Scientists Correlate Lead Exposure with Intelligence Summary

  • Last updated on November 10, 2022

Herbert L. Needleman reported in the New England Journal of Medicine that his research had shown a correlation between the intelligence and academic performance of children and the levels of lead found in their bodies.

Summary of Event

In the March, 1979, issue of the New England Journal of Medicine, Herbert L. Needleman published an article titled “Deficits in Psychologic and Classroom Performance of Children with Elevated Dentine Lead Levels,” in which he defined standards of exposure to lead and lead measurements in the body and correlated these with children’s behavior and academic performance. Scientists who are responsible for determining the risk factors associated with levels of exposure to particular substances often have difficulty correlating exposure concentration with functional alterations. They must define the threshold for adverse health effects and identify the organs most likely to experience early effects. The early effects are often unclear and could have a variety of causes. Prior to publication of Needleman’s paper, little was known about the concentrations of lead required to elicit physiological effects. Blood-level measurements do not necessarily indicate long-term storage-site levels, nor can they reveal how long an individual has been exposed. Lead;exposure effects "Deficits in Psychologic and Classroom Performance of Children with Elevated Lead Levels"[Deficits in Psychologic and Classroom Performance] (Needleman) [kw]Scientists Correlate Lead Exposure with Intelligence (Mar., 1979) [kw]Lead Exposure with Intelligence, Scientists Correlate (Mar., 1979) [kw]Intelligence, Scientists Correlate Lead Exposure with (Mar., 1979) Lead;exposure effects "Deficits in Psychologic and Classroom Performance of Children with Elevated Lead Levels"[Deficits in Psychologic and Classroom Performance] (Needleman) [g]North America;Mar., 1979: Scientists Correlate Lead Exposure with Intelligence[03510] [g]United States;Mar., 1979: Scientists Correlate Lead Exposure with Intelligence[03510] [c]Health and medicine;Mar., 1979: Scientists Correlate Lead Exposure with Intelligence[03510] [c]Environmental issues;Mar., 1979: Scientists Correlate Lead Exposure with Intelligence[03510] Needleman, Herbert L.

Needleman helped clarify the problem by defining the issues that need to be addressed. He pointed out that the particular organ that first shows a critical concentration of a metal being studied may be dependent on the route of exposure or on some aspect of the population subsegment being studied. Concentration of the metal in the organ can be defined as critical when undesirable functional changes, whether reversible or irreversible, first begin to appear. The critical effect can be defined as the point in the relationship between dose and effects at which the critical organ concentration is reached; critical effect may or may not play an important role in the health of the whole organism.

Needleman used these definitions to help address the issues of lead exposure. He believed that most previous lead studies suffered from four major problems. First, most researchers used blood lead, a relatively short-term marker of exposure, to classify subjects; blood returns to normal after exposure to lead has ceased and thus does not account for stored lead. Second, outcome measures were often insensitive and could be missed in screening tests. Third, the research had the problem of biased volunteer groups—that is, only individuals with significant exposure or high blood levels may have volunteered for study. Fourth, many other variables unidentified at the time of a study can affect the physiological and psychological effects of lead.

Needleman’s research group used deciduous teeth (first teeth or baby teeth) as the index of exposure. Dental tissue is a fixed storage site for lead, and studies of teeth can reveal exposure over time. The teeth were collected from first- and second-grade students from two towns in Massachusetts. A neuropsychological study was extended to the students with the highest measurements of tooth lead and those with the lowest. Prerequisites of participation in the study were that the children had not earlier been diagnosed as having had lead poisoning, that they were not low-birth-weight infants, and that their parents spoke English as a first language. All the children were rated by their teachers on an eleven-point behavioral scale that reflected such things as whether a child was easily distracted, could follow directions, or could work independently.

The researchers found significant differences between the low-lead and high-lead subjects with regard to certain parental variables. The high-lead subjects tended to have fathers of a lower socioeconomic status and mothers with significantly less education who tended to have been younger at the time of the child’s birth.

Children with high lead levels were shown to have lower full-scale intelligence quotients (IQs), lower verbal IQs, and lower performance IQs. They performed less well on sentence-repetition tests and tests of reaction time. Their teachers had rated these children negatively in all of the behavioral characteristics examined. High-lead subjects were more likely than low-lead subjects to be classified as distractible, dependent, disorganized, hyperactive, impulsive, and easily frustrated. They were also more likely to be classified as daydreamers, unable to follow simple directions or sequences.

Significance

Lead was discovered at least as far back as 3500 b.c.e. Because the soft metal was easy to handle and had so many desirable characteristics, it was widely used for crockery and cooking. The Romans used lead pipes to create their water-carrying systems, and lead vessels were used to boil grapes to make a syrup used as a preservative in wine. The toxic effects of wine prepared in lead vessels were recognized in the second century b.c.e. The Greeks and Romans were aware of the toxic effects of lead but continued to use lead vessels and to drink wine contaminated with lead.

