Lead poisoning effects and health outcomes

Lead Poisoning

The History and Impact of Lead Poisoning on Children and Adults

First get the child out of the lead, then get the lead out of the child. -- John W. Graef, M.D., Chief Emeritus, Lead and Toxicity Program, Children's Hospital, Boston, 1997

Department of Health and Human Services Centers for Disease Control reports that lead poisoning represents one of the most common and preventable pediatric health problems in the United States today. While the problem has existed for hundreds of years, researchers are just beginning to understand the actual implications of lead poisoning on the health and well-being of children and adults. In this regard, this paper provides a review of the scholarly and peer-reviewed literature to identify the history of lead poisoning and its various applications, the incidence of the problem today, followed by a discussion of the epidemiology of lead poisoning and the pathophysiological and biochemical basis of lead poisoning in children and adults. Recommendations for preventing lead poisoning in the United States and abroad are followed by a summary of the research and salient conclusions.

Review and Discussion

History of Lead Poisoning.

While lead has been used for various commercial applications for hundreds of years (Miksic, Yap, & Younan, 1994), the dangers associated with its use have only been identified fairly recently. According to Kessel and O'Connor (1997), the average level of lead in children's blood is lower today than it was before lead paint was prohibited from use in homes 25 years ago, and the phasing out of lead in gasoline; nevertheless, the problem of lead poisoning has been the focus of an increasing amount of research because it has been determined that lower and lower blood levels of lead can still adversely affect children's health, particularly the development of the brain. In addition, Kessel and O'Connor report that (1997), "The major cause of lead poisoning is lead-based paint. A child does not, however, have to eat paint chips to become lead poisoned. Any time lead-based paint is disrupted there is a potential for lead poisoning -- whether the paint is rubbed against, sanded or scraped, chewed on, or simply falling off" (p. 3). "Over the last two decades, atmospheric concentrations of lead have decreased significantly around the globe as more and more nations have chosen to remove tetraethylead from gasoline. However, humans may also be exposed to Pb through contaminated food, water, and house dust and through industrial activities such as metal recycling and the battery industry" (Barbosa, Gerlach, Parsons & Tanus-Santos, 2005, p. 1669). Furthermore, as Ryan, Huet, and MacIntosh (2000) emphasize, human activities have significantly changed the natural distribution of lead in the environment, resulting in potentially elevated concentrations of this metal in a number of environmental media; not surprisingly, the occurrence of lead in drinking water is regarded as another important pathway for potential exposure for citizens of the United States and many other countries today (Ryan et al., 2000).

Notwithstanding the costs in human terms, the economic consequences of lead poisoning are staggering. According to Fahs, Landrigan, Lipton, Schechter and Schwartz (2002), "Total annual costs are estimated to be $43.4 billion for lead poisoning. This estimate is likely low because it considers only four categories of illness, incorporates conservative assumptions, ignores costs of pain suffering, and does not include late complications for which etiologic associations are poorly quantified" (p. 721). Furthermore, the costs associated with lead poisoning remain inordinately disproportionate compared to the limited amount of resources devoted to the research, tracking, and prevention of lead poisoning today (Fahs, 2002).

Incidence of the Problem.

According to Dugbatey, Evans, Lienhop and Stelzer (1995), "Childhood lead poisoning is one of the most common preventable pediatric problems in the United States today" (p. 6). In spite of its preventable quality, the consequences of lead poisoning can be severe and even life-threatening and there are millions of children at risk all over the world today, including in the United States. In this regard, Kessel and O'Connor (1997) report that, "Each year, more than one-and-a-half million children have elevated blood lead levels in this country. In most cases of lead poisoning there are no obvious symptoms to alert a parent. Yet these children are at risk for behavioral problems, learning disabilities, and reduced IQ because of exposure to this invisible toxin, which is all around us" (p. 3). Likewise, the U.S. Department of Housing and Urban Development estimates that 3.8 million homes inhabited by children continue to have high levels of lead in dust and lead-based paint in poor condition (Croskey et al., 2005).

While there has been some recent progress made in preventing lead poisoning, lead-based paint that has been improperly managed in older homes remains the greatest source of exposure for children in the United States today (Goldman, 1997). The U.S. Secretary of the Department of Health and Human Services recently characterized lead poisoning as being the leading environmental threat to the health of nation's children; a survey in 1992 estimated that more than three million tons of lead in the form of lead-based paints remained a potential threat in American dwellings built prior to 1980 (Goldman, 1997).

Epidemiology of Lead Poisoning.

