Arie Haagen-Smit, the Chemist Who Figured Out Los Angeles Smog

Before he solved the most famous pollution mystery in America, Arie Jan Haagen-Smit spent his days trying to work out why a pineapple tastes like a pineapple. He was a Dutch biochemist, born in Utrecht in 1900, who had come to the California Institute of Technology in the late 1930s and made a name studying plant hormones and the volatile chemicals that give fruit its smell. His laboratory method was simple to describe and slow to carry out: pull enormous volumes of air or vapour through a cold trap, freeze out the tiny amounts of chemicals in it, and identify them. Around 1950 the county asked him to turn that equipment on the brown air outside his window. A man who had spent years teasing out the scent of fruit was about to find the recipe for smog.

In short: In the early 1950s Haagen-Smit showed that Los Angeles smog was not mainly smoke but ozone and other chemicals cooked by sunlight out of car exhaust and refinery vapours. Oil and car companies fought the finding for years. His science became the basis for California's vehicle rules and for the state air board he first chaired, and California's power to set stricter car standards is now one of the central fights between the blue states and the Trump administration.

The day Los Angeles thought it was under attack

The smog arrived in force during the Second World War. In July 1943 a thick, eye-stinging haze settled over downtown Los Angeles, so bad that some residents feared a chemical attack. It was not an attack. It was the city itself, which had grown into a sprawl of factories, refineries and a fast-rising number of cars, sitting in a basin ringed by mountains, under a lid of warm air that kept pollution from rising away.

For years after that the official explanation focused on what people could see: smoke. Backyard trash incinerators, factory chimneys and a synthetic rubber plant downtown all came under suspicion. Los Angeles County created an air pollution control district in 1947, one of the first in the country, and it went after smokestacks with real energy. The smog did not go away. Something else was happening, and nobody knew what.

The clue was in the symptoms. Los Angeles smog did not behave like the coal smoke of London or Pittsburgh. It made eyes sting and burn. It cracked rubber. It damaged crops in a particular way, leaving silvery, bronzed leaves on spinach and other plants. And it was worst on bright, sunny days, which made no sense if the problem were smoke alone.

Rubber bands, spinach and a cold trap

Haagen-Smit's approach was the one he had used on fruit. His team drew huge quantities of Los Angeles air through traps cooled to very low temperatures and studied what condensed. They found organic peroxides and other oxidized compounds, chemicals that were not being emitted directly by any smokestack. They were being made in the air.

Then came the experiments that turned a theory into a demonstration. In the lab, Haagen-Smit mixed hydrocarbons, the unburned fuel and vapours that come from car tailpipes and petroleum operations, with nitrogen oxides, which come from high-temperature combustion in engines and power plants. He exposed the mixture to light. The result was ozone and a brew of irritants that reproduced the symptoms: plant damage like the damage in the fields, and rubber that cracked under strain. Ozone cracks stretched rubber quickly, and a stretched rubber strip became one of the cheapest air quality instruments in history.

He published the work in the early 1950s, most famously in a 1952 paper on the chemistry and physiology of Los Angeles smog. The finding changed the target. Smog in Los Angeles was photochemical. The main sources were not the visible chimneys but the invisible exhaust of millions of cars, plus the leaks and vapours of an oil industry that operated across the basin.

The industry's counterattack

This was not welcome news to two of the most powerful industries in California. The oil industry funded research, through a respected independent institute, that disputed Haagen-Smit's conclusions and suggested the chemistry was more complicated and the role of petroleum smaller. Car makers in Detroit argued for years that their products were not the main problem, and that the technology to clean up exhaust did not exist or would be too expensive.

It is worth stating their best case honestly. In the early 1950s atmospheric chemistry was a young field, and it was reasonable to ask for more evidence before rebuilding an entire transportation system around one lab's results. Some of the early critiques pointed at real gaps in the data. Good science invites challenge, and Haagen-Smit answered his critics with more measurements rather than more adjectives.

What turned the argument was that his findings kept being confirmed. Other researchers reproduced the chemistry. The pattern of the worst smog matched traffic and sunlight. By the end of the 1950s the photochemical explanation was the consensus, and the debate moved on to the harder question of what to do about cars.

There is a later chapter to that story that deserves attention. In 1969 the federal government sued the major American automakers, alleging that they had conspired for years to slow the development and use of emission control devices. The case was settled with a consent decree in which the companies agreed to stop such cooperation, without admitting the charge. Readers can weigh that settlement as they like; it shows that the resistance Haagen-Smit faced was organized, and that it lasted.

From the lab to the board

California moved first, because it had to. The state created a motor vehicle pollution control board in 1960 and required basic crankcase controls on new cars in the early 1960s, years before federal law did anything comparable. In 1967, under Governor Ronald Reagan, the state merged its efforts into a new California Air Resources Board, and Haagen-Smit became its first chairman.

That fact surprises people who know Reagan mainly from his later attacks on regulation. In the late 1960s the smog over Los Angeles was a bipartisan grievance, and the Republican governor signed the law that created one of the most powerful environmental agencies in the world. Haagen-Smit led it in its early years, bringing a scientist's insistence on measurement into a political job.

The board's most important tool was the authority to set vehicle emission standards. When Congress wrote the national Clean Air Act in 1970, it mostly barred states from setting their own car standards, because car makers did not want fifty different rulebooks. But it carved out an exception for California, which had regulated cars first. California can set stricter standards if the federal EPA grants a waiver, and other states can choose to adopt California's rules instead of the federal ones. That two-standard design, one national and one Californian that others can join, became the engine of American car regulation for the next half century.

Haagen-Smit stepped away from the chairmanship in the early 1970s and died in 1977. The state air board's highest honour for clean air work carries his name.

Why California's waiver became a front line

For decades the waiver worked as a kind of laboratory. California set a tougher rule; car makers built the technology to meet it; the federal government later adopted something similar for the whole country. Catalytic converters, onboard diagnostics and low-emission vehicle programs all followed some version of that path. Because California is such a large car market, and because a dozen or more other states have adopted its rules at various times, manufacturers often found it simpler to build to the California standard everywhere.

In the climate era the waiver took on new weight. California's rules on greenhouse gases from cars, and its plan to phase out the sale of new gasoline-only passenger cars by 2035, became the most ambitious vehicle climate policy in the country. That made the waiver a target.

During his first term, President Trump's EPA withdrew part of a California waiver, and the Biden administration restored it. In 2025, in the second Trump term, Congress used the Congressional Review Act to pass resolutions overturning the EPA waivers for California's newest vehicle rules, including the one behind the 2035 sales target, and the President signed them in June 2025. Before those votes, the Government Accountability Office had concluded that waivers were not the kind of rules that law covers, and many legal scholars agreed. California went to court. The outcome of that litigation is one to follow on the Court Watch page rather than to forecast here.

The case for letting states lead

Here's the argument, and it is one progressives should make without embarrassment. Federalism is not a conservative property. The ability of a state to try a stricter rule, show that it works and let others follow is one of the oldest ideas in American government. Justice Louis Brandeis called the states laboratories of democracy nearly a century ago, and California's air board has been the most productive lab in the building.

The other side's case deserves a fair hearing. Car makers and some economists argue that a single national standard is more efficient, that California's mandates push expensive vehicles on buyers who cannot afford them, and that one state should not effectively set policy for drivers in states that voted differently. Those are real concerns, especially about affordability. They are best answered not by abandoning the waiver but by pairing clean car rules with help for working families: used electric vehicle credits, charging in apartment buildings and rural areas, and transit that makes a car less necessary.

Progressives have made things harder for themselves here too. California's own housing and permitting rules have made it slow and expensive to build the dense neighbourhoods and transit lines that would cut driving the most. A state that wants to lead on clean air has to lead on building as well.

The neighbourhoods closest to the freeways

Haagen-Smit's discovery has an environmental justice edge that he probably did not foresee. Ozone forms over hours and spreads across a region. But the raw ingredients, and the diesel soot that travels with them, are thickest beside freeways, ports and warehouses. In Southern California, the neighbourhoods along the 710 freeway, around the ports of Los Angeles and Long Beach, and in the warehouse belt of the Inland Empire breathe far more truck exhaust than the hillside suburbs. Those communities are largely Latino and working class.

California's air board has tried, unevenly, to respond. State law in 2017 created a program that directs extra monitoring and emission reduction plans to the communities with the heaviest pollution burden, chosen in part by residents themselves. Community groups have pushed the board on trucks and port equipment, and in turn the board has written rules for both, some of which now depend on federal approvals that the current administration has resisted.

The chemist in the cold-trap lab would recognize the basic problem. The air in a basin does not respect city lines, but the worst of it settles where the traffic is heaviest, and that is rarely where the people who design the freeways live.

Measuring the progress, and the distance left

Anyone who lived in Los Angeles in the 1960s and 1970s remembers the smog alerts: days when schools kept children inside at recess and health officials told people with heart and lung disease to stay home. Those alerts were common for decades. They are now rare. The basin's peak ozone levels have fallen dramatically since Haagen-Smit's day, even though the region has many more people and many more cars. Cleaner engines, reformulated gasoline, vapour controls at gas stations and refineries, and rules on everything from paint to lawn equipment each took a slice out of the problem.

That is the good news, and it should be said loudly, because the people who fought those rules predicted economic ruin that never came. Southern California kept growing. The car industry adapted. Engineers found solutions that executives had insisted did not exist.

The bad news is that the job is not finished. In the American Lung Association's annual air quality reports, the Los Angeles area has ranked at or near the top of the nation for ozone pollution year after year. The basin's geography has not changed: the mountains and the warm lid of air still trap whatever is emitted below. Wildfire smoke now adds a new burden on many summer and autumn days, and the heavy trucks serving the ports and warehouses remain a stubborn source of nitrogen oxides, one of the two ingredients Haagen-Smit put in his flask.

It is worth noticing what kind of victory this was. Nobody banned the car. The state set a target based on health, measured the air relentlessly and tightened the rules a step at a time as technology allowed. That is the method of a scientist, and it is the method the waiver protects.

What you can do

• See how your state compares on clean electricity and how the national temperature record and Mauna Loa carbon dioxide readings have moved on the climate data page. Car rules and power rules are two halves of the same air problem.
• Use the polluters page to check EPA-regulated facilities in your ZIP code, including warehouses and fuel terminals, along with their violation and penalty history.
• If you live in a state that follows California's vehicle rules, ask your state legislators where they stand on defending them. You can find them, with phone numbers and recent votes, through the representatives lookup.
• Keep track of the conflict between state governments and the White House on the blue states topic page, which gathers coverage of waivers, lawsuits and state laws.

Haagen-Smit's career began with fruit and ended with a state agency that changed the cars of the world. The lesson for anyone watching the waiver fight is that the science came first, the industry fought it, and the public, once it understood, chose to breathe.