| When I was a child, a speck of lead escaped the tailpipe of a passing car, drifted through the air and lodged deep in my lungs. There it dissolved across a tissue-thin membrane into my bloodstream. Then the toxic metal atoms became microscopic wrecking balls in my body. Some, hitching a ride on red blood cells, slipped past the blood-brain barrier, tangling neural switches and repaving neurons. Others lodged deep inside my kidneys, liver and heart, leaving behind injured cells and blood vessels. But the vast majority, masquerading as calcium, were entombed in the mineral lattice of my growing skeleton. I never suffered acute lead poisoning, but like roughly 125 million children born between 1951 and 1980, a period of extraordinary lead exposure in the United States, I am invisibly scarred by the damage the heavy metal left behind. As I age, the lead in my bones may reemerge to deliver one last blow. Growing up in the 1980s, I was the last generation to live through the highest period of lead exposure in U.S. history: Exhaust from cars fueled by leaded gasoline, as well as paint chips and lead pipes, added to the toxic burden in my body. Trillions of lead atoms are now part of my being. For more than 90 percent of the population born between 1951 and 1980, blood lead levels exceeded 5 micrograms per deciliter, a level we now know is associated with lower IQ, attention and behavior problems, and smaller brain volume. In the late 1970s, the peak of the lead crisis, levels in children under 5 averaged 16 micrograms per deciliter, nearly five times higher than today’s threshold for triggering public health intervention. Before national monitoring began in 1976, lead levels were probably much higher. That arguably makes lead exposure the largest quiet mass poisoning in human history, said Jennifer Weuve, an epidemiologist at Boston University’s School of Public Health. The United States burned enough leaded gasoline during the three decades after 1950 to sprinkle the equivalent of nearly 2,000 of pounds of the heavy metal along every mile of roadway in the nation (many urban roads received far more). Lead in paint, batteries and construction added to this relentless outdoors exposure. Unfortunately, we now know even minuscule amounts of the element harm the human body: Blood-lead levels above just two micrograms per deciliter, roughly equivalent to dissolving a BB-size lead pellet in a large backyard pool, are associated with developmental harm. IQ drops sharply at even the lowest levels of exposure. Thankfully, lead pollution has plummeted in the U.S., although it’s by no means gone. Children’s blood levels are down more than 96 percent from their measured peak in the 1970s. (Tragically, 800 million children globally still have elevated blood lead levels.) But lead poisoning is not a one-time event: The metal may remain in your blood for a few months after exposure, but 70 to 95 percent of what isn’t excreted is locked up in your bones, often for decades. That effectively turns your skeleton into a time bomb. The body’s steady remodeling of old bone into new returns stored lead to the bloodstream, allowing it to harm your organs. Pregnancy, breastfeeding, menopause and inactivity significantly accelerate this process, as does normal aging, when bone density declines. One study linked menopause to significantly higher blood lead levels in women. The release of lead from the skeleton doesn’t lead to acute poisoning or, at least initially, obvious symptoms. But studies of health records suggest that lead burned in gasoline or mixed into paint long ago can harm us decades later. One disturbing clue arrived this year in the peer-reviewed journal Alzheimer’s & Dementia. Analyzing nearly three decades of national health surveys and Medicare data showed that people estimated to have the highest levels of lead in their bones were roughly twice as likely to develop dementia than those with the lowest levels. They were three times more likely to develop Alzheimer’s disease. The researchers behind the study say it’s too early to pin the blame solely on bone lead, which represents a lifetime of exposure. But it suggests we may be entering a critical moment of cognitive decline for generations that grew up during peak lead. “We’re going to see an increase,” said Robert Wright, a pediatrician and medical toxicologist at New York City’s Icahn School of Medicine at Mount Sinai, on rates of dementia linked to lead exposure. “I think we’re about to find out how much.” | 