Today, researchers are well versed with the epidemiology of lead poisoning, although some recent findings have changed some long-held views about its etology. For instance, researchers have determined that when lead enters the body, the substance can be insinuated into the human body via a number of pathways depending on its source and, by extension, its bioavailability (Barbosa et al., 2005). In their study, "Lessons from a Primary-Prevention Program for Lead Poisoning among Inner-City Children," Croskey and her colleagues report that, "From lead-based paint, indoor dust, soil, and probably contaminated water pipes, children living in poor areas are exposed to lead through ingestion, inhalation, or both. Lead-based paint continues to be a problem in older homes and therefore is the major contributor to lead poisoning through ingestion of paint chips or ingestion of dust and soil contaminated with lead paint" (p. 15).

The percentage of lead that is absorbed by the human body primarily depends on the physical and chemical form, particularly particle size and the solubility, of the specific lead-based compound involved. Likewise, some other important factors are specific to the exposed subject, including age, sex, nutritional status and, to a lesser extent, genetic background (Barbosa et al., 2005). One of the earliest toxicokinetics studies reported that once absorbed into the blood compartment, lead has a mean biological half-life of about 40 days in adult males; the half-life in children and in pregnant women has been reported to be longer, because of bone remodeling but another study was unable to confirm this finding (Barbosa et al., 2005).

Similar to a number of other so-called "bone-seeking" elements, lead from blood is incorporated into calcified tissues such as bone and teeth where it is capable of remaining for years. Citing research by Rabinowitz (1991), Barbosa and his colleagues report that the half-life of Pb in bone (bone-Pb) ranges from 10 to 30 years, but the use of the term "half-life" to describe the biological clearance of Pb from bone implicitly makes assumptions about the kinetics of the process by which Pb is released. "Some researchers prefer to use the term 'residence time' to avoid implying more precision than what can be directly determined. From calcified tissue stores, Pb is slowly released, depending on bone turnover rates, which in turn are a function of the type of bone, whether compact (slow turnover) or trabecular (rapid turnover)" (Barbosa et al., 2005).

The studies to date have found that the release rate of Pb from bone varies with age and intensity of exposure, and the transfer of Pb from blood to other compartments determined to be much faster than previous estimates have suggested with the overall clearance rate from blood (sum of rates from blood to cortical bone, to trabecular bone and to other tissue), implying a half-life of 10-12 days (Brito et al. 2005). These latest findings emphasize the difference between the overall clearance viewed from outside, when no allowance can be made for recirculation, and the actual transfer rates identified by researchers conducted thus far (Barbosa et al., 2005).

Pathophysiological and Biochemical Basis of Lead Poisoning in Children and Adults.

The symptoms of lead poisoning are difficult to identify and there is the potential for neurological damage without any discernable symptoms; the only accurate method of determining whether a child has been lead poisoned is through a blood lead level screening (Kessel & O'Connor, 1997). "Because a child is at greatest risk of lead poisoning between the ages of 12 and 36 months, this is the most critical time to screen your child for lead" (Kessel & O'Connor, 1997, p. 33). Even though lead poisoning can be seriously harmful, lead poisoning frequently remains undetected because there are no obvious symptoms involved, a feature that has caused some to refer to lead poisoning as the "silent disease" (Kessel & O'Connor, 1997). Even fairly severe cases show symptoms that are difficult to identify because they are the same for many other diseases (i.e., irritability, dizziness, stomach pains, constipation, vomiting, muscle weakness, and lack of appetite) (Kessel & O'Connor, 1997).

The results of a study by Dolinoy and her colleagues notes that healthcare professionals have long recognized the threats represented by severe lead poisoning; however, since the late 1970s, there has been a growing body of research that indicates that lead also causes asymptomatic effects at levels far below thresholds previously considered safe. According to this authors, "The adverse effects of lead, including learning and behavioral disorders (e.g., attention deficit disorder and attention deficit hyperactivity disorder), hearing impairment, decreased intelligence quotient, and decreased attention span, are particularly harmful in children and often become apparent during puberty -- long after exposure has caused irreversible impacts" (Dolinoy et al., 2002, p. 947). In response, the Centers for Disease Control and Prevention have incrementally lowered the threshold for lead levels regarded as dangerous in children by fully 88% (from 60 to 10 [micro]g/dL) over the past 40 years; in addition, more recent investigations have found that cognitive deficits may occur at blood lead levels as low as 5 [micro]g/dL (Dolinoy et al., 2002).

