reported from Africa, Asia and South America.

Although the occurrence of severe lead poisoning has largely receded in many countries, occupational exposure to lead resulting in moderate and clinically symptomatic poisoning is still common. Among adults, exposure is normally greatest for those who come into closest contact with lead in production processes. Workers are exposed to lead in many occupations, including motor vehicle assembly, panel beating, battery manufacture and recovery, soldering lead mining and smelting, lead alloy production, and in the glass, plastic, printing, ceramics and paint industries.

In most highly industrialized countries, stricter controls and improvements in industrial methods have helped to ensure that occupational lead poisoning is less prevalent than formerly. In developing countries, however, it remains a problem of potentially huge dimensions.

Table 2 indicates occupations and operations in which lead may be a hazard for workers. In developing countries, occupational lead exposure is commonly unregulated and little monitoring of exposure is carried out. The potential for hazardous exposure to lead during smelting and refining of the metal is well recognized, both for primary new metal and secondary metal, i.e. scrap lead, smelters and refineries. Small domestic secondary smelters in many countries are typically located close to people's homes. The lead fumes and dust generated from such operations can pose and exceptional health hazard to children and adults living nearby.

Workers are also at particular risk in battery manufacture, demolition work, welding, pottery and ceramic ware production (often a home-based occupation involving women and children), small business repairing automobile radiators, and the production of jewellery and decorative items by artisans. The last-mentioned category is of particular concern since the work is predominantly carried out at home or in unregulated workshops, often by women and children. Although adults are mainly involved, in many countries, especially those with developing industries, and in small home-based industries, there is little distinction between home and the workplace, and children are consequently exposed to lead. Because of the transfer of lead to the fetus in utero and the introduction of lead into people's homes on clothing, where young children thus become exposed, problems of occupational exposure become community problems.

In most developed countries, concerted efforts have led to a reduction in the introduction of lead into the ambient environment in recent years, reflecting a decline in the commeruse of lead, particularly in petrol. Blood lead levels in the general population in these countries have fallen dramatically over the past 20 years, thanks to the phasing out of lead from petrol and the reduction of environmental exposure to the metal. This trend, illustrated by the examples given below, is likely to continue.

In the USA between 1976 and 1991 the mean blood lead level of persons aged 1-74 years dropped by 78%, from 12.8 µg/dl to 2.8 µg/dl. Mean blood lead levels of children aged 1-5 years declined by 77% (from 13.7 µg/dl to 3.2 µg/dl) for non-Hispanic white children and by 72% (from 20.2 µg/dl to 5.6 µg/dl) for non-Hispanic black children. The prevalence of blood lead levels of >10 µg/dl for children aged 1-5 years declined from 85.0% to 5.5% for non-Hispanic white children, and from 97.7% to 20.6% for non-Hispanic black children. Similar declines were found in population subgroups defined by age, sex, race/ethnicity, income level and urban status. The major cause of the observed decline in blood lead levels was the removal of lead from petrol.

The Third National Health and Nutrition Examination Survey, a representative survey of the civilian, non-institutionalized population of the USA, showed that, up to 1994, the overall man blood lead level in people aged >1 year was 2.3 µg/dl; 2.2% of the population had levels of 10 µg/dl, the level of health concern for children, or above. Among American children aged 1-5 years the mean blood lead level was 2.7 µg/dl, with 890 000 of such children (4.4%) having elevated blood lead levels. Sociodemographic factors associated with higher blood lead levels in children were non-Hispanic black race/ethnicity, low income and living in older housing. It was concluded that programmes for the prevention of lead poisoning should target high-risk persons, such as children living in old houses, belonging to minority groups, and living in families with low incomes.

In Australia, blood samples were taken during 1995-96 from 1575 children aged 1-4 years. The geometric mean blood lead level was 5.05 µg/dl. For 115 of these children (i.e. 7.3%) the blood lead level was >10 µg/dl. Only 27 children (1.7%) had levels >15 µg/dl. It is difficult, however, to assess the trends of lead exposure in Australia in relation to the control of sources because the available data are limited.

