One of America's great public health achievements in the 20th century was removing lead—an extremely useful but incredibly toxic metal—from gasoline, paint, water pipes and food cans. Children are particularly vulnerable to the damage the element inflicts on nerve cells and the brain. Swallowing very large amounts can trigger convulsions and ultimately kill someone in a matter of days, but eating or inhaling a little lead here and there over longer periods can result in lower IQ, hearing loss, and behavioral problems, such as hyperactivity. Indeed, many researchers think there is no safe level of lead for children.
The more scientists learn about the dangers of lead, however, the more they realize that they may have underestimated how even small amounts of the element poison adults. Studies conducted over the past 20 years have documented a wide range of subtle, long-term medical issues—from an increased risk of high blood pressure and heart disease to various kidney and immune system problems—in men and women who were exposed either to the metal on the job or to lingering traces in soil, in air and in buildings constructed when lead paint was still in use.
The new insights raise concerns for older generations that accumulated lead in their bones during the leaded-gas-and-paint era. Although most of that lead is locked away in the skeleton for years to decades, the metal can leak back into the blood in small increments as people age and lose bone density. And many people currently working in such industries as metal smelting, lead-battery manufacturing and building renovation continue to routinely absorb the toxic element.
Under regulations that have not been updated since they were first established in 1978, the Occupational Safety and Health Administration (OSHA) permits blood lead concentrations in workers of 40 to 60 micrograms per deciliter (mcg/dL), depending on circumstances. Yet a 2012 scientific review from the U.S. National Toxicology Program linked concentrations between 5 and 10 mcg/dL—the rough equivalent of half a grain of salt per cup of blood—to elevated blood pressure, among other problems. “We haven't done a darn thing about what's going on with exposures for adults,” says Ellen Silbergeld, an environmental health scientist at the Johns Hopkins Bloomberg School of Public Health. “We have an occupational lead standard in the U.S. that has not been changed for 35 years. It's outrageous.”
Even today lead remains a prevalent and formidable environmental contaminant. The wind can whip up old lead dust from paint or gas emissions that settled into soil, explains environmental toxicologist Russell Flegal of the University of California, Santa Cruz. However the metal gets from one's surroundings into one's body, it takes its toll on living tissues in two key ways. It interferes with the production of the oxygen-toting molecule hemoglobin in red blood cells, and it mimics the behavior of calcium, without any of calcium's benefits. Brain and nerve cells depend on calcium to transmit their electrical signals; when lead barges in, it garbles the usual communication between neurons.
Studying lead's long-term effects at low concentrations became possible only after the U.S. phased the metal out of gasoline between 1973 and 1996, which resulted in a sharp drop in the amount of lead in people's blood. Average concentrations around the country fell from 13 mcg/dL in the late 1970s to 1.12 mcg/dL as of 2010. With the help of increasingly sensitive instruments and better statistical methods for studying large populations, researchers have learned that tiny doses of lead can harm an individual's health even if they do not cause any overt symptoms.
Research to date has associated small amounts of lead stored in bones—around 10 or 20 micrograms per gram of tissue, some of which may leak into the blood over time—with a dulling of mental acuity in the elderly equivalent to cognitive decline in three to five years of aging, notes Marc Weisskopf, a Harvard University epidemiologist. Yet untangling lead's influence from that of normal aging is tricky.