Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

Classic Toxicology No Longer Works

The dose makes the poison is the most basic principle of toxicology. The first chapter in every toxicology textbook discusses how the response to a poison depends on how much of the poison you are exposed to (often referred to as the “dose”). This principle assumes that chemicals act by overwhelming the body’s defenses at higher doses. It also assumes that at some lower dose, there is no harm or no effect.

But this basic principle may no longer hold true as what we know about exposures to toxic chemicals is changing. Some time ago, Linda Birnbaum, former director of the National Institute for Environmental Health Sciences, wrote an overview paper on the changing science in the field of environmental health. She wrote in this paper that the antiquated perception of how chemicals act is being replaced by a “better understanding of the actual characteristics of modern environmental chemicals.”

These characteristics include the timing and vulnerability of exposure, exposures to mixtures, effects at low doses, and genetic alterations called “epigenetics.” Birnbaum addressed each of these frontiers of toxicology and discussed how each affects our understanding of the link between the environment and public health.  

It’s clear now that the body’s response to toxic chemicals is complex and not as predictable as classic toxicology would lead us to believe. Chemicals such as bisphenol A (BPA) and dioxin cause adverse effects at low doses, not predictable by classic toxicology. No longer can you safely assume that at some low dose there is no harm. Birnbaum makes it clear that within traditional toxicity “… [testing] of effects at some high doses are no longer adequate to [determining] the full spectrum of response from a given chemical.” What’s more, the effects observed at low doses may be different than those observed at higher doses.

We also now know that there are critical “windows of exposure” during development that determine effects – the same exposure at different times can result in different effects. There are also enhanced periods of vulnerability during prenatal development that “program” the bodies’ physiology leading to diseases in later life. According to Birnbaum, this phenomenon has been demonstrated for hypertension, coronary heart disease, and neurological and cognitive development.

Another advance in our understanding of toxicology comes from the field of epigenetics, or the study of changes in DNA expression that are independent of the DNA sequence itself. We now know that exposures to chemicals can alter the normal triggering mechanism that turns genes on or off that determines a person’s observable traits, such as blue eyes or susceptibility to diabetes or other illnesses. Says Birnbaum, “[We’re] born with our genes, but epigenetic changes occur because of environmental influences during development and throughout life.”

These advances in toxicology raise many questions about the adequacy of traditional methods of assessing the impact of exposure to chemicals like BPA or dioxin, not to mention mixtures. Regulatory agencies have not integrated this new science into the methods they use to assess public health risks and it’s likely to take some time before traditional toxicologists accept this new thinking and integrate it into their methods for assessing public health risks. In the meantime, recognize that some of what public health officials are telling you is antiquated and no longer applicable to the world we live in.

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