Restoring the foundation: Tracking chemical exposures and human health.

ubiquity of toxic substances in the environment continues to be a significant public health concern. Body burdens of chemicals such as dioxins, methyl mercury, PCBs, and a range of the " usual suspects " are the increasingly represented norm rather than the exception in the U.S. population. In the recently released National Exposure Report (National Center for Environmental Health 2003), which provides an inventory of 116 chemicals commonly encountered in the U.S. population, it appears that the levels of these key chemicals have declined over time. Although this constitutes a key success in primary pollution prevention, serious concerns remain regarding the exposure of the population to a wide spectrum of chemicals in the environment. Many chemicals are persistent toxic substances that bioaccumu-late and biomagnify and are thus expected to be detectable in the human population. Others, however, cannot be detected long after the period of exposure. For the majority of chemicals, there is a paucity of environmental population, monitoring, and health effects data. At current funding levels, it would take 1,000 years to adequately document the health effects of the chemicals commonly encountered in commerce and industry. In contrast, a limited number of substances are among the most intensively studied (i.e., the usual suspects). The literature in the broad areas of exposure, toxicity, and epidemiology for these chemicals, as summarized in the Agency for Toxic Substances and Disease Registry's (ATSDR) toxicologic profiles , approaches 1,000 pages of peer-reviewed information. Despite this wealth of data for each of these chemicals, significant uncertainties remain regarding the overall health impact of these chemicals. Although uncertainties exist with respect to health effects for well-characterized chemicals, even more daunting is the issue of documenting environmental exposures in the human population. Exposure data represent the weak or missing link in our efforts to characterize human health hazards due to chemical exposures. Environmental human exposures are much more complex and generally even less well defined than are associated health effects. Limited data exist that reliably characterize the range of human exposures, the number of chemicals involved in such exposures, both in combination and temporally, and the fact that these exposures occur across different and/or multiple routes and durations. This is further complicated by limitations of analytical technology. These issues led the National Research Council to conclude in 1991 that " critical information on the distribution of exposure and health effects associated with hazardous wastes [is] still lacking …