Thomas Midgley Jr., a mechanical engineer working under Charles F. Kettering at General Motors’ research division, discovered that tetraethyl lead could be added to gasoline to stop engine knocking on December 9, 1921. The decision to mass-produce and market it involved GM, Standard Oil of New Jersey, and DuPont, which together formed a joint venture called the Ethyl Corporation. What followed was one of the most consequential public health decisions of the 20th century, one that took over 70 years to fully reverse.
Kettering, Midgley, and the Search for Anti-Knock Fuel
Engine knocking was a serious problem in early automobiles. When fuel ignited prematurely inside the cylinder, it created a metallic rattling sound and damaged engines over time. Charles Kettering, who had already invented the electric starter motor for cars, led GM’s research effort to solve this starting around 1916. He assigned Thomas Midgley and another researcher, T.A. Boyd, to find a chemical additive that could prevent premature ignition.
Midgley and his team worked systematically through the periodic table of elements, testing compound after compound. They noticed a pattern: elements toward the heavier end of the carbon group, including tin and lead, showed the most promise. On the morning of December 9, 1921, when chemists delivered a small amount of tetraethyl lead to the lab, they added it to the fuel of a one-cylinder test engine. The knocking stopped completely.
The chemistry behind this was straightforward. When tetraethyl lead burned inside the engine, it broke down into tiny particles of lead monoxide. These particles destroyed the peroxides and other reactive molecules that cause fuel to ignite too early. In effect, lead acted as a chemical sponge, absorbing the chain reactions responsible for knocking before they could get out of control.
Why Lead Won Over Safer Alternatives
Lead was not the only option Midgley and Boyd found. Their own research identified ethyl alcohol (ethanol) and benzene as “very promising allies” to petroleum. Kettering himself saw ethanol as a “fuel of the future” that could keep American cars running regardless of what happened to oil supplies. During the entire period from 1916 to 1925, the GM team pursued two parallel tracks: leaded gasoline as a short-term efficiency booster, and ethanol as a long-term alternative.
But ethanol had a problem from GM’s perspective: it couldn’t be patented. Any farmer could produce it. Tetraethyl lead, on the other hand, was a proprietary chemical compound that GM could control and profit from. That financial incentive shaped the outcome. GM partnered with Standard Oil of New Jersey in 1924 to form the Ethyl Corporation, a joint venture specifically created to sell tetraethyl lead additives. DuPont, which had extensive experience handling hazardous chemicals, became the primary manufacturer.
Workers Started Dying Almost Immediately
The dangers of lead were already well understood in the 1920s. Lead poisoning had been documented for centuries. But the production of tetraethyl lead concentrated the exposure to extreme levels. At a Standard Oil refinery in Bayway, New Jersey, the consequences were devastating: of the 49 workers at the facility, 32 were hospitalized for lead poisoning and 5 died. Their symptoms included fits of rage, memory loss, confusion, delirium, and convulsions. Workers at the plant reportedly called the product “loony gas” because of the psychiatric symptoms it caused.
These deaths made national headlines and led to a temporary suspension of leaded gasoline sales. But after an industry-influenced review, production resumed. The Ethyl Corporation and its corporate backers argued that the small amount of lead in each gallon posed no risk to the general public, only to workers in poorly ventilated factories. This framing allowed leaded gasoline to remain on the market for decades.
How One Scientist Proved the Global Damage
The scale of lead’s environmental contamination remained hidden until the 1960s, when geochemist Clair Cameron Patterson at the California Institute of Technology made a startling discovery. Patterson had originally set out to determine the age of the Earth by measuring lead isotopes in ancient rocks. But his measurements kept getting contaminated by lead in the surrounding environment, which led him to investigate where all this lead was coming from.
By analyzing ice cores drilled from the Greenland ice sheet, Patterson and his colleagues found that lead concentrations in the atmosphere had increased roughly 300-fold over the past 3,000 years compared to natural background levels. Virtually all of that excess could be traced to industrial emissions. In the most recent century alone, lead levels in Arctic ice had risen at least 10-fold, tracking almost perfectly with the rise of leaded gasoline use worldwide. Patterson demonstrated that volcanic eruptions and sea spray, the natural sources the industry had pointed to, could account for less than 1% of the excess lead in the atmosphere.
Patterson spent years fighting the lead industry’s attempts to discredit his work. The Ethyl Corporation tried to cut his research funding and challenged his findings publicly. But his data held up, and it became the scientific foundation for eventually removing lead from gasoline.
The Long Road to a Global Ban
The U.S. Environmental Protection Agency began working to reduce lead in gasoline soon after the agency was created in 1970. The first reduction standards came in 1973, calling for a gradual decrease to one-tenth of a gram of lead per gallon by 1986. New cars were required to use catalytic converters, which needed unleaded fuel to function, creating a parallel market pressure. On January 1, 1996, the Clean Air Act officially banned the sale of leaded fuel for on-road vehicles in the United States.
The rest of the world took longer. The United Nations Environment Programme led a 19-year global campaign to eliminate leaded petrol country by country. The last holdout was Algeria, which finally stopped selling leaded gasoline in July 2021. That marked the end of a product that had been pumped into cars on every continent for nearly a century.
The Scale of the Consequences
Between the 1920s and the mid-1990s, an estimated 7 million tons of lead were burned in American gasoline alone. That lead didn’t disappear. It settled into soil near roadways, drifted into water supplies, and was inhaled by billions of people worldwide. Lead exposure is particularly harmful to developing brains, and researchers have since linked the leaded gasoline era to measurable drops in population-wide IQ scores, increased rates of violent crime, and widespread cardiovascular damage in exposed generations.
Thomas Midgley, the man who discovered tetraethyl lead’s anti-knock properties, never faced consequences for his role. He went on to develop chlorofluorocarbons (CFCs), the refrigerant chemicals later found to destroy the ozone layer. One environmental historian called him the individual who “had more impact on the atmosphere than any other single organism in Earth’s history.” Midgley died in 1944, decades before the full consequences of either invention were understood.