Letters

Environmental lead and children's intelligence

EDITOR,--Stuart J Pocock has always been profoundly sceptical of a causal relation between lead and IQ.¹ Accordingly, the recent paper by him and colleagues deserves a cautious welcome, if only because the data show conclusively that a doubling of body lead from 10 to 20 µg/dl of blood (0.48 to 0.97 µmol/l) or 5 to 10 µg/g of tooth is associated with an IQ deficit of 1-2 points after allowance for confounding variables.² Unfortunately, the "standards of objectivity and critical appraisal" that the authors find so lacking in others are crucially absent from the discussion of their paper, in which, seemingly, any hypothesis is entertained other than the obvious--namely, that lead reduces IQ.

Although the authors concede that this is plausible, they dismiss neuropsychological studies in animals as providing "only indirect support" but fail to mention the reproducibility of such data.³ Nor do they consider studies in humans that have shown biochemical effects at blood lead concentrations well within the range studied epidemiologically--studies that include the effects of lead on enzyme systems central to neurological function.⁴

It needs to be remembered that the natural (that is, pre-technological) blood lead concentration in children has been calculated at 0.016 µg/dl (0.0008 µmol/l)⁵ and that the concentrations typically found in modern children are greater by three orders of magnitude. Furthermore, it requires only a quadrupling of the concentration to bring children into the range associated with neuropathy, coma, and ultimately death. For no other neurotoxin is there such a narrow gap between what is typical and what is toxic, so no one should be surprised when small deficits in IQ are shown by epidemiological studies.

It is unfortunate that these studies have to rely on tooth lead content or blood lead concentration as an indirect measure of exposure to lead on the far side of the blood-brain barrier. Perhaps Pocock and colleagues should turn their attention to the extent to which epidemiological studies underestimate the true impact of lead on children's intelligence.

Former chairman, Campaign for Lead Free Air St Thomas's Hospital, London SE1 7EH

Robin Russell Jones 

1. Pocock J, Ashby D, Smith M. Lead exposure and children's intellectual performance. Int J Epidemiol 1987;16:57-67. [Abstract/Free Full Text]
2. Pocock J, Smith M, Baghurst P. Environmental lead and children's intelligence: a systematic review of the epidemiological evidence. BMJ 1994;309:1189-97. (5 November.) [Abstract/Free Full Text]
3. Silbergeld E. Experimental studies of lead neurotoxicity: implications of mechanisms, dose response and reversibility. In: Rutter M, Russell Jones R, eds. Lead versus health. Sources and effects of low level lead exposure. Chichester: John Wiley, 1983:191-216.
4. Blair J, Hilburn M, Leeming R, McIntosh M, Moore M. Lead and tetrahydrobiopterin metabolism: possible effects on IQ. Lancet 1982;i:464.
5. Flegal A, Smith D. Lead levels in preindustrial humans. N Engl J Med 1992;326:1293-4. [Medline]