Lead Poisoning: A Review
IMAGE: CC BY-SA 3.0. “Lead warning on a gas pump at Keeler’s Korner, Lynnwood, Washington. Keeler’s Korner, a former grocery store and gas station (built 1927) listed on the National Register of Historic Places, listed 1982, NRHP listing #82004287.”
The crisis in Flint, Michigan, has brought lead poisoning to the forefront of North American public health discourse. However, as one of the oldest hazards known to public health, lead has been studied extensively, and numerous policies have been implemented to reduce exposure to lead. Lead has been implicated in many adverse health outcomes including neurodevelopmental effects (O’Halloran & Spickett, 1992), anemia and kidney diseases (Goyer, 1990). The damage can be permanent in young children, and it causes a variety of learning disabilities and cognitive problems depending on the level of exposure (O’Halloran & Spickett, 1992, CDC 2005).
The objective of lead reduction programs and policies has been to reduce human contact with the sources of lead. These interventions depend on how the population of interest has been conceptualized. For instance, programs that focus on reducing exposures for young children who are being exposed to lead or those that attempt to monitor products that may contain lead represents two different approaches to public health interventions. This article reviews the epidemiology of lead exposure and discusses the conceptualizations that have led to the several public health policies and programs for lead reduction.
Background
Lead was a ubiquitous metal during the industrial revolution until the 21st century (Candelone, Hong, Pellone, & Boutron, 1995; Hernberg, 2000). While it was known from early observations that high levels of exposure to lead results in lead poisoning, the effects of sub-clinical exposure (i.e. without showing symptoms) to lead were only researched in the last 50 years (DC. Bellinger & Bellinger, 2006). Currently, the Centre for Disease Control and Prevention (CDC) recommends medical intervention for blood lead level above 10 ug/dL in young children; however, there is evidence of harmful effects of lead at lower blood levels,
Lead can be absorbed through inhalation, oral, and dermal exposure. Historically, the use of leaded gasoline resulted in exposure through inhalation. With the phasing out of leaded gasoline, other sources, such as drinking water, food, and products have become the major contributors to lead exposure in Canada (Health Canada, 2013). For young children, due to their interaction with the environment and their frequent hand and mouth contact, the exposure could come through non-food items, such as lead paints and contaminated dirt or dust (Health Canada, 2013; Horton, Mortensen, Iossifova, Wald, & Burgess, 2013). In Canada and the US, the children likely to be at risk of lead poisoning live in old houses with lead paint, and these homes may require renovation (CDC, 2012b; Health Canada, 2013). Moreover, industrial activities are still significant sources of lead exposure throughout the world (Horton et al., 2013).
The epidemiology of lead poisoning reflects a distribution shaped by socio-economic status and marginalized identities, reflecting the history of environmental racism in policies (for a review of the history of environmental racism in the US see Mohai, Pellow, Roberts 2009). In Canada, since 1971, the blood lead levels have decreased by 70% (Health Canada, 2013). However, low income, immigrant, and racial groups who reside in old housing are at greater risk of lead poisoning and have higher blood lead levels compared to other Canadians (Health Canada, 2013). A similar pattern of skewed distribution towards marginalized groups is visible in the US. (DC. Bellinger & Bellinger, 2006; Hore et al. 2016). Unfortunately, there are no nationally representative surveys of lead concentrations based on gender, ethnicity, and immigration status in Canada to describe the extent of health inequities between different groups (Chakravartty, Wiseman, Cole 2014).
Globally, the burden of lead poisoning persists, and it accounts for 0.6% of the global burden of disease (WHO, 2010). The 2010 case of lead poisoning in northern Nigeria illustrates that severe lead poisoning is still a global issue, and public health interventions are needed. According to Medicine Sans Frontier (MSF, 2012), unsafe mining practices resulted in numerous cases of lead poisoning. The average blood lead level was 127 ug/dL, contributing to the mortality rate of 43% in one of the villages (MSF, 2012). This case highlights the need for strong public health action in all countries and global regulations on lead-containing products and industrial uses.
