The cans of baby formula invaded Brian Jackson’s Dartmouth College lab late in 2010. His team picked up an armful of popular brands at the food co-op in Hanover, N.H. Then another armload. Eventually Jackson had a cabinet full of the brightly labeled canisters. Today, he still keeps a few in his office. Not as clutter — that’s not his style. He just likes to keep his toxicology evidence close at hand. A 47-year-old analytical chemist with sandy-gray hair and blue eyes, Jackson has a chemist’s passion for the picky details of analysis, the skill his colleagues tapped when they asked him to investigate a disturbing possibility: that baby foods and formulas made with rice might contain arsenic, a known carcinogen. Ingested even at the trace levels the scientists suspected, devastating health outcomes could result. In a first round of tests, arsenic levels in all the products Jackson’s group studied fell within the 10 parts per billion safety limit the EPA sets for water. (There is no limit for arsenic for most foods.) But a short time later, while shopping at the co-op, Jackson noticed two brands of toddler formula, both high-end organic products, that his team had missed on the first sweep. This time, to the team’s surprise, the arsenic readings flew off the chart. “My first thought,” Jackson says, “was that I’d better reanalyze these samples in case I’d screwed up.” His second thought, after confirming the readings, was to wonder: What made the arsenic levels spike in those two cans? In answering that question, Jackson traced not just the story of the metal-loving rice plant, but also the tangled and troubling path of a notorious poison through our past and present. A naturally occurring metallic element, arsenic permeates the Earth’s crust. Glinting silver-gray in rocks and soils, it mixes with other minerals as it seeps into water supplies, drifts on the dusty plumes of volcanic eruptions and travels on the wind. It also spreads through industrial use, from mining to agriculture. Arsenic coils like a dark smoke through our history. The word derives from the ancient Greek arsenikon, meaning “potent.” It was used to describe the compound arsenic trioxide, which can be lethal at 100 milligrams, about one-fiftieth of a teaspoon. Arsenic trioxide is famously tasteless and odorless, which helped make it one of the most frequently used homicidal poisons in history. But in recent years, studies have revealed that exposure to far smaller doses poses a more subtle — but also insidious — threat. The pure element arsenic mixes into many compounds, either organic (in chemical lingo, meaning that it contains carbon) or inorganic (without carbon). And even at concentrations of parts per billion (ppb), closer to a drop in a swimming pool than a drop in a teacup, long-term exposure to inorganic arsenic — generally considered the most toxic form — has been linked to an increased risk of cancer and other life-threatening illnesses. Although arsenic hasn’t been studied in as much detail as other toxins found in industrial materials, such as mercury or PCBs, scientists say it underscores the finding that minute exposures to such substances can do great harm. At low doses, arsenic doesn’t overwhelm body systems immediately or even cause death over the course of months. Rather, explains Dartmouth molecular toxicologist Joshua Hamilton, chronic exposure to trace arsenic inflicts damage at a cellular level, increasing the body’s vulnerability to a wide array of sicknesses, including cancer, cardiovascular disease and diabetes. While trace arsenic won’t kill on its own, he says, it “seems to make everything worse.” For decades, officials have focused on trace arsenic in drinking water as the chemical’s primary public health threat; in 2001, the EPA dropped the limit for arsenic in water from 50 ppb to 10 ppb. But in the past few years, regulators have also begun to worry about exposure from foods and beverages. This summer, concerned about arsenic in pesticide residues found in imported juices, the FDA announced it will limit the amount of arsenic allowed in apple juice to 10 ppb, the same amount permitted in water. The FDA has also investigated arsenic’s presence in other foods. Chicken, for example, has come under scrutiny because of the longtime use of an arsenic additive in poultry feed. But the top-priority food on the list is rice, which became a focus when researchers realized that it takes up inorganic arsenic from soil far more efficiently than other grains. A July study revealed the first evidence directly linking consumption of rice containing arsenic to genetic damage in humans. Such findings are especially alarming because rice is a major part of the diet in certain communities, such as those with an Asian heritage, and because rice is a staple for infants and young children, whose developing bodies and brains are especially vulnerable to harm. It’s that last concern that sparked the formula studies in Jackson’s Dartmouth lab.