Lead was added to gasoline to solve a destructive engine problem called “knocking,” a phenomenon where fuel ignites too early inside the cylinder and damages the engine. A chemical compound called tetraethyl lead (TEL), first mixed into gasoline in the early 1920s, turned out to be remarkably effective at preventing this. It was also cheap to produce and, crucially, patentable. That combination of engineering performance and corporate profit motives kept lead in gasoline for nearly 80 years, even as evidence of its health dangers mounted.
The Engine Problem Lead Was Meant to Fix
In a healthy engine, fuel and air mix together in a cylinder, then ignite at exactly the right moment when a spark plug fires. But in early 20th-century engines, the fuel mixture would sometimes ignite prematurely, before the spark plug fired, or in multiple spots at once. This created a sharp, metallic pinging sound, which mechanics called “knocking.” It wasn’t just noise. Knocking generated intense pressure waves inside the cylinder that could crack pistons, damage bearings, and destroy engines over time.
Knocking was especially problematic because automakers wanted to build more powerful engines. Higher compression ratios extract more energy from fuel, but they also make knocking worse. The gasoline available in the 1910s and 1920s simply couldn’t handle the engines that engineers wanted to design. The industry needed either better fuel or something to add to existing fuel that would prevent premature ignition.
How Lead Stops Knocking
When tetraethyl lead burns inside an engine, it breaks down and forms tiny particles of lead monoxide. These solid particles act as a chemical sponge, absorbing the reactive molecules that trigger premature ignition. Specifically, they neutralize peroxides and other unstable compounds that build up in the fuel-air mixture before the spark plug fires. Without those reactive molecules reaching a critical concentration, the chain reaction that causes knocking never gets started.
The process is surprisingly physical. The lead monoxide particles floating in the combustion chamber provide a surface where those reactive molecules land and are destroyed, rather than bouncing around and triggering early ignition. Research published in combustion science journals confirmed that this solid-phase reaction accounts for essentially all of TEL’s antiknock effectiveness. Just a few grams of lead per gallon could raise a fuel’s octane rating enough to allow significantly higher compression engines to run smoothly.
Why Lead Won Over Safer Alternatives
Lead wasn’t the only compound that could prevent knocking. Ethanol worked well, and early automakers knew it. Henry Ford designed the original Model T to run on ethanol. Benzene and other aromatic hydrocarbons also raised octane. But lead had two advantages that had nothing to do with chemistry: it was cheaper to produce, and General Motors could patent it.
Thomas Midgley, an engineer working for Charles Kettering’s research company DELCO (which became a General Motors subsidiary), identified tetraethyl lead as an antiknock additive in 1921. The innovation was quickly patented, and General Motors and Standard Oil jointly established the Ethyl Corporation to produce and market it. Ethanol, by contrast, could be made by anyone with access to crops and a distillery. There was no way to build a monopoly around it. Standard Oil was also, as historical records show, reluctant to encourage the manufacture and sale of a competitive fuel produced by an industry unrelated to petroleum. The petroleum industry controlled the fuels market from that point forward.
So the choice of lead over ethanol wasn’t purely technical. It was a business decision that prioritized a patentable, petroleum-compatible additive over a renewable alternative that would have been far less profitable and far less harmful.
The Health Consequences
Lead is a potent neurotoxin, and burning it in gasoline sent it straight into the air that people breathed. During the peak era of leaded gasoline use, from the late 1960s through the early 1980s, the average blood lead level in the U.S. population was routinely three to five times higher than the current threshold for clinical concern (3.5 micrograms per deciliter). A 2022 study in the Proceedings of the National Academy of Sciences estimated that more than half of the U.S. population alive today was exposed to harmful lead levels during early childhood.
The dangers were apparent almost immediately. Workers at TEL manufacturing plants in the 1920s suffered hallucinations, convulsions, and death. But the Ethyl Corporation aggressively pushed back against safety concerns, and regulatory agencies largely deferred to industry. It took decades for public health science to build an overwhelming enough case to force action.
Why Lead Was Finally Removed
The beginning of the end for leaded gasoline came not from health concerns alone, but from an engineering conflict. In the 1970s, new Clean Air Act regulations required cars to have catalytic converters to reduce tailpipe pollution. Lead destroys catalytic converters. It coats the catalyst surface with layers of lead sulfate and other lead compounds, forming a shell that blocks the chemical reactions the converter needs to perform. Cars with catalytic converters simply could not run on leaded fuel without their emission controls failing within thousands of miles.
This forced the U.S. to begin phasing lead out of gasoline starting in 1975. As lead disappeared from fuel, population blood lead levels dropped in almost perfect parallel, providing some of the clearest epidemiological evidence linking leaded gasoline to widespread lead exposure. The U.S. completed its ban on leaded road gasoline in 1996.
Globally, the phase-out took much longer. The United Nations Environment Programme led a 20-year campaign to eliminate leaded fuel worldwide. Algeria was the last country on Earth where drivers could still buy leaded gasoline. In 2021, after its state-owned oil company stopped production and the country decontaminated its storage and distribution networks, Algeria confirmed that service stations were no longer selling the fuel. It happened 99 years and seven months after TEL’s invention.
Lead Still in Aviation Fuel
One significant exception remains. Small piston-engine aircraft in the United States still run on 100LL (100 octane, low lead) aviation gasoline. It is now the largest remaining source of airborne lead emissions in the country. The FAA has set a goal of eliminating leaded aviation fuel by the end of 2030, with an extended deadline of 2032 for Alaska, where small aircraft are critical to transportation infrastructure. The transition depends on developing and certifying unleaded replacement fuels that work safely across the existing fleet of general aviation aircraft. Until those fuels are widely available, airports are not permitted to restrict the sale of 100LL.