IMPLICATIONS OF NATURAL LEVELS OF METALS IN SOILS TO CDC'S EXPOSURE REPORT CARD RESULTS

In March, 2001, the Centers for Disease Control (CDC) released the first annual National Report on Human Exposure to Environmental Chemicals. Among other chemicals, CDC measured levels of several naturally occurring metals in either blood or urine. The metals measured in urine include Sb, Ba, Be, Cd, Cs, Co, Pb, Mo, Pt, Tl, W, and U, while the metals measured in blood include Pb, Cd, and Hg. The CDC report provides distributional information (levels of metals in select percentiles of the population) as well as average values. What the CDC report fails to provide is any context to understand whether the measured levels are consistent with natural levels of the metals in the environment or whether human exposure at the observed levels requires an anthropogenic component as well.

The human routes of exposure to all these metals is similar, and include ingestion of water, ingestion of soil, and ingestion of food containing metals. Food may contain metals either as a result of plant uptake from soil, or as a result of animal ingestion of soil. Inhalation of air containing metals is another possible route of human exposure, but it is likely to provide minimal exposure to most metals relative to the ingestion of water, soil, and food. Background levels of metals are well known in both soil and water, and significant information is available for some of the metals concerning their levels in our diet.

Understanding the range of metal concentrations in the environmental media to which we are exposed allows estimation of the amount of metals we would expect to see in blood or urine, based on simplistic assumptions concerning absorption of the metals and their distribution between body compartments. Using these broad estimation techniques we are able to distinguish where the observed levels are consistent with our natural environment vs where additional exposure beyond background levels is required to explain the measured levels. Where the range of exposures is broadest in the population (e.g. for mercury) it is likely that a combination of natural and anthropogenic sources are required. Where the range of exposures is narrowest (cesium, thallium, cobalt) it appears that measurements of human exposure may be consistent with our natural environment.