FCEHA RESEARCH PROGRAM #1 (METAL TOXICITY)

Topic: Childhood Exposure To Lead

1) Introduction – Lead is highly toxic, especially to children under 2 years of age. In the second year of life, the still unfinished maturation of the brain coincides with peak blood-lead concentration as children explore their environments with increasing ambulatory and oral intensity. Lead not only diminishes intellectual capacity, but it also causes loss of hearing, reduces hand-eye coordination, impairs the ability to pay attention, and creates a propensity toward violence, diminishes the ability to handle stress, and may lead to violent outbursts. In addition, lead can harm a child’s kidneys, bone marrow, and other body systems. At higher levels, lead can cause coma, convulsions, and death.

2) Biochemistry - The diversity of these mechanisms is such that virtually all cells are affected by this metal. Lead has a high affinity for many calcium-binding proteins (e.g., calmodulin, troponin). Once bound to such molecules, it may activate or inhibit processes that are normally calcium-mediated, thus disrupting normal intracellular calcium-relayed messages. Lead may also affect neurotransmitter release, distorting intercellular communication. Lead can adversely affect enzyme function. Lead has an affinity for sulfhydryl and amide groups, and it can bind to enzymes that contain those groups, altering their configuration and diminishing their activities. Many of these changes occur at extremely low concentrations of lead. The relationship between lead and enzymes of the heme pathway, which is found in all cells, has been well studied. At least three of the seven enzymes in this pathway are poisoned by lead, resulting in diminished production of heme and in the buildup of substrates that may be toxic when present in excessive amounts.

3) Subclinical Effects - Perhaps the most critical subclinical effects of lead poisoning are observed in the cognition and behavior of children. The effect on cognition is most pronounced when exposures occur in utero or in the first years of life. Lead also affects behavior in school. Teachers reported that students with elevated tooth lead levels were more inattentative, hyperactive, disorganized, and less able to follow directions than those without elevated levels of lead. In epidemiological studies of large populations, elevated lead levels have been found to be associated with decreased growth, decreased hearing acuity, and elevated blood pressure. Most recently, studies worldwide demonstrated that some asymptomatic children elevated bone lead concentrations were reported to be associated with lower IQ scores, more language difficulties, behavior disorders, increased attentional dysfunction, aggression, and delinquency.

4) Clinical Effects - Children with extremely high concentrations of lead in their blood may complain of headaches, abdominal pain, loss of appetite, constipation, and display clumsiness, agitation, or decreased activity and somnolence. These are premonitory symptoms of central nervous system involvement and may rapidly proceed to vomiting, stupor, and convulsions. Because symptoms may be nonspecific (malaise or irritability) or nonexistent at lesser concentrations of blood lead, the diagnosis of lead intoxication in these children depends on the measurements of blood lead.

5) Exposure to Lead – With the removal of lead from gasoline beginning in the mid-1970s, airborne lead concentrations dropped, and a reduction in the average blood lead levels followed. Sources of exposure include lead-based paint and industrial sites and smelters that use or produce lead-containing materials. Lead-contaminated dust, soil, and water; lead-containing materials used in parental occupations or hobbies; and lead-containing ceramic ware and traditional remedies all contribute to childhood lead exposure. Lead-contaminated house dust, ingested in the course of normal hand-to-mouth activity, is of major significance. House dust is most often contaminated by lead-based paint in the home, when such paint is peeling, deteriorating, or scattered about during home renovation or preparation of painted surfaces for repainting. Although use of lead in paint intended for household use has been almost eliminated since 1977, many houses continue to have surfaces that contain lead. Approximately 70% of the houses built before 1980 contain some lead-based paint. There are about 2 to 3 million housing units in the United States built before 1950 in which children live with deteriorated lead paint. These children are at the highest risk for lead poisoning. According to the Alliance to End Childhood Lead Poisoning (AECLP), Miami-Dade County had 10 times more homes – 15,000 – with a high risk of lead exposure than Broward County, and 3 times more than any other Florida county. Alliance’s calculations are based on the number of homes in each county built before 1950 – the period when lead paint was widely used. A recent pilot study showed that more than half of the houses in Miami, particularly in Liberty City, Little Haiti and Little Havana exhibited unacceptably high levels of lead dust and soil in areas where children live and play. We tend to forget that soil contaminated with lead is the legacy of decades of use of lead additives in gasoline (for more than 70 years) and paints (for more than 80 years). There is a common perception that lead-based paints alone account for the amount of lead in the environment. In reality, both sources of lead contribute to the problem as just revealed by the results of this study in Miami.

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