Generally speaking, young people that have been lead poisoned to not present with any specific set of symptoms that identify a child as lead poisoned; possible neuropsychological symptoms vary from individual to individual, but the change in intelligence appears to be fairly consistent throughout a number of studies (Kessel & O'Connor, 1997). According to Barbosa, Tanus-Santos, Gerlach, and Parsons (2005), lead concentration in whole blood (BPb) is the most popular biomarker used to monitor lead exposure. For this purpose, the U.S. Centers for Disease Control and Prevention and the World Health Organization both define a BPb of 10 [micro]g/dL (0.48 [micro]mol/L) as being the threshold of concern for lead poisoning among young children; recent studies, though, have identified the potential for adverse health effects, including intellectual impairment in young children, at BPb levels < 10 [micro]g/dL, indicating that there is no safe level of exposure (Barbosa et al., 2005).

Likewise, a study by Fahs and his associates (2002) suggests that there is no safe level of exposure to lead. These researchers determined that the mean blood level of lead in the birth cohort of children age 5 years was reported in 1997 to be 2.7 [micro] g/dL; at that time, the estimated number of 5-year-old children in the United States was 1,960,200 boys and 1,869,800 girls (Fahs et al., 2002). According to these authors, "At this age, there is no significant difference between boys and girls in blood lead level. Application here of an IQ reduction of 0.25 IQ points/[micro] g/dL assumes implicitly that there is no threshold blood lead level below which cognitive effects are not seen" (Fahs et al., 2002, p. 721). The authors conclude that this assumption appears reasonable because cognitive deficits have been associated with all ranges of blood lead concentration studied, and no evidence of a threshold has been identified to date (Fahs et al., 2002).

In their study, Associations among Lead Dose Biomarkers, Uric Acid and Renal Function in Korean Lead Workers," Ahn and his colleagues report that gout has been common among lead-poisoned individuals; in recent years, though, links between various measures of lead dose and serum uric acid (urate) levels have also been reported in studies of occupationally exposed populations as well as in general population studies (Ahn et al., 2005). According to these authors, "These associations are present at much lower lead doses than those associated with gout in historical lead poisoning. Lead exposure also increases the risk for adverse renal outcomes. Lead has been reported to cause nephrotoxicity by several mechanisms, although it is not known which of these is the predominant pathway" (p. 36).

Recommendations to Eliminate Lead Poisoning as a Public Health Hazard.

By any measure, complex problems require complex solutions and the elimination of sources of lead poisoning as a public health hazard at home and abroad is no different. According to Dolinoy, Miranda, and Overstreet (2002), "Environmental threats to children's health -- especially low-level lead exposure -- are complex and multifaceted; consequently, mitigation of these threats has proven costly and insufficient and has produced economic and racial disparities in exposure among populations" (p. 947). Fortunately, a virtual consensus emerged from the literature review that suggests community education initiatives represent one of the best front-line approaches, particularly in developing nations although the same types of programs have proven effective in urban U.S. settings as well. For example, "Providing increased awareness is a tool to fight lead poisoning" (Kessel & O'Connor, 1997, p. 2). To this end, in response to the growing need for more public education, the Environmental Protection Agency and the Department of Housing and Urban Development jointly promulgated a regulation in March 1996 that provided for disclosure of possible hazards in lead-based paint at the time when any home is sold or rented in the United States; the rule implemented section 1018 of the Residential Lead-Based Paint Hazard Reduction Act of 1992, which was designed to protect families from exposure to lead from paint, dust and soil (Goldman, 1997).

In fact, while most researchers routinely call for the need for additional studies in a given area, some experts are suggesting that although additional studies are needed concerning the specific operation of lead in the human body, the scientific community already knows everything that needs to be known to eliminate this threat right now: "The knowledge that currently exists in the scientific community about the sources and pathways of lead exposure and about possible ways of preventing the ensuing poisoning is sufficient to facilitate the development of programs that should permanently eradicate this disease" (Dugbatey et al., 1995, p. 7). The authors caution, though, that because the sources and pathways of childhood lead poisoning are attributable to the constellation of community and individual behaviors and activities, a well-coordinated and multi-sectoral program planning for such interventions is imperative (Dugbatey et al., 1995). Furthermore, according to Croskey and her associates, there is an abundance of valuable educational materials for use in addressing the dangers associated with lead poisoning: "A host of traditional educational materials are available in the form of publications, videos, slides, and mass media endeavors. The key problem seems to be that the people for whom these materials are intended do not use them effectively" (Croskey et al., 2005, p. 15).