Read about more of the author’s findings in Experimental Man, published by John Wiley and Sons. When the halibut on my hook breaks the surface, writhing in a splash of seawater off the coast of Bolinas, California, I am thinking less of this fish’s fate than of my own. Considering that I plan to kill and eat it, this might seem cruel. Yet inside the fat and muscle cells of this flat, odd-looking creature is a substance as poisonous to me as it is to him: methylmercury, the most common form of mercury that builds up inside people (and fish). At the right dose and duration of exposure, mercury can impair a person’s memory, ability to learn, and behavior; it can also damage the heart and immune system. Even in small quantities, this heavy metal can cause birth defects in fetuses exposed in the womb and in breast-fed newborns whose mothers’ milk is laced with it.
Scientists have assured me that one serving of halibut contains nowhere near a dosage that might cause harm. These are the same scientists, though, who admit that no one knows for sure what the threshold dose is that causes mercury to subtly poison cells in the brain and the liver, two organs where it tends to accumulate.
As frightening as that sounds, most of us were born with a defense against exposure to mercury, initiated by specific sequences of genetic code that cause most people to expel the metal in 30 to 40 days. Not everyone carries this natural resistance, however. A small minority of people carry a genetic mutation that apparently causes their cells to retain mercury for far longer—in rare cases up to 190 days—greatly increasing the chance for cellular damage.
Such genetic differences may explain why some people are more susceptible to mercury poisoning than others. This possibility is driving a nascent but growing effort among scientists to link the impact of mercury and other environmental factors (everything from pollutants and diet to the sun’s ultraviolet rays) to the individual genetic proclivities that each of us is born with. “Toxicologists say that ‘the dose makes the poison,’” says mercury expert Jane Hightower, who practices internal medicine in San Francisco, “but it’s clear that some people are more sensitive to even small exposures than others.”
For lack of a better term, I’ll call this new science human envirogenomics, the fusing of environmental toxicology and genetics, two fields that until recently didn’t interact much with each other. Yet researchers are finding that the interplay of the two makes us who we are and often determines whether we are healthy or sick. “Recent increases in chronic diseases like childhood asthma and autism cannot be due to major shifts in the human gene pool,” says physician and geneticist Francis Collins, former director of the National Human Genome Research Institute. While acknowledging that changes in diagnostic criteria and heightened awareness may play a role, Collins says that much of the increase “must be due to changes in the environment, which may produce disease in genetically predisposed persons.” One day, envirogenomics could provide clues to a person’s sensitivity to environmental toxins (such as mercury) and the potential for damage based on that person’s genes. Doctors might then better understand how to prevent such harm and how to treat patients exposed to deleterious chemicals.