Target for Interventions
The primary goal of public health interventions about lead exposure is to remove it from the human environment. Policies and programs in public health have been designed to achieve this aim in numerous ways. For example, policies on regulating mining and smelting, banning of lead in gasoline, paint, cans, pipes, and food and other products were designed to protect the whole population from exposure to lead (Campbell & Osterhoudt, 2000). Other public health interventions focus on specific high-risk groups. High-risk groups can be conceptualized in 2 different ways. The high-risk classification can either be understood as age groups that are more at risk of exposure and harmful outcomes of lead poisoning or as individuals having blood lead concentration above a certain threshold that is deemed dangerous. Young children below the age of 5 are considered a high-risk group and are often the target for interventions. In the US, the CDC also recommends children of families who are the recipient of Medicaid to be tested for lead, a narrow definition representing low-income families – although less than 25% of the eligible children receive the test (Campbell & Osterhoudt, 2000; CDC, 2005). For secondary prevention interventions, the high-risk group is often defined as children whose blood lead levels are above a certain threshold. While both these conceptualizations of the high-risk groups followed a set of justifications, they have disadvantages associated with them.
The conceptualization of high-risk as above-a-certain-threshold of blood lead levels has been the dominant public health approach. Interestingly, the threshold changed dramatically as the understanding of the effects of lead improved. According to Bellinger & Bellinger (2006), the threshold for blood lead levels – described as the level of concern – was at 60ug/dL in 1960. This was changed to 40 ug/dL in 1971, 30 ug/dL in 1975, 25 ug/dL in 1985, and for now settling at 10 ug/dL since 1991. However, no concentration of lead seems to be safe (for evidence of harmful effects of lead in levels as low as 1ug/dL see David C Bellinger, 2008; CDC, 2012b; Health Canada, 2013; WHO, 2010). In the case of Flint, Michigan, the recent study on lead exposure in children defined elevated blood lead levels as results above 5 ug/dL (Hanna-Attisha et al. 2016). Bellinger & Bellinger (2015) note that from blood lead levels below 10ug/dL to 150ug/dL there is a range of health effects associated with lead exposure. The historical trend of changing this threshold demonstrate that focusing on only the high-risk, conceptualized as blood lead levels above a certain threshold, will not result in eliminating lead from the environment and a population health benefit. This conceptualization of high-risk groups can be attributed to pressures from the industries that produce lead and marginalization of affected communities (DC. Bellinger & Bellinger, 2006).
The second conceptualization of high-risk is to focus on specific age groups. The reason to target young children is due to a higher rate of gastrointestinal absorption and less effective renal excretion (Health Canada, 2013). Children also eat non-food items, such as dirt or chipped paint, that can be contaminated with lead (Health Canada, 2013). For these reasons, children are more likely to accumulate lead compared to adults. Furthermore, the bulk of literature on the harmful effects of lead poisoning pertains to the detrimental effects on neurodevelopmental (Health Canada, 2013). The neurological effects are particularly damaging to children, and the consequences are considered to be irreversible, pointing to the need to focus on young children (CDC, 2005, 2012b; Health Canada, 2013).
One of the challenges of focusing on children is the transfer of lead from mother to child (O’Halloran & Spickett, 1992). Due to the mobilization of lead from the bone, the mother can transfer lead to the fetus or later on to the infant via breast milk. Therefore, focusing on only children and removing lead from their environments may not minimize the harm from lead exposure. Furthermore, blood lead concentration does not only have neurodevelopmental effects. Cardiovascular, renal, neurodegenerative, and reproductive health conditions are associated with higher levels of blood lead (Health Canada, 2013). These other health effects can be particularly damaging in susceptible populations, such as those with kidney diseases, hypertension, or diabetes (Health Canada, 2013). Therefore, attention on the removal of lead hazards from the environment can also benefit harmful effects of lead later in life.
