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.”

Chronic Damage
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.

In contrast, the evidence for lead's subtle effects on the heart is much stronger. Studies over the past 10 years have linked various concentrations of lead in the blood below the 40 mcg/dL threshold permitted by OSHA to high blood pressure, which is a well-known risk factor for cardiovascular disease.

Lead likely raises blood pressure through several biochemical processes. Not only does the metal directly damage the delicate layer of cells lining blood vessels, it also hinders our cells from counteracting “free radicals”—highly reactive molecules that can harm various tissues, including those found in the circulatory system. Lead also inactivates a molecule known as nitric oxide and impairs the kidney, both of which are essential to regulating blood pressure. Small amounts of lead raise blood pressure by 1 percent or less, but even such tiny changes turn out to have greater repercussions than initially recognized for people on the border of hypertension and heart trouble.

In one investigation published in 2006, Johns Hopkins's Silbergeld and her colleagues at Tulane University looked at data collected from 13,946 men and women in the National Health and Nutrition Examination Survey between 1988 and 1994. After 12 years, individuals who initially had 3.63 mcg/dL were one and a half times more likely to have died from heart attack, stroke or other cardiovascular problems than those with levels below 1.93 mcg/dL. An elevated risk of cardiovascular death was still detectable at concentrations as low as 2 mcg/dL. What remains unclear, however, is whether such health risks arise primarily from these very low exposure levels, or if past, higher lead exposures in the older adults surveyed also play a role.

Given that cardiovascular disorders are such a common cause of premature death, anything that increases their risk even a little will translate to many more deaths in the population, experts maintain. One ballpark estimate from the Institute for Health Metrics and Evaluation in Seattle attributes 20,000 of the 670,000 annual cardiovascular deaths in the U.S. and Canada to lead exposure.

Although fewer workers encounter very high amounts of lead on the job these days, many of them still have too much lead in their blood, given the latest health research. Tens of thousands of workers still have levels exceeding 10 mcg/dL, at which adverse effects can occur, according to the National Institute for Occupational Safety and Health, a division of the Centers for Disease Control and Prevention. Consequently, public health experts have urgently called on OSHA to revise its regulations, but so far the agency has taken no action. “It's a profound disappointment and disservice to American workers,” says Howard Hu, a physician and epidemiologist at the University of Toronto.

OSHA head David Michaels declined an interview request from Scientific American but e-mailed a statement via a spokesperson: “We recognize that our standard is outdated; this is true of many of OSHA's other chemical standards,” Michaels wrote. “Unfortunately, OSHA's standard-setting process is extremely slow, and there is little that can be done under our current law to speed up that process.”

So it has fallen to others to push awareness and change. In 2010 the CDC made a blood lead level of 10 mcg/dL or higher in adults a “notifiable” condition, which means that physicians and state health departments must report any results that exceed that limit to federal health authorities. This decision followed recommendations published in 2007 by several experts on lead poisoning, which advised doctors to remove workers who have blood lead levels of 20 to 30 mcg/dL from further exposure until their levels drop below 15 mcg/dL.

A handful of health organizations have adopted those guidelines or a modified version, including the American College of Occupational and Environmental Medicine and the California Department of Public Health, which is currently working with the California Division of Occupational Safety and Health to toughen the state's lead standards. And many companies have taken their own initiative to reduce on-the-job lead exposures to far below OSHA standards, including RSR Corporation, a major lead-battery recycler. Progress is afoot, but the pace is much too slow for far too many people whose health is on the line or has already been permanently damaged.