Justification of an Air Quality Standard for Lead

25. For the purposes of setting an Air Quality Standard for lead the Panel have concluded that the critical health effect is on the intelligence of young children. A Standard which provides adequate protection against impairment of intelligence in children will also prevent the other toxic effects that have been linked with higher exposures.
26. Currently there is no convincing evidence of a threshold exposure to lead below which no effect on intelligence occurs. Therefore, in setting a standard the aim should be to identify a level at which any effect on intelligence is likely to be so small as to be negligible. The limit of accuracy for measurement of intelligence in individuals is 1 IQ point, and the Panel has taken the view that a lead concentration in the air that might cause an average fall in population IQ of 1 point should be regarded as unacceptable.
27. Epidemiological studies suggest that an increase in the concentration of lead in blood from 10 to 20 µg/dl is associated with an average reduction in population IQ of about 2 points. In the absence of evidence to the contrary, it is reasonable to assume that smaller increases in blood lead will have correspondingly smaller effects, and that therefore a rise in blood lead of about 5 µg/dl would be associated with a fall of about 1 point in population IQ.
28. The relationship between airborne concentrations of lead and the blood lead of children is complex. It is determined not only by absorption from the air that the child breathes but also by ingestion of lead in food and drink and deposited from the air onto surfaces as dust which the child may then transfer to the mouth. Available data indicate, however, that to increase blood lead concentrations by an average of 5 µg/dl, the airborne concentration of lead must be increased by about 1 µg/m³.
29. Lead serves no useful biological function so ideally there would be no lead in air. If lead concentrations in air increased from zero to 1 µg/m³, the above paragraphs indicate that health effects in children (shown as an average reduction in population IQ of about 1 point) would be detectable. Thus, the Panel have concluded that the standard could not be set at a concentration greater than 1 µg/m³.
30. Even at 1 µg/m³, there would be very little margin of safety. Consequently, we consider this figure should be reduced by a safety factor of 50% to take into account uncertainties in the relationship between blood lead concentrations and change in IQ and between air lead concentration and blood lead concentration. Bearing in mind that there may be some variation in the susceptibility of children to lead, we further consider that an additional 50% safety factor should be introduced to protect the most vulnerable. Thus the Panel recommends a concentration of lead in air of 0.25 µg/m³ as an Air Quality Standard at which we believe any effects on health of children will be so small as to be undetectable and at which the vulnerable will be protected.
31. Lead at the concentrations found in the general environment in the United Kingdom does not pose a short term danger to health, but has an effect through long term exposure, since it is the total amount of lead that accumulates in the body that is important in determining adverse effects on the developing brain. The Panel, therefore, recommend that this Standard be applied as an annual average concentration.
32. When this Standard is exceeded any risks to health are likely to be greater if the individual also has relatively high exposure to lead from other sources such as lead plumbing. In these circumstances, risk could be reduced not only by lowering airborne concentrations but also by controlling the other sources of exposure.