Overview (as listed on page):
What is lead?
Burden of Disease
Sources of Exposures (current and overtime)
Vulnerable populations: Children
Exposure Doses and Health Effects
Clinical Symptoms and Diagnoses
Frequently Asked Questions
Resources/ Ongoing Initiatives
What is lead?
Lead is a naturally occurring heavy metal (bluish-grey in color, but tarnishes quite easily in the air into a darker gray color) that can be found in the Earth's crust, especially where volcanic activities and geochemical weathering occur. Within the environment, lead can be found in atmospheric suspended particles, water, soil and biota in general. It has a low melting point, is easily molded and shaped, and can be combined with other metals to form alloys. For these reasons, lead has been used by humans for millennia and is widespread today in a multitude of products. Human activities related to lead release into the environment include, but are not limited to, mining, smelting, refining and informal recycling of lead, use of leaded petrol, production and recycling of lead-acid batteries and paints, soldering, ceramics manufacturing, electronic waste, and lead use in (old) water pipes. Within the environment, lead can be found in atmospheric suspended particles, water, soil and biota in general.
Lead is listed as one of the 10 major chemicals of public health concern in the World Health Organization Chemical Safety Agenda; it imperative to note that the Joint Food and Agriculture Organization of the United Nations (FAO)/ World Health Organization (WHO) Expert Committee on Food Additives (JEFCA) concluded that a previously established tolerable intake level of 25 µg/kg body weight is no longer health protective, and it is not possible to establish a new provisional tolerable weekly intake.
Lead can be found in various forms:
Elemental lead/ lead ores: Lead ores comprise ~ 0.002% of the earth's crust. They include lead sulfide (mainly), lead sulfate, lead carbonate, lead chloroarsenate, and lead chlorophsophate. Upon isolation of lead from these naturally occurring ores, lead has an atomic weight of 207.2. a density of 11.34 g/cm3,and a melting point of 621.43 ?F (327.46 ?C).
Inorganic lead: Most inorganic forms of lead are white lead (a carbonate compound), yellow lead (Lead chromate, lead monoxide), or red lead (lead tetroxide). This is the form generally found in old paint, soil, dust, and various consumer products.
Organic lead: This form of lead is extremely dangerous — it can be absorbed through the skin and is much more toxic to the brain and central nervous system than inorganic lead. Tetra-ethyl lead is the form of lead used in leaded petrol (currently only still used in 9 countries); the combustion of organic lead results in the release of lead into the atmosphere.
*It is imperative to note that all forms of lead are toxic to humans as well as most other living organisms. There is no safe level of lead exposure/blood lead level.
Burden of Disease
DALYS, or disability-adjusted life years, are the metric used by the WHO to assess the global burden of disease; they are defined by the sum of years of life lost due to death and to disability as a result of a particular disease or condition — in this case: lead exposure. The total burden of disease attributable to lead amounts to about 9 million DALYs (disability-adjusted life years)! This DALY estimate stems primarily from mental retardation (measured by IQ); however, raised blood pressure (which also increases the risk of ischemic heart disease), stroke, hypertensive disease, as well as other cardiovascular diseases also contributed to this DALY estimate (WHO). Additionally, in 2004, it was estimated that lead exposure was responsible for 143,000 deaths and 0.6% of the global burden of disease. These estimates appear to be on the lower end of the spectrum, and since their publication, there has been considerable evidence indicating that these figures actually underestimate the burden of disease and costs attributable to low-level lead toxicity. This may be the case because in addition to the apparent IQ deficits and cardiovascular impediments accounted for in the DALY estimate, there are various subclinical neurotoxic health effects resulting from lead exposure. These effects mainly manifest in subtle intellectual impairments that may not result in a decrease large enough to warrant the "mental retardation" label; however, many individuals may not experience their full potential as a result of lead exposure (and may never come to know it). Due to this phenomenon, lead exposure/toxicity is not as high of a priority in terms of public perception (thus governmental intervention) in many countries — especially those that are low-income. Collectively, a population exposed to lead may depict a lower level of IQ than a population that is not.
Since the phase-out of leaded petrol, lead paint has been one of the largest sources of exposure to lead in children causing approximately 600, 000 new cases of intellectual disability among children every year. Blood lead levels vary widely from country to country and region to region, but continuously the highest blood lead levels (also attributable to the largest burden of disease) from lead exposures are found in low-income countries. In 2004, it was estimated that of the children with elevated blood lead levels (BLLs), approximately 90% came from low-income regions. While the numbers presented are undoubtedly alarming, it is imperative to note that cases across the globe have alluded that with proper regulation of leaded entities, BLL in the respective populations decreases; thus it can be deduced that negative neurotoxicological impacts are most likely also decreased in that population. An example of this phenomenon is the regulation of lead use in gasoline in the United States. Upon the implementation of regulations prohibiting leaded gasoline in 1974, the mean BLL of the US residents have dropped approximately 78%, from 12.8 µg/dl to 3.2 µg/dl (Pirkle, et al).