Between 1978 and 1988, marked decrease in the average blood lead levels of adults were noted in many countries, including Belgium, Federal Republic of Germany, New Zealand, Sweden, and the United Kingdom. Recently, lead exposure was examined among the population of Barcelona and the changes that had occurred during the previous 10 years were evaluated. The blood lead levels in a random sample of 694 healthy subjects in the age range 0-65 years were 4.06 µg/dl (umbilical cord), 8.9 µg/dl (children), and 7.8 µg/dl (adults). Over this 10-year period there had been a reduction of more than 50% in blood lead levels, reflecting a decrease in the lead concentration in the ambient air.

Lead continues to be a significant public health problem in developing countries, where there are considerable variations in the sources and pathways of exposure. For example, in many Latin American countries, leaded paint is not a significant source of recurrent exposure, whereas lead-glazed ceramics are such a source. Exposure attributable to miscellaneous sources may be even more significant than universal exposure associated with leaded petrol, especially for people living in poverty. Exposure to lead from led mining, smelting, battery factories and cottage industries is a significant environmental hazard in developing countries.

In Jamaica a survey was conducted to determine the distribution and determinants of environmental and blood lead levels in populations living near conventional and cottage lead smelters. Geometric mean blood lead levels in exposed groups were nearly twice as high as those in unexposed groups; 44% of exposed under 6-year-olds had blood lead levels >25 µg/dl. In Berat and Tirana, Albania, the mean observed blood lead levels in 84 preschool children living less than 2 km from a battery plant was 43.4 µg/dl, significantly higher than the value of 15.0 µg/dl in 45 preschool children living more than 2 km from the plant; 98% of preschool children and 82% of schoolchildren had blood lead levels >10 µg/dl.

In China, childhood lead poisoning may be widespread as a result of rapid industrialization and the use of leaded petrol. Children residing in industrial areas and in areas with heavy traffic had average blood lead levels of 21.8-67.9 µg/dl. The proportion of blood lead levels > 10 µg/dl ranged from 64.9% to 99.5%. Even abut 50% of children living in noindustrialized areas had blood lead value >10 µg/dl. There is also evidence of an increase in blood lead levels among non-smoking women between 1983 and 1998, associated with a rapid increase in the number of motor vehicles. The problem of lead exposure in children is particularly significant in small towns with numerous small factories.

The situation is similar in other countries where industrialization is occurring. In Dhaka, lead concentrations in airborne particulate matter averaged 453 µg/m³ during the low rainfall season of November to January. The mean blood lead concentration among 93 randomly selected rickshaw pullers in India was 53 µg/dl. Also in India, direct testing for blood lead was carried out randomly on 2031 children and adults in five cities with high population densities where leaded petrol had contributed to environmental lead levels. Approximately 51% had levels >10 µg/dl, and 13% had values >20 µg/dl. The proportion of children with levels >10 µg/dl ranged from 40% in Bangalore to 62% in Mumbai.

A cross-sectional study was carried out on a random sample of 200 children aged under 5 years living in an area of Mexico City. Samples of floor, window and street dust, paint, soil, water and glazed ceramics were obtained from the participants' households, as well as blood samples and dirt from their hands. Blood lead levels ranged from 1 µg/dl to 31 µg/dl, the mean being 9.9 µg/dl. Among children aged >18 months, 44% had blood lead levels exceeding 10 µg/dl. Except for glazed ceramic, the led content of environmental samples was low. The major predictors of blood lead levels were the lead content of the glazed ceramics used in the preparation of children's food, exposure to airborne lead from vehicle emissions, and the lead content of dirt from the children's hands.

African children may be particularly predisposed to environmental lead exposure because of their lifestyle and sociocological factors. However, the true picture of childhood lead poisoning in Africa remains unclear. Petrol sold in most African countries contains 0.5-0.8 µg/l lead, which may be among the highest levels in the world. In urban and rural areas and near mining centres, average atmospheric lead concentrations reach 0.5-0.3 µg/m³ and exceed 1000 µg/g in dust and soils. In addition to automotive and industrial sources, the following are hazards in individual households: cottage industries and the burning of paper products, discarded rubber, battery casings, and painted wood for cooking and heating. Lead paint, lead solder and lead cosmetics are unregulated in some countries.