The Industrial Revolution increased the demand for lead. Lead became an occupational hazard not only for those who worked in lead industries but also for their children. Female lead workers and the wives of lead workers had elevated levels of sterility, miscarriage, premature delivery, and stillbirths. Among the children who were normally delivered to lead workers, there was greater likelihood of infant mortality, retarded growth, and slow development. Eventually, laws were passed in most industrialized nations that prevented the employment of women in the lead industry.

In 1892, scientific investigators identified cases of lead poisoning in children that were not associated with parental exposure. Most of these children had been exposed to large quantities of lead and had severe neurological impairment. Scientists examining the source of lead in these children called attention to lead in house dust and in the lead paint used in the children’s homes. These observations were ignored until the 1970’s, however, and in spite of the recognition of lead as a toxic compound, little was done initially to prevent its use in the United States. Needleman’s research focused attention on the long-term irreversible effects of lead exposure, and subsequent efforts concentrated on prevention of exposure.

The major sources of environmental lead in the United States include hazardous-waste incinerators, lead smelters, furnaces that burn used motor oil, and battery manufacturing plants, which release lead particles into the atmosphere. Other sources of lead include interior paint in many older homes, groundwater contaminated by lead leached from landfills, drinking water contaminated by plumbing containing lead solder, imported dishes and cookware, and vegetables and fruits grown in soil contaminated by lead.

It was recognized that, because children are more vulnerable than adults to the harmful effects of lead exposure, it is important to limit the amounts of lead children might be exposed to in their homes. Although the Lead-Based Paint Poisoning Prevention Act Lead-Based Paint Poisoning Prevention Act (1971)[Lead Based Paint Poisoning Prevention Act] was passed in 1971, it was not until 1978 that paints containing lead were banned by the U.S. Consumer Product Safety Commission. Consumer Product Safety Commission;lead-containing paint[lead containing paint] A 1986 study by the Environmental Protection Agency (EPA) found that large numbers of children in the United States under the age of six had unsafe blood levels of lead and so were at risk for problems with their mental, physical, and emotional development. In 1986, the U.S. government revised its air-quality standards, lowering the amount of lead allowed in the air. It also banned the use of solder containing lead. The government later made concerted efforts to lower the amount of lead in the environment by requiring automobile manufacturers to produce cars that run on lead-free gasoline.

Although all these efforts proved successful in decreasing the amount of lead in the atmosphere and in homes across the United States, lead poisoning remained a major problem for children living in inner cities, where old homes that had not been painted for many years still exposed the children to lead-based paints. Attention to this problem achieved some progress in the last decade of the twentieth century, however. According to the Centers for Disease Control and Prevention, the proportion of American children with elevated lead levels in their blood declined from 4.4 percent in the early 1990’s to 1.6 percent in 2002. Lead;exposure effects "Deficits in Psychologic and Classroom Performance of Children with Elevated Lead Levels"[Deficits in Psychologic and Classroom Performance] (Needleman)

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Centers for Disease Control. Preventing Lead Poisoning in Young Children: A Statement by the Centers for Disease Control. Atlanta: Author, 1991. Government pamphlet provides thorough coverage of the topic. Includes information on sources of lead exposure, screening methods, and the roles of government agencies in reducing lead risks.
  • citation-type="booksimple"

    xlink:type="simple">Kumar, Vinay, Ramzi S. Cotran, and Stanley L. Robbins. Basic Pathology. 7th ed. Philadelphia: W. B. Saunders, 2005. College-level textbook includes a discussion of the pathological effects of lead. Gives a clear presentation of the subject.
  • citation-type="booksimple"

    xlink:type="simple">McCance, Kathryn L., and Sue E. Huether. Pathophysiology: The Biologic Basis for Disease in Adults and Children. 5th ed. St. Louis: C. V. Mosby, 2005. Comprehensive text includes discussion of the effects of lead on the nervous system. The schematic presentation of the effect of lead in children is particularly well presented.
  • citation-type="booksimple"

    xlink:type="simple">Needleman, Herbert L., ed. Human Lead Exposure. Boca Raton, Fla.: CRC Press, 1992. Collection of technical articles addresses the effects of continued human exposure to lead and the adverse effects of lead poisoning. Includes bibliographic references and index.
  • citation-type="booksimple"

    xlink:type="simple">Warren, Christian. Brush with Death: A Social History of Lead Poisoning. Baltimore: The Johns Hopkins University Press, 2000. Explores the social costs of lead exposure throughout U.S. history. Describes the shifts in attitudes and changes in technologies that came about after Needleman’s findings were published. Includes index.

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