Sources of Exposures (current and historical)
Prior to human exploitation, people were not exposed to lead; however, once lead is introduced into the biosphere, it persists and bioaccumulates in the biosphere. Evidence that environmental lead levels have risen — and continue to rise — is depicted in geochemical data obtained from the accumulation of lead in the Greenland ice cap over the past three millennia. By far, the greatest increase in lead accumulation occurred in the 20th century and is attributed to the combustion of leaded petrol (which is deemed the largest contributor to global environmental lead contamination). Additionally, investigations of human skeletal remains indicate that the body burden of today's populations is 500-1000 times greater than that of their pre-industrial predecessors.
As previously mentioned, a multitude of sources and products can account for lead exposure, thus the toxic nature of lead has been apparent for quite some time. Dating back to 2000 BC, during the Roman times, lead was utilized in water pipes, earthenware containers and wine storage vessels, and the use of a leaded syrup, called sapa, to sweeten wine. Additionally, occupational exposures leading to lead poisoning have been reported since 370 BC and were quite apparent among industrial workers — especially those in the fields of smelting, painting, plumbing, printing — in the 19th and 20th century. Lead toxicity in children was first recognized as a pediatric disease in Australia over 100 years ago, and after extensive investigation, the cause of this 'disease' was deemed to be lead-based residential paint used on the children's homes.
Today, major sources of lead exposure include (but are not limited to):
- leaded petrol (car exhausts if gasoline is leaded release lead particles into the air)
- lead from an active industry, such as mining (especially in soils surrounding the area)
- lead-based paints and pigments; ceramic glazes
- lead solder in food cans; candy wrappers
- water pipes/ drinking-water systems (lead solder and lead pipes)
- leaded consumer products:
- traditional medicines/ folk remedies
- lead-containing waste; electronic waste
- historical contamination from former industrial sites
- food chain (via soil)
- lead-acid batteries (manufacturing and recycling)
Placenta: Prenatal Life
During embryonic/fetal development (and early postnatal life), lead can travel from an exposed mother into her offspring's body. Throughout the pregnancy (from the moment of conception), lead that has accumulated in a woman's bones is removed from her bones and passes freely from mother to child. Maternal and fetal blood lead levels are virtually identical; once in the fetal circulation, lead readily enters the developing brain through the immature blood-brain barrier thus inducing a multitude of detrimental health effects.
Ingestion is the primary pathway of exposure for lead, especially amongst children. Children's hand to mouth behavior is the most common cause of this phenomenon. Approximately 10-70% of ingested lead is absorbed by the body (~50% in children and ~10% for adults).
Inhalation is the second major pathway of lead exposure; however, unlike ingestion, almost all inhaled lead is absorbed into the body (making this exposure route a bit more serious). Lead can be found in particular matter 10 µm in diameter or smaller, thus traveling deep into the airways and increasing the exposure rate.
Dermal lead absorption is not found to be a significant route of exposure for the general public (although it may occur through contaminated cosmetics). Organic lead is most likely to be absorbed by the skin, and this is often only an issue amongst workers that come across lead in this form.
As of today, there is no safe level of human lead exposure, and no threshold level below which lead causes no injury to the developing human brain. Lead poisoning can potentially impact every organ system in the body and thus is associated with a wide range of toxic manifestations. It is particularly harmful due to its ability to persist and bioaccumulate in the body. In humans, the principal organs affected are the central and peripheral nervous system and the cardiovascular (brain and nerves), gastrointestinal (GI tract), renal (kidneys), endocrine (hormone secretion), immune, and hematological (blood) systems.
Vulnerable populations: Children
Children are at particularly high danger of lead toxicity (from conception till adult-life). Not only do they have the greatest risk of exposure, but their bodies are more susceptible. In comparison to adults, children have windows of vulnerability that have no adult-life counterparts, these include: embryonic and fetal development and early postnatal life.
Children's risk of exposure is much greater due to their tendencies to:
- eat more food, drink more water, and breathe more air per unit of body weight.
- curiously explore their surroundings and engage in detrimental hand-to-mouth behavior as well as pics (an abnormal extreme of hand-to-mouth behavior).
- spend more time in a single environment.
- have an increased risk (as compared to adults) of nutritional deficiencies that lead to a higher rate of lead absorption.
- have more future life years, therefore a longer time to develop delayed consequences of early exposures, potentially even including dementia.
- lack control over the circumstances of their environment.
They tend to be biologically more susceptible because:
- prenatally, any lead that may have accumulated in a woman's bones is removed from her bones and travels to her baby through blood; once in the fetal circulation, lead readily enters the developing brain (due to an immature blood-brain barrier) and causes developmental delays/impairments.
- a child's developing brain undergoes rapid growth, development, and differentiation; lead interferes with this complex, yet delicate process.
- early lead exposure can 're-program' genes, thus altered gene expression can occur and an associated increase in risk of disease is likely.
- Gastrointestinal absorption of lead is enhanced in childhood (50% as compared to 10% in adults) — in addition to the previously mentioned larger food intake per body weight
- their immune systems are still developing; if lead interference occurs, this can result in immune dysfunction later in life — adverse may be latent and not emerge until the immune system is stressed.