In Cape Province, South Africa, over 90% of the children in some urban and rural communities have blood lead levels >10 µg/dl. The mean blood lead level in inner-city, first-grade schoolchildren in the country was 18 µg/dl; 13% of mixed race children, but not white children, had blood lead levels >25 µg/dl. Over 90% of children had blood lead levels >10 µg/dl/. The levels of lead in blood, air and dust samples taken from schools close to roads carrying heavy traffic were higher than in those from schools further away.

Raised blood lead values were also associated with dusty houses, houses in a poor state of repair, overcrowding, low parental educational and income levels, and other factors related to family structure and socioeconomic status. Social factors were thought to be important in predisposing children to poisoning by lead in the environment. Childhood lead poisoning appeared to be a widespread urban health problem throughout Africa.

The level of exposure to lead is, however, falling in some developing countries because of the reduced use of lead in petrol and elsewhere. In Thailand, for example, leaded petrol was phased out during the period 1984-96 and was associated with a marked decline in atmospheric lead levels in Bangkok. A recent survey of 1000 children aged 6-72 months in Chiang Mai, Thailand, revealed that their average blood lead level was 4.2 µg/dl; only 4.6% of the children had blood lead levels >10 µg/dl. Between February and September 1994 the blood lead levels in maternal and cord blood of 37 pregnant women from urban areas, 53 from periurban areas and 28 from rural areas of Chiang Mai were rather low and did not differ significantly: the geometric mean levels were 4.16 µg/dl and 3.32 µg/dl; 4.12 µg/dl and 3.11 µg/dl; and 4.50 µg/dl and 4.12 µg/dl respectively. A total of 4.2% of maternal blood samples and 1.7% of cord blood samples had lead levels >10 µg/dl. In Bangkok, where air pollution levels were higher than in Chiang Mai, 5.2% and 2.4% of 500 pregnant women and their newborn babies, respectively, had blood lead levels >10 µg/dl. These values compared favourably with those determined in a previous study in Bangkok before the introduction of unleaded petrol.

Exposure to environment lead is clearly a major public health hazard of global dimensions. As measures to control the transfer of lead to the environment are implemented in most developed countries through, for example, the phasing out of lead in fuel, paints and other consumer products, and tighter control of industrial emissions, environmental exposure to lead can, in general, be expected to continue to decline. However, because of rapid industrialization and the persistence of lead in the environment, exposure is likely to remain a significant public health problem in most developing countries for many years. Much work needs to be done to identify and treat children with elevated blood lead levels and reduce lead exposure in the community.

Screening, monitoring, intervention and evaluation are critical for the development of rational, cost-effective and science-based public health policies aimed at achieving these goals.

Among the many international conventions that have acknowledged the importance of exposure to lead as a key public health issue are the following:

Public health measures should continue to be directed to the reduction and prevention of exposure to lead by reducing the use of the metal and its compounds and by minimizing lead-containing emissions that result in human exposures. This can be achieved by:

Specific recommendations on national policy and implementation measures to reduce the impact of lead on human health in developing countries have been drawn up by the Executive Committee of the International Conference on Lead Poisoning.

Nearly 100 countries, mostly developing, still use leaded petrol. Evidence from Europe, Japan, Mexico, and the USA suggests that phasing out leaded petrol is the most effective way of reducing the general population's exposure to lead, although other sources are also important. Many developing countries have been actively engaged in lead reduction programmes, particularly in respect of leaded petrol, "the mistake of the twentieth century". Bangladesh, China, Egypt, Haiti, Honduras, Hungary, India, Kuwait, Nicaragua, Malaysia, and Thailand have, for examp, made dramatic efforts to phase out leaded petrol in recent years. Success in this endeavour requires government commitment, incentive policies, a broad consensus among stakeholders and public understanding, acceptance and support.

Because of their experience, many international bodies are well equipped to provide assistance with tackling the various dimensions of exposure to lead in the environment. Concerted efforts on a worldwide basis can overcome this menace.