Selected chemical and physical properties of soils and gut physiological processes that influence lead bioavailability

Abstract

This paper considers selected field examples of physical and chemical properties of soil and some of the interactions with gut physiological processes that are related to lead bioavailability. The blood lead response to quantity of lead in mining and milling environments compared with urban and lead smelter conditions appears to be different. The emphasis of this paper is to understand the complexity of the urban environment. Bioavailability appears to be related to physical and chemical qualities other than mere quantity of lead. Particle size is one physical quality that influences bioavailability. Compared to intact lead-based paint, small particle emissions from vehicles govern the general soil lead pattern in urban environments. Lead has accumulated in soils in proportion to city size, with the inner-city generally measuring the highest lead levels. The soil lead situation is further exacerbated by the chemical influence of other toxic substances such as zinc. In several cities, zinc levels of 1, 500 ppm and higher, plus acid conditions (pH 5.4 and lower) have been observed. This condition is phytotoxic to plants and the deficiency of plant cover increases the likelihood for soil lead ingestion. After ingestion, nutritional status becomes an important factor with both iron and calcium deficiencies increasing lead bioavailability. To complement the other discussions of the Gl tract and bioavailability in this volume, the following physiological responses of the gut that either increase or decrease soil lead bioavailability are described: (1) The role of the ʼnormal’ microbial flora in altering baseline gut function, (2) effect of pH, (3) intestinal transit time, (4) role of mucus, and (5) barriers to lead transport. Physiologically there are nine physical and/or chemical barriers to soil lead absorption which tend to decrease bioavailability: any breakdown of or increased permeability in these barriers would have the opposite effect. The addition of a soil amendment, such as pathogen free processed sludge, would be expected to be a practical means for reducing soil lead bioavailability. The amendment should serve to bind lead and thus increase effective particle size. It would also have the benefit of improving plant growth as shown in the laboratory. Further study is needed to conduct toxicity testing and undertake field evaluation. © 1991 Taylor and Francis Group, LLC.