Exposure Doses and Health Effects
As previously stated, there is no safe level of human lead exposure, and no threshold level below which lead causes no injury to the developing human brain. Lead can impact all organ systems in the body. Below is a basic visual that highlights the blood lead levels (BLL) at which certain health deficits occur.
Motor axons in the peripheral nervous system are a major target of lead toxicity. Lead induces pathological alterations such as segmental demyelination and axonal degeneration. Extensor muscle palsy with wrist and ankle drop has been a well-known classic indicator of peripheral neurological lead toxicity — however, it is imperative to note that these symptoms only occur with chronic lead poisoning and are rare in acute lead exposure.
Lead toxicity in the central nervous system may be a bit more difficult to detect as it is generally causes asymptomatic impairment of neurobehavioral function in children. Early studies of the association between lead and intelligence quotient (IQ) reveal that clinically asymptomatic children with elevated BLLs experienced a four to five point deficit in mean verbal IQ scores compared with children from the same communities with lower BLLs. Today, on the basis of multiple studies in several countries, it is estimated that about a quarter to half of an IQ point is lost for each 1µg/dl increase in the BLL during preschool years for children who have BLLs in the range of 10-20 µg/dl.
Additionally, various studies reveal that children may never fully recover from high levels of lead exposures. Children who had previously suffered from lead poisoning were associated with subsequent negative behavioral outcomes including lower intellectual abilities, shorter attention spans, longer reaction times, and an increase in delinquent activity (WHO).
The classical manifestation of lead toxicity in red blood cells is anemia. The severity and prevalence of lead-induced anemia correlate directly with the BLL. Children (especially those who are younger and may be iron deficient) are at a higher risk of lead-induced anemia.
Lead causes proximal tubular injury in the kidneys that can progress to fibrosis. Additionally, lead accumulation in the kidneys can lead to hyperuricemia and gout. BLLs higher than 40 µg/dl are associated with nephropathy and renal failure, while individuals with diabetes and hypertension are at an increased risk of renal dysfunction at lower exposures to lead.
Long-term, high-dose exposure to lead has been associated with an increased risk of hypertension and stroke. More recently, several studies have found evidence that increased lead absorption, even at relatively low levels is also associated with elevation in blood pressure (across general populations, not occupational exposure).
The immune system is also adversely affected by relatively low levels of exposure to lead — that is, lower than 10 µg/dl.
Reproductive Function Deficits
The reproductive system is also adversely affected by relatively low levels of exposure to lead — that is, lower than 10 µg/dl. Reproductive deficits include damaged sperm and lower sperm count in males.
Clinical Symptoms and Diagnoses
Relatively low-dose exposure to lead can cause harmful effects not evident in a standard clinical examination; this is known as subclinical toxicity. Although not clinically obvious, the subclinical toxic effects of lead can be quite damaging. On the other hand, intense, acute, high-dose exposure to lead can cause symptomatic poisoning in children (acute clinical toxicity). Common symptoms of lead poisoning include (but are not limited to): gastrointestinal, neurological, and musculoskeletal difficulties. These symptoms often manifest in colic, constipation, fatigue, anemia, and neurological deficits. In the most severe cases, fatal acute encephalopathy with ataxia, coma, and convulsions can occur. Generally, children who survive acute lead poisoning have permanent and clinically apparent deficits in their neurodevelopmental function.
Physical examinations may reveal lead lines in teeth, gout (joints), and hypertension — to name a few. Additionally, laboratory inspections can expose elevated blood lead levels and/ or anemia. Radiological tests can further indicate the presence of high levels of lead in the body by displaying lead in long bones, thus indicating chronic lead poisoning.
Frequently Asked Questions
Q: If there is lead in my house, how do I protect my family?
A: Dry, undamaged paint is not a concern; during moves, demolition, or destruction to the wall surfaces, paint chips (depending on the size) may then be ingested or inhaled (especially by children). Heating or burning lead paints also generate toxic fumes.
Q: How much lead can cause lead poisoning in my child?
A: NO lead exposure is considered safe, even minimal levels can have a detrimental impact.
Q: Are there alternatives to leaded paint?
A: Yes, alternatives to lead in paint do exist and cleaner substitutes for lead-based pigments have been in use for some time now. However, there is still an immense lack of awareness on the issue of lead and an absence of mandatory standards for lead in paints.
Q: Many people have grown up around leaded gasoline and with lead painted homes, why the great concern now?
A: Moderate lead poisoning poses no visible symptoms; many individuals may have experienced subtle neurological damage, and thus do not reach their full potential due to earlier exposures. Also, as the age of leaded paint increase, the likelihood of it chipping, peeling, and chalking also increases. Young children growing up in their parents' older homes are at a greater threat there than their parents were.
**Lead toxicity is harmful to everyone, but has a much larger toll in children; health deficits include: impaired cognitive, motor, behavioral, and physical functions — SOME OF WHICH ARE PERMANENT AND IRREVERSIBLE.
**LEAD POISONING IS PREVENTABLE; prevention is much easier, less costly, and more effective than treatment.
5. Pirkle, et al. The decline in blood lead levels in the United States. The National Health and Nutrition Examination Surveys (NHANES). The Journal of the American Medical Association. 1994; 272(4):284-291.