How Alice Hamilton helped the world see a hidden poison

In 2010, early in my time as CDC director, I visited Nigeria to strengthen our collaboration on disease control. Doctors told me of a terrible outbreak. In villages of the Anka area of Zamfara, a state in northern Nigeria, dozens of severe abdominal pain, headache, and seizures. It was a tragedy; it was also a mystery.

The most likely cause of an increase in childhood deaths is malaria, which kills more than 400,000 children a year and can cause seizures. But there was no report of the spiking fevers that characterize malaria. Lack of fever also made pneumonia, the most common cause of childhood death, less likely. The absence of diarrhea and dehydration ruled out cholera and other intestinal infections, the third most common cause of death. Seizures can be a symptom of meningitis, which at the time spread rapidly in the region, but, again, the lack of fever made this unlikely.

Investigation found that none of the above caused the sudden increase in deaths. It took time and specialized laboratory testing to conclude something that would have been obvious to public health experts a hundred years ago: The combination of abdominal pain, headaches, seizures, and death is a classic presentation of lead poisoning.

Lead stores energy in batteries, shields against radiation, gives glass crystal its brilliance, and makes paint more luminous and cars run more smoothly. Unfortunately, it’s toxic. Lead accumulates in our bones, brains, and other organs. It causes cognitive impairment, hyperactivity, and other behavioral problems in children and is associated with delinquency and violence, lower income, reduced fertility, and increased heart and kidney disease. More than 2,000 years ago, Vitruvius, a Roman architect and engineer, warned against using lead pipes to carry drinking water. Despite his warning, lead poisoning may have contributed to the downfall of the Roman Empire through the use of lead in wine, bowls, cups, and cosmetics. The harms of lead were largely forgotten or invisible until the 20th century.

In the Anka region, impoverished villagers had taken soil home from abandoned illegal mines to refine it for traces of gold ore. What they didn’t know was that the soil was full of lead. Médecins Sans Frontières (MSF) and local public health staff, who learned of the outbreak when tracking meningitis in the area, conducted a detailed investigation. They drew blood from hundreds of children; every child had lead poisoning and in 85%, levels were so high they exceeded the limit of their portable machine. Samples flown to the CDC in Atlanta demonstrated extraordinarily high levels — nearly every child urgently needed chelation to reduce the effects of lead poisoning. In the following years, MSF provided chelation to thousands of children and the government and community organizations undertook the complex job of environmental remediation, laying fresh soil over acres of contaminated land to prevent continued poisoning.

The Nigerian incident was one of a series of tragedies that resulted from the invisibility of lead poisoning. A century earlier, one doctor’s pioneering work used the superpower of seeing the invisible to track industry-related lead poisoning. Her studies revealed lead’s dangers and protected workers, but government and industry ignored her warnings, with almost unimaginably devastating results.

A preventable mass poisoning

Alice Hamilton, an exceptional and tenacious physician born in 1869, created the field of worker health in the United States. She grew up in a cultured family in Fort Wayne, Indiana; her sister Edith became a best-selling author on Greek and Roman thought. Dr. Hamilton joined the social justice advocate Jane Addams’s Hull House Settlement in Chicago. There she met a who’s who of reformers and intellectuals, including the suffragist Susan B. Anthony, the education reformer John Dewey, the activist Mother Jones, future Supreme Court Justice Felix Frankfurter, prominent socialist Eugene V. Debs, and Upton Sinclair, author of The Jungle — a book largely responsible for the first food safety measures in the United States. Support for workers was integral to Hull House, where, Hamilton wrote, “one got into the labor movement as a matter of course, without realizing how or when.”

Hamilton used the superpower of surveillance to see past obvious injuries and find invisible toxins. Mangled arms and lost fingers were dramatic but harmed far fewer workers than “the less spectacular hazard of sickness from some industrial poison.” She wrote: “Accidents in industry are striking, dramatic, and the cause is clear, while occupational disease comes on slowly and insidiously, and the responsibility [of the companies employing the workers] is more easily evaded.”

Hamilton was meticulous. She searched for every case and reviewed every step in industrial production processes to identify how exposures occurred and figure out how to stop them. Challenged by a factory owner who believed that none of his workers had developed lead poisoning, she scoured local hospitals, spoke with social workers, and visited pharmacists. She identified 22 workers who had left the factory due to disability from lead poisoning and therefore were invisible to the factory owner. Hamilton advocated for government regulation and organized worker action to reduce risks. When neither regulation nor worker demands were possible, she used astonishing tact to persuade factory owners and managers to implement safety procedures. Hamilton made factory owners see invisible cases and take action to prevent them; the U.S. government would be much harder to convince.

Hamilton investigated and controlled lead poisoning and other hazards in workplaces for decades. She had become the first woman appointed to the faculty of Harvard University when, on October 26, 1924, severe illness struck workers at a Standard Oil Company lab in New Jersey. The lab had been developing processes to add lead to gasoline. Within a week, 40 of 49 workers were either dead or severely ill. The deaths triggered intense media attention. Despite initial denials from the company, which blamed the illness on alcoholism or other worker behaviors, investigation revealed that the cause was lead poisoning. In the aftermath, hearings investigated the potential risks of adding lead to gasoline.

