BOSTON—Exposure to lead—so toxic—is a problem of the past, right? Wrong. Since the U.S. took lead out of gasoline in 1976 and banned lead paint in 1978, most health scientists, regulators and the public have considered the problem largely solved. But ongoing testing shows that even though the average concentration of lead in the American bloodstream has dropped by a factor of 10 since the late 1970s, the levels are still two orders of magnitude higher than natural human levels, which have been determined by studying skeletal remains of native Americans dating to before the industrial revolution.

Equally problematic, recent health studies have shown that exposure levels previously thought to be “safe” were too high. Scientists from various disciplines are advising the Environmental Protection Agency and health departments to lower the concentration deemed acceptable in the bloodstream, which today averages 1.3 micrograms per deciliter but can be much higher for many individuals. The change is warranted because the latest set of long-term tests done over decades has revealed that many of the health complications from lead arise even at low exposures. Higher levels are not necessary to instigate damage to the body or brain, Joel Schwartz of the Harvard School of Public Health told a somewhat surprised crowd on Feb. 16 here at the annual American Association for the Advancement of Science (AAAS) meeting. Excessive lead exposure correlates with a host of ills, including impaired cognition, attention deficit disorder and lower academic test scores for children, psychiatric disorders, and increased blood pressure, hypertension and arrhythmia.

Lead is also increasingly implicated in dementia in the elderly. As we age, our bones demineralize and release calcium (which is why calcium supplements are often recommended, especially for women). “But the bones also release lead,” which accumulates in our skeletons over a lifetime, Schwartz said. “We don’t know if the brain can adapt to the higher levels” of lead in the bloodstream, he said, calling for new research to find out.

The ramifications of lead exposure are financial as well, costing the U.S. about $209 billion a year, said Jessica Reyes, an economist at Amherst College. The bill includes everything from direct medical costs to a heightened need for special education classes and incarcerations for violent crime, which also correlates with higher lead exposure.

The ongoing trouble with lead exposure is not to be confused with lead poisoning, which has dropped significantly in developed countries, including the U.S. The latter condition is caused by acute exposure at high concentrations, which can occur from eating lead paint chips. But all the other problems “are more like chronic diseases that build over time,” said A. Russell Flegal of the University of California, Santa Cruz. “We need to start thinking about the risks in that way.”

Lead is still prevalent in our environment for many reasons. Because lead does not degrade, heavy emissions from the past accumulate in soil. Winds, especially during drought—like that afflicting the Midwest for the past year or so—kick it up as dust, and runoff from heavy rains and flooding can re-suspend the particles in the atmosphere. Trees take up soil particles, too, but when forests burn in wildfires, as has been occurring more frequently worldwide with global warming in recent years, that lead is released back into the air. Fires also release lead from old houses and buildings coated with lead paint that was applied prior to the U.S. ban. Lead smelting and refining is still an enormous industry worldwide, sending more of the metal into the environment. Aviation gas used in planes still contains lead.

Lead is still present in drinking water in many communities, where it can leach from lead pipes in homes, apartment buildings and municipal water system, or from brass fittings or solder used in plumbing. Another 25,000 to 30,000 tons of lead enters the U.S. environment each year from hunting and shooting-range ammunition, fishing-line weights, discarded batteries and electronic waste, said Mark Pokras at Tufts University.

Coal-burning power plants in developed nations also generate some lead in emissions and more so in ash, and the steep rise in coal power in China has boosted levels worldwide because regulations are more lax. Larger lead particles fall to the ground within about 200 meters of the source (including tailpipes, by the way), but the smaller particles, about 0.5 micron in size, can remain airborne for a week before they settle out. According to Flegal, lead particles from China have been found in rainfall in Santa Cruz, Calif.

Many steps can be taken nationwide to further reduce lead levels. Tougher emissions laws can be imposed. Lead paint, still sold in China, for example, can be banned in that country, or for import by other countries. Lead pipes and old lead paint can be removed. A high tax could be imposed on products containing lead, and lead in ammunition and fishing weights could be replaced with substitutes—although materials such as tungsten have not performed well in bullets. A different view about prevention is needed, too. For years, U.S. regulators have focused primarily on reducing lead poisoning, and they have succeeded. “So now we have to stop thinking about the problem as a small number of people who have an acute exposure, and start thinking about the problem as a large number of people who have a chronic exposure,” Schwartz said.

Cost analyses might help push regulators into action, Reyes said. “Perhaps we will find that an X-amount of reduction in lead exposure equates with an X-amount of rise in test scores” [which has been shown in Massachusetts], she said. “Or perhaps we will find that a certain amount of reduction equates with a certain reduction in health-care costs.”