Interventions
The public health interventions for lead exposure are well researched in the past 50 years. Interventions exist for primary, secondary, and tertiary prevention, targeting the whole population and high-risk groups. First, for primary prevention, limiting known sources of exposure is well understood and during the past 50 years, policies have been implemented in many countries. However, not all countries have universally adopted these measures; for example, while harmful effects of leaded gasoline were known since early 20th century, the US only started phasing out leaded gasoline in 1977 (DC. Bellinger & Bellinger, 2006). According to the data from National Health and Nutrition Examination Survey (NHANES II), the blood concentration of lead and phasing out of gasoline are associated with one another, demonstrating the importance of policies that remove lead from the environment (Needleman, 2004).
As discussed in Campbell & Osterhoudt (2000) following the implementation of the policies, primary prevention can focus on enforcement as well as remove lead from the environment. For example, removal of lead paints from old homes and identifying homes in need of such services can be part of these strategies (Rabito, White, & Shorter, 2004). Since this approach can be costly, secondary prevention can focus on households that have high blood lead levels in children (Campbell & Osterhoudt, 2000). However, this secondary approach is criticized due to the irreversible impact of early lead exposure on the children (DC. Bellinger & Bellinger, 2006; CDC, 2012b). Other examples include removal of lead from drinking water (CDC, 2012a), nutritional intervention to improve iron and calcium levels in the body to reduce uptake and enhance removal of lead (Campbell & Osterhoudt, 2000), public health educational campaigns, and dust removal (Campbell & Osterhoudt, 2000; Yeoh et al., 2014).
Second, as described above, the secondary prevention strategies are criticized for not adequately preventing irreversible damage to children. Specifically, these interventions differentiate from primary prevention through the use of a screening procedure (Campbell & Osterhoudt, 2000). Blood lead screening is focused on groups that are defined as high-risk. In this conceptualization of high-risk, the children are defined as those above a certain threshold (>10 ug/dL since 1992 (CDC, 2005)) who live in old housing, and have a certain behavioral history or a history of lead exposure in their household (Campbell & Osterhoudt, 2000). Following identification, the interventions are often education, dust removal, cleaning, or lead abatement strategies (Campbell & Osterhoudt, 2000; Yeoh et al., 2014). However, according to a recent meta-analysis of randomized control trials on education and dust removal interventions, it was concluded that these interventions are not effective as lead prevention strategies (Yeoh et al., 2014). Lastly, tertiary prevention is concerns individuals with elevated blood lead levels. The current recommendation by the CDC for blood levels above 45 ug/dL is the provision of chelation therapy, but its efficacy in reversing the harmful effects of lead poisoning is not clear (Rogan et al., 2001).
While each of these strategies may be appropriate for a certain population, the context of lead exposure is important to consider. As demonstrated by the current body of evidence, policies banning specific types of lead products have shifted the distribution of blood lead concentrations to lower levels. Therefore, if countries lack such policies, focus on these broad policies that need to be prioritized to shift the distribution. The challenge with these population-based policies is in phasing out the previously used products, i.e. changing the lead pipes, removing lead paint, etc. This part of primary prevention requires funding and resources making the programs harder to conduct. However, as demonstrated by Rabito et al. (2004), the case for cost-effectiveness of undertaking such initiatives can be made. As discussed in the conceptualization of high-risk groups, the benefit of interventions with screening processes to find high-risk individuals can be limited in scope. The case for population approach, as opposed to screening, can be argued based on the broad range of health consequences that exposure to lead can have on individuals, leading to high costs on the health care system. Screening while valuable in generating evidence; needs to be used in conjunction with population-based interventions. Furthermore, lack of effectiveness of individual or family-oriented interventions such as education and dust cleaning emphasizes the need for primary prevention on a population level to reduce the burden of lead poisoning.
Conclusion
With the industrial revolution and increased in resource extraction, the use of lead in different products increased followed suit, leading to the high concentration of lead in the environment and consequently in humans (Candelone et al., 1995). In the past century, many public health interventions have been implemented in different countries and considerably reduced the blood lead levels in humans. The most successful policies recognize that the impact of lead is spectral; the ensuing harmful effects exist at virtually all concentrations in humans and as such, there is the need for policies and interventions that will lower blood lead levels among the whole population. The case of Flint demonstrates the need for officials in municipalities and public health departments to be vigilant about the sources of environmental hazards and maintain the infrastructure required to curb the impact of these chemicals.
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