Low Level Lead Exposure Subclinical Effects
New Research shows no threshold

The blood lead levels of children in most developed and many developing nations have declined.  For instance, the mean blood lead level among children in the United States according to data collected from 1999-2002 among children aged 1-5 years was 1.9 ?/I>g/dL compared to 15 ug/dL among children in the 1976-1980 survey{（429 Centers for Disease Control and Prevention (CDC). 2005;430 Pirkle,J.L. 1994;  ）}.  At these levels of lead exposure subclinical effects on the central nervous system (CNS) are the most common effects and represent an important public health issue. [NG1] The best-studied effect is cognitive impairment, measured by IQ tests. The strength of this association and its time course have been observed to be similar in multiple studies in several countries{{434 Schwartz,J. 1994; }}.  From review of several studies, it is estimated that about one quarter to one half of an 揑Q?point is lost for each 1 mcg/dL increase in the BLL over measurements during preschool years(Shwartz, 1994; Pocock et al., 1994).  Canfield et al recently extended the relationship between blood lead concentration and IQ to blood lead concentrations less than 10 ug/dL{{43 Canfield,R.L. 2003; }}. They observed a decrease in IQ of more than 7 points over the first 10 ug/dL of lifetime average blood lead concentration and the slope of this curve was greatest for a given change in exposure at the lowest blood lead levels.[NG2]   An international pooled analysis of data from 7 cohorts confirmed these findings (See Figure 2) {（247 Lanphear, Bruce 2004; ）}.  At the moment, however, these data have not yet been incorporated into policy, and the CDC, WHO and AAP currently use 10 ug/dL as the blood lead concentration of concern{（420 American Academy of Pediatrics Committee on Environmental Health. 2005; 435 Centers for Disease Control and Prevention 2002; NEED WHO REFERENCE ）}.

The effect is persistent

Debate continues over the nature, magnitude and persistence of the adverse effects on human health of low-level exposure to environmental lead.  However, the accumulated epidemiological evidence indicates that such exposure in early childhood causes a discernible deficit in cognitive development during the immediately ensuing childhood years (Tong et al. 2000).  Several studies have demonstrated an inverse relationship between early childhood exposure to lead and performance on tests of cognitive function and neurobehavior 10, 15 and 20 years after the blood lead levels were measured (Amler, 2001; Williamson 2003).  Early exposures have also been linked to juvenile delinquency, hyperactivity, inattentiveness, and increased rates of failure to graduate high school (Needleman et al, 1994; Sciarillo et al, 1992; Needleman; 1990; Needleman, 1996, Dietrich et al., 2001).  

Certain environments can be protective.

Whether early interventions to reduce lead in the body is capable of reversing its effects on the central nervous system has received limited attention.  Analysis of the cognitive testing of children followed in some of the large cohorts, do demonstrate improved scores with declining blood lead levels (Ruff et al, 1993; Tong et al, 1998; Liu et al, 2002).  However, there is some evidence from a sophisticated animal study which demonstrates that intellectually stimulating environments not only overcome the adverse effects of lead but are in fact protective against the damage lead causes at the cellular level (Schneider et al,2003 ).  This finding is buttressed by a reanalysis of data from the Boston cohort of lead exposed children in the author describes socioeconomic status as an effect modifier, as opposed to a confounder (Bellinger et al, 2005).  In other words, children who are more likely to develop in an intellectually nourishing environment  are protected from some of the impacts that lead has on intelligence.  

Other CNS effects

Other aspects of brain or nerve function, especially behavior, also may be affected. Teachers reported that students with elevated tooth lead concentrations were more inattentive, hyperactive, disorganized, and less able to follow directions (Needleman et al, 1979; Sciarillo et al; 1992).  Additional follow-up of some of those children showed higher rates of failure to graduate from high school, reading disabilities, and greater absenteeism in the final year of high school (Needleman et al., 1990).  Elevated bone lead concentrations are associated with increased attentional dysfunction, aggression, and delinquency (Needleman et al., 1996).  In children followed from infancy with blood lead measurements, self-reported delinquent behavior at 15 to 17 years of age increased with both prenatal and postnatal lead exposure (Dietrich et al., 2001), and bone lead, thought to represent cumulative dose, is higher in adjudicated delinquents (Needleman et al., 2002).  These data imply that the effects of lead exposure are long lasting and perhaps permanent. Subclinical effects on both hearing (Schwartz and Otto, 1991) and balance (Bhattacharya et al., 1990) may occur at commonly encountered blood lead concentrations(AAP, 2005).