The Effects of Lead on Human Health

20. Lead is absorbed into the body both through the stomach and intestines after being taken in through the mouth, and through the lungs when breathed in from the air. Once absorbed it spreads around the body and accumulates particularly in bone, teeth, skin and muscle. In these tissues it is relatively stable and released only over months or years. A small proportion, around 2%, is found in blood and it is this fraction that is biologically active and leads to harmful effects. Removal of lead takes place slowly through the kidneys, the concentration in the blood halving over the course of about several weeks in the absence of further uptake.
21. The toxic effects of lead are a consequence of its ability to inhibit the actions of certain enzymes and to damage chemicals in the nuclei of cells. In workers with high exposure a rare but serious manifestation of lead poisoning is acute brain damage, causing delirium and fits. Severe poisoning can also induce many other symptoms, and damage to organs such as the kidney can occur when concentrations in the blood exceed 100µg/dl*. At somewhat lower concentrations, above about 80 µg/dl, colicky intestinal pains may be a feature. Above about 50 µg/dl anaemia can arise due to an inability to produce haemoglobin, the blood pigment that is responsible for carrying oxygen. Reversible effects on the kidneys and male reproductive organs have been described at blood concentrations greater than 40 µg/dl, as have effects on nerve functions in the limbs at concentrations above 30 µg/dl. Finally, above a level of 10 µg/dl studies of large groups of children have shown subtle evidence of changes in brain development, and this is also the lowest concentration at which biochemical evidence of interference with blood pigment synthesis has been described.
22. The most substantial evidence of effects of low levels of lead on health relates to effects on the central nervous system and, in particular, on the developing brain of children. Investigations have concentrated on average effects on populations. The end point most commonly measured has been the intelligence quotient (IQ), an index expressed in relation to the average (which is arbitrarily scored at 100) of the population as a whole. In any population the IQs of individuals are distributed around this figure so that those less intelligent than the average score less than 100, and those more intelligent score more than 100. Any substance that damages the brain might be expected to reduce the average IQ in the exposed population, an effect that would not be noticeable in people of average intelligence but that would increase the numbers of individuals with low intelligence and decrease the numbers of very intelligent people in that population.
23. Many studies have investigated the relationships between blood lead (or sometimes tooth lead) in children and IQ. While few of these studies have taken account of all other factors that might have been associated with lead exposure and have independently influenced intelligence (technically called confounders), the results of them all taken together suggest that there is an inverse relationship between blood lead and intelligence; that is, the higher the average blood lead concentration in a population, the lower that population's average IQ. In these studies it has not proved possible to show evidence of a threshold concentration in blood below which lead has no effects at a population level, although adverse effects in individuals have not yet been demonstrated below about 10 µg/dl. Some controversy surrounds these studies because of their inability to take account of all possible confounding factors, and it remains possible that the association between IQ and blood lead is not a causative one. Perhaps, for example, children of lower average intelligence are more likely to be exposed to lead because of their habits and environment. However, the studies have shown consistent results and are biologically plausible since lead is a known nervous system poison and can damage the brains of experimental animals. The Panel have therefore taken the prudent view that elevated lead concentrations in the blood do have the potential to cause damage to the developing brains of children.
24. The evidence from these studies of populations of young children suggests that the developing brain of a child from the time of birth up to the age of 5 years is at its most vulnerable and there may be a loss of up to about 2 IQ points on average for a rise in blood lead from 10 to 20 µg/dl. In view of this probable reduction in IQ and its effects on the numbers of people in the population with high and low intelligence mentioned in paragraph 22, there is a case for further action to reduce average blood lead levels in populations. As mentioned above, air lead concentrations are not the only determinants of blood lead in a population, since the major proportion of intake is derived from the diet. However, there is published evidence that reduction in air lead concentrations occurring over a period of increasing use of unleaded petrol has been associated with consistent reductions in the exposed population's blood lead concentrations, and the Panel have concluded that controls on airborne lead can reduce the risk to the health of the population through more than inhalation alone.