DOWNWIND, AND DOWNSTREAM: THE LINGERING IMPACTS OF CITIES ON THEIR ENVIRONMENT

The past and future of cities are inextricably linked, a linkage that can be seen clearly in the long-term impacts of urban geochemical legacies. As loci of population as well as the means of employment and industry to support these populations, cities have a long history of co-locating contaminating practices and people, sometimes with negative implications for human health. An emerging area of environmental health research is to understand the impacts of chronic exposures and exposure mixtures—these impacts are very poorly studied, yet have materialized as perhaps the greatest threat to large-scale population health. Understanding the role of environmental vectors of exposure requires a clear understanding of biogeochemical cycling, a particular challenge in cities with multiple source, sink, and flow pathways. The large impact that cities have on their environment, both downwind from atmospheric pathways and downstream from hydrologic ones, needs to be accurately characterized to provide planning for both mitigation and health screening in cities. Two examples are provided here, that build on W. Berry Lyon’s outstanding work on metal biogeochemistry.

Acute exposure to lead (Pb), a powerful neurotoxin to which children are particularly susceptible, has largely been eliminated in the U.S. and other countries through policy-based restrictions on leaded gasoline and lead-based paints. But these legacy Pb sources are still around in the form of surface soil Pb contamination, a common problem in cities and one that has only recently emerged as a pernicious and widespread chronic exposure mechanism in cities. Some urban soils are also contaminated with another neurotoxin, mercury (Hg), although very little work has been done to understand human exposures to low levels of this element in soils. The most documented human exposure to Hg is through fish consumption, so eating fish caught in urban areas presents risks for above average dietary Hg exposure. The potential double impact of chronic exposure to these two neurotoxins is pronounced in cities. Many aspects of the dose-response curves for individual elements and mixtures are poorly understood, especially at lower levels, leaving unanswered several interesting and provocative questions about environmental impacts on neurological and developmental disorders.