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The "just traces" argument is misleading on so many levels. Current sludge regulations have standards for only a handful of metals and nitrogen. And even these standards are much more lenient than those of other industrialized countries that land apply sludges. For example, no amount of lead-not even trace amounts-should be deliberately applied to garden soil or where children play. Yet EPA/USDA claim that 400 parts per million of lead in soil where children play is safe. Urban sludges contain a lot more than just toxic metals. It is an unpredictable complex mixture of thousands of industrial chemical compounds as well as harmful biological agents. Toxicologists are finding that the principle " the dose makes the poison" may be too simplistic when measuring the health and environmental risks of complex mixtures. The principle does not address synergistic effects, interactions, and the toxicity of breakdown products. The 2002 National Academy of Sciences biosolids report recognized this problem and warned that chemical-by-chemical risk assessment will not gauge the true risks of land application because the degree of complexity and uncertainty "requires some form of active health and environmental tracking." "Traditional toxicology assumes that there is a direct linear relationship between dose and effect. Not so for endocrine disrupting chemicals (EDCs)." Sewage sludge is full of EDCs. "Low doses--trace amounts-- of EDCs work in ways that are totally unpredicted by traditional toxicology," according to an internationally renowned EDC expert." For every EDC we test, there will be a non-monotonic response. See http://www.nature.com/news/toxicology-the-learning-curve-1.11644?WT.ec_id=NEWS-20121030 In other words, EDC's can harm organisms in parts per trillion-- depending on the time of exposure. Other unregulated chemicals such as dioxins and PCBs, ubiquitous in land applied sewage sludge, also harm organisms in part per trillion. But they magnify in the food chain, and concentrate, for example, in milk from cows that graze on sludge-treated pastures.