Lead: Assessing the environmental burden of disease
Environmental burden of disease series No. 2
This guide outlines a method for estimating the disease burden at national, city or local levels caused by environmental exposure to lead. The method described in the guide assesses lead exposure from the distribution of lead levels in blood samples representative of the study population. For children, the exposure distribution is then converted into a distribution of IQ points lost, resulting mild mental retardation, and into rates of anaemia and gastrointestinal symptoms. For adults, the exposure distribution is converted mainly into increased blood pressure, which may cause various cardiovascular diseases, including ischaemic heart disease and cerebrovascular disease.
The method can be used to highlight the health impacts on population subgroups at particular risk, provided that exposure in these subgroups is assessed separately. Such subgroups can include children of lower-income families living in degraded housing, people living around “hotspots” (such as certain industrial activities), or occupational groups. Global estimates of the disease burden from lead are given for 14 different regions and several age groups (Annex 4). It is estimated that lead exposure accounts for almost one percent of the global burden of disease, and most of the exposure affects children in the developing world.
To select the most suitable interventions for reducing the disease burden, an inventory of the main sources of exposure to lead is required. In countries where it is still used, leaded gasoline will likely be a major contributor to exposure, either directly through the air, or indirectly through food and dust. Other sources may also be locally relevant, such as the use of leaded ceramics. Workplace exposures and cottage industries can affect workers and their families and neighborhoods, although the emphasis here is on environmental exposure. In developed countries, where leaded gasoline has usually been phased out, the highest environmental exposures to lead generally affect children of lower-income families living in degraded housing.
The burden of disease caused by relatively low and widespread exposures to lead is often underestimated by policy makers. For example, loss of IQ points is not considered a disease per se, yet it reflects subtle neurological impairment that will be most marked on the social and psychological development of children who already have a low IQ score. Subtle effects on IQ loss are expected from blood lead levels as low as 5 µg/dl and the effects gradually increase with increasing levels of lead in the blood. The effects of losing IQ points will be greater in children with an IQ score just above 69 (with mild mental retardation defined as an IQ score between 50–69), than in children with a higher IQ. At higher levels, lead exposure also leads to gastrointestinal symptoms and anaemia (about 20% of children are affected when blood lead levels exceed 60–70 µg/dl). In adults, relatively low levels of lead exposure (5 µg/dl) may increase blood pressure, which can then lead to cardiovascular diseases.
Quantification of the health impacts in a given country or region by methods such as that presented in this guide provides an opportunity to highlight the magnitude of disease burden that could be avoided. The results of such an analysis can therefore present important arguments for policy action to reduce the exposure to this risk factor, and thus reduce the current burden of disease of the population.