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His name was Theophrastus Philippus Aureolus Bombastus von Hohenheim and his contemporaries called him Paracelsus, but history knows him by this title: the Father of Toxicology. Paracelsus was a 16th century Swiss physician who profoundly influenced our understanding of how chemicals affect the body. His dictum, “The dose makes the poison,” helped explain that even toxic substances could be safe as long as the amount ingested remained below a certain threshold. It’s still a major principle of modern toxicology, and it’s why the Food and Drug Administration (FDA), the Environmental Protection Agency and other government safety offices fight with industry to find that safe level below which the toxins that are part of modern life can be tolerated.

But as science advances, we can detect smaller and smaller doses of chemicals in the human body – as small as one part per trillion, or about one-twentieth of a drop of water in an Olympic-sized swimming pool. A Centers for Disease Control and Prevention biomonitoring survey found that Americans have traces of 212 environmental chemicals in their bodies – including so-called endocrine disrupters like bisphenol-A (BPA), which may have a major impact on human health even though the dose is barely perceptible. But our ability to detect chemicals outpaces our ability to understand exactly what exposure means for us – which puts regulatory agencies in a tight spot, especially when the chemicals in question are widely used in modern life and are hard to replace.