Based on her meticulous studies of industrial uses of lead, Hamilton argued forcefully against the automobile and gasoline companies’ plan to add lead to fuel: “I am not one of those who believe that the use of this leaded gasoline can ever be made safe.” She saw that industrial pollutants such as lead weren’t just a problem for workers but would seep into the environment — and into people’s bodies: “You may control conditions within a factory, but how are you going to control the whole country?”

Her colleague Yandell Henderson predicted, with uncanny accuracy, that “conditions would grow worse so gradually and the development of lead poisoning will come on so insidiously … that leaded gasoline will be in nearly universal use and large numbers of cars would have been sold … before the public and the government awaken to the situation.” Gradual worsening is another route to invisibility: Low-level exposures are less visible but can cause much more widespread harm than dramatic cases of lead poisoning.

It’s almost impossible to convey the extent of this tragedy. The government ignored Hamilton’s warning, and for 50 years lead spewed from every car’s tailpipe, contaminating the air, soil, and bodies of generations. By the 1970s, the average American child had a blood lead level higher than what today triggers urgent investigation. Millions grew up less intelligent, more violent, poorer, and more likely to die from heart disease, stroke, and kidney failure. 

Imagine a country just like the United States in a parallel universe where the government heeded Hamilton’s 1925 warning and banned lead in gasoline. In this lead-free America, children grow up with their minds and bodies not damaged by this poison. They score five points higher on intelligence tests, earn 5–10% more throughout their lives, and live in safer communities. Their health care system faces less chronic disease. The tragedy was self- inflicted. Hamilton showed the danger; government and industry chose profit over public health.

Halting exposure to lead required not only scientific knowledge but also advocacy. France, Belgium, and Austria banned indoor lead paint by 1909. In 1922, the League of Nations called for bans on lead paint indoors, and more than 20 countries did so in the following decades. New York City banned lead in gasoline in 1923 — but only for a few years. Because of industry opposition, the United States didn’t ban lead in paint until 1978 and in gasoline until the 1990s. Leaded gasoline has now been banned for on-road vehicles in every country in the world.

In recent decades, the United States has made enormous progress reducing lead poisoning. By 2025, fewer than one in 1,000 children had a level of 15 micrograms per deciliter or more — the average level of the 1970s. But there’s unfinished business. Now that we see how harmful this invisible toxin is, we must work to eliminate lead paint globally, control lead from batteries and cookware, prevent adulteration of spices and cosmetics with lead, and protect workers and communities.

Hamilton protected workers because she forced factory owners, managers, and governments to see previously invisible occupational exposures, but the government couldn’t perceive — or was not willing to stop — the broader harm from leaded gasoline. Unfortunately, lead isn’t the only invisible and deadly toxin.

The poisons that remain invisible

Although we’ve made progress preventing lead poisoning, we don’t yet have adequate systems to see and control many continuing threats to our health. Other invisible toxins, including tiny soot particles (called PM 2.5) cause millions of heart attacks and cancers globally and up to 100,000 deaths each year in the U.S. Cancer, neurological diseases such as amyotrophic lateral sclerosis (ALS, aka Lou Gehrig’s disease), Parkinson’s disease — which killed my father — and many other health conditions arise mysteriously. More and more young people are dying from colon and stomach cancer; toxins in our environment are likely responsible. Maybe it’s chemicals that change the bacterial populations living inside of us. Maybe unrecognized toxins such as nanoparticles are the cause.

Widely used toxins known as endocrine disruptors interfere with the hormonal system, potentially causing cancer, obesity, reproductive problems, and more. These toxins include industrial solvents such as dioxins — dangerous chemicals created as by-products during manufacturing — as well as pesticides and some plastics. The increasing use of these toxins in recent decades may result in extensive health harms. Increasing evidence now indicates that so-called “forever chemicals” such as PFAS (per- and polyfluoroalkyl substances) that persist in our bodies and in the environment increase the risk of a wide range of illnesses. 

The CDC and other entities monitor some exposure levels, particularly through an extraordinarily important examination survey that for decades has used highly accurate tests to detect chemicals in blood and urine. But this doesn’t reduce exposures substantially because the surveillance isn’t analyzed in conjunction with long-term illness data, and findings are too rarely used to remove dangerous products from the market and environment.

The 1920s debate about lead in gasoline highlighted a fundamental question: Must public health prove that a substance is harmful, or does industry need to prove it’s safe? Hamilton hoped “the day is not far off when we shall take the next step and investigate a new danger in industry before it is put into use.” A century later, that day has still not arrived: Companies routinely put chemicals into widespread use without information on safety.

To reduce toxic exposures and balance economic and health priorities, we need data to help others see and act on harmful but invisible toxins. We must monitor not only illnesses but also exposures. We must require laboratory, animal, and epidemiologic research and insist that industries prove new chemicals are safe before selling them to the public.