Lead was added to gasoline to solve a specific engineering problem: engine knock. In the early 1920s, car engines would misfire when the fuel-air mixture inside the cylinder ignited too early, creating a rattling sound and, over time, serious engine damage. In 1921, a General Motors engineer named Thomas Midgley Jr. discovered that adding tiny amounts of a lead compound to gasoline stopped the knocking almost entirely. That discovery shaped the next seven decades of automotive fuel, even as evidence mounted that it was poisoning millions of people.
The Problem Lead Was Meant to Fix
An internal combustion engine works by compressing a mixture of fuel and air, then igniting it at precisely the right moment. When that timing is off and the mixture detonates too early, you get “knock,” a harsh metallic rattling that robs the engine of power and can eventually destroy internal components. In the early days of the automobile industry, knock was a widespread problem that limited how powerful engines could be. Higher-compression engines, which squeeze the fuel-air mixture more tightly for greater efficiency, were especially prone to it.
Midgley had spent six years testing potential solutions, starting with electrical fixes for the ignition system before turning to chemical additives. His team screened roughly 33,000 substances in total. Lead turned out to be the most effective by far, requiring the smallest concentration to work: just 1 part lead compound to 1,300 parts gasoline by weight was enough to suppress knock. That tiny amount made it cheap to produce, and it worked in virtually any engine.
Why Lead Won Over Safer Alternatives
Ethanol, which is grain alcohol, was already known to reduce knock in the 1920s. It was safe, renewable, and widely available. But it had a critical disadvantage from a business perspective: you can’t patent ethanol. The lead compound Midgley used, tetraethyl lead, could be patented, giving General Motors and its partners exclusive control over the product and its profits. The combination of extreme effectiveness at low doses and commercial exclusivity made lead the preferred choice, despite the fact that lead’s toxicity to humans was already well established in other industries.
Early Warnings That Were Ignored
The dangers of leaded gasoline became horrifyingly clear almost immediately. In October 1924, at Standard Oil’s Bayway plant in Elizabeth, New Jersey, five workers died and 35 others developed severe neurological symptoms over the course of just five days. Of the 49 workers in the tetraethyl lead processing facility, 40 were severely poisoned. Workers at the plant called the product “insanity gas.” One lab worker, Ernest Oelgert, began hallucinating on a Thursday, grew severely paranoid by Friday, and was dead by Sunday. Others were carried away in straitjackets, convulsing and screaming about visions.
Standard Oil later acknowledged that the company already knew the compound had caused deaths and mental breakdowns before those 49 workers were ever exposed to it. Despite this, production and sale of leaded gasoline continued. The industry argued that the tiny concentrations in consumer fuel posed no meaningful risk, a claim that would take decades to disprove in the public policy arena, even as the science was already clear.
What Leaded Gas Did to Public Health
For most of the 20th century, every car on the road was spraying fine lead particles out of its tailpipe. Lead accumulated in soil near highways, in dust, and in the bloodstreams of virtually everyone alive. Unlike many toxins, lead has no safe level of exposure. It damages the brain and nervous system, particularly in children, reducing IQ, impairing attention, and increasing behavioral problems.
The scale of the contamination only became measurable once the phase-out began. Between February 1976 and February 1980, as the United States started reducing lead in gasoline, average blood lead levels in the American population dropped 36.7%, falling from 15.8 micrograms per deciliter to 10.0. That dramatic decline from a partial reduction in just one source of exposure revealed how much of the population’s lead burden had been coming directly from car exhaust.
The Long Road to a Ban
The U.S. Environmental Protection Agency first required a phase-out of lead in gasoline in 1973, but the process was slow and fiercely contested by the oil and automotive industries. The EPA didn’t take its final step in eliminating leaded gasoline for road vehicles until January 1996, more than two decades after the process began and 72 years after those first worker deaths in New Jersey.
Globally, the timeline stretched even longer. Country after country phased out leaded fuel through the 1990s and 2000s, but it wasn’t until 2021 that the last holdout, Algeria, finally banned leaded gasoline for cars. That made it a full century from Midgley’s discovery to the worldwide elimination of his product from road fuel.
Leaded Fuel Is Still in Use
One transportation sector still burns leaded fuel today: small piston-engine aircraft. Aviation gasoline, known as 100LL (100-octane low lead), remains the standard fuel for propeller-driven planes in the United States. It’s dyed blue to distinguish it from other fuels. The FAA has acknowledged that aviation gasoline is now the only transportation fuel in the country that contains tetraethyl lead, and the agency is actively working on a transition plan to unleaded alternatives. The same knock-prevention logic from 1921 still applies: these aircraft engines run at high power settings where detonation can be catastrophic, and finding a safe replacement that matches the performance of leaded fuel has taken years of testing.