---
title: "Midgley: Two inventions and the air"
publication: Awescroll
number: "004"
url: https://awescroll.com/004-midgley
published: 2026-10-05
updated: 2026-10-05
reading_time: 9 min
language: en
---

# Midgley

Two inventions and the air.

*No. 004 · 9 min read · 5 October 2026*

In 1921 a mechanical engineer in Dayton, Ohio, found what made car engines run smoothly: a few grams of a lead compound in every gallon of petrol. Nine years later he found the perfect gas for refrigerators: safe to breathe, impossible to set on fire.

Both worked. Both went everywhere, until there was some of each in the air over every part of the planet. It took half a century to learn what each of them did there.

His name was Thomas Midgley Jr.


---

*02 · The knock*

*DAYTON, OHIO, 1916*

## Engines knocked.

Midgley had studied mechanical engineering at Cornell. In 1916, aged 27, he went to work for the inventor Charles Kettering, whose laboratory in Dayton later became General Motors Research. Kettering gave him a problem nobody had solved: the knock.

![Studio portrait of Thomas Midgley Jr., a balding man in round wire glasses, a dark suit and a spotted tie.](https://awescroll.com/photos/midgley-portrait.jpg)
*Thomas Midgley Jr. in a studio portrait from the 1930s, the decade he presented Freon. (Blank & Stoller, New York, 1930s · Science History Institute, no known copyright)*

In an engine, the spark lights the fuel and a flame is meant to sweep across the cylinder in order. Squeeze the mixture harder, for more power, and the last of it explodes on its own before the flame arrives. The engine pings, shakes and loses its power. Knock set a ceiling on how strong and efficient a car could be.

> **HOW IT WORKS**: The cylinder on screen burns its charge twice: once smoothly, from the spark plug outward, and once knocking, with the far corner going off by itself. It is a drawing of the idea, not of a 1920s engine.


---

*03 · 9 December 1921*

*9 DECEMBER 1921*

## The knock stopped.

Midgley tried things for five years. Iodine worked, but it was corrosive and expensive. Aniline worked too. Selenium and tellurium compounds worked better still, but stank even worse. On the morning of 9 December 1921, his team fed a little tetraethyllead into the laboratory&\#39;s one-cylinder engine.

A few grams a gallon were enough. The engine ran smooth. Carroll Hochwalt made the sample and Thomas Boyd ran the test; the credit went to Midgley and Kettering.

> **THE NUMBER**: How many compounds did they try first? Accounts range from 143 to 33,000. The 33,000 comes from a 1927 sales pamphlet by the company that sold the additive. The decisive series of tests covered 24 compounds. The laboratory notebooks are not in any archive.


---

*04 · Ethyl*

*TETRAETHYLLEAD, Pb(C₂H₅)₄*

## They called it Ethyl.

One atom of lead, held by four ethyl groups. It was not new: a German chemist had first made it in the 1850s, and it was known to be a poison. In March 1922, before any of it was sold, Pierre du Pont wrote that it was "very poisonous if absorbed through the skin, resulting in lead poisoning almost immediately."

Kettering proposed the name: Ethyl. The word lead was left out of it. The first leaded petrol went on sale in Dayton in February 1923. A month later Midgley wrote to his boss that they could count on "a minimum of 20 percent of the gas sold in the country," at three cents a gallon to them.

In August 1924 General Motors and Standard Oil of New Jersey set up the Ethyl Gasoline Corporation. Midgley was its vice president.


---

*05 · Fresh air*

*19 JANUARY 1923*

## His own lungs.

After about a year of working with organic lead, Midgley cancelled a series of talks. He wrote why:

After about a year&\#39;s work in organic lead I find that my lungs have been affected and that it is necessary to drop all work and get a large supply of fresh air. (THOMAS MIDGLEY JR., LETTER OF 19 JANUARY 1923)

He went to Miami to recover. From there he wrote to an oil engineer that poisoning the public was "almost impossible": nobody would keep getting their hands covered in leaded petrol, and the exhaust did not contain enough lead to worry about. What he feared was legislation, "fostered by competition and fanatical health cranks."


---

*06 · Loony gas*

*THE PLANTS, 1923 TO 1925*

## The men who made it.

The first to be harmed were the men who made it. Workers at the plants began to hallucinate, shake and fall into fits. They called it loony gas. At Du Pont&\#39;s works in Deepwater, New Jersey, the tetraethyllead building became the House of Butterflies, after the insects the poisoned men saw.

- APRIL 1924 · Dayton, Ohio: General Motors' pilot plant. Midgley later admitted two deaths and more than 50 workers "under observation". (2)
- 1923 TO 1925 · Deepwater, New Jersey: Du Pont's plant. Eight deaths reported by mid-1925; one historian counts ten. (8)
- OCTOBER 1924 · Bayway, New Jersey: Standard Oil's works. Five men died within about a week; dozens more were taken to hospital. (5)

One Bayway worker, Joseph Leslie, spent the next forty years in psychiatric hospitals. Most of his family believed he was dead; only his wife, and later his son, knew where he was.

> **THE COUNT**: At least fifteen workers died making tetraethyllead between 1923 and 1925. Historians differ on the exact numbers at each plant; the ledger gives the counts reported at the time, and William Kovarik&\#39;s later count is higher.


---

*07 · 26 Broadway*

*NEW YORK, 30 OCTOBER 1924*

## He washed his hands in it.

Days after the Bayway deaths, Standard Oil called the press to its offices at 26 Broadway. Midgley poured the liquid over his hands and dried them with a handkerchief. Then, according to the New York Times, he held a bottle of the liquid under his nose for about a minute, to show that a short exposure did no harm.

He liked the demonstration and repeated it elsewhere. One account quotes him:

I&\#39;m not taking any chance whatever. Nor would I take any chance doing that every day. (THOMAS MIDGLEY JR., AS QUOTED BY JAMIE LINCOLN KITMAN, 2000)

That day, New York City banned the sale of leaded petrol. Other cities followed.

> **THE MYTH**: A popular version says he claimed he could do it every day without harm. The words he was quoted as saying point the other way.


---

*08 · A gift of God*

*WASHINGTON, 20 MAY 1925*

## "An apparent gift of God."

The industry paused sales and asked the Surgeon General for a hearing. It lasted one day. Frank Howard of Standard Oil called the additive "this apparent gift of God", which, he said, would let a gallon of petrol go perhaps half as far again.

Before the hearing, Yandell Henderson of Yale had called it "probably the greatest single question in the field of public health that has ever faced the American public." Alice Hamilton of Harvard urged the industry to find something else: "Men who could discover the fuel value of tetraethyl certainly could invent or discover something equally efficient and in no way dangerous."

A committee studied 252 drivers and garage workers and reported in January 1926 that it had found "no good grounds" to ban leaded petrol. It added a warning: "this investigation must not be allowed to lapse." It lapsed. For about forty years the research on the additive&\#39;s health effects was paid for by the industry.


---

*09 · Patterson*

*PASADENA, 1953 TO 1969*

## Lead was everywhere.

Clair Patterson, a geochemist, set out to measure the age of the Earth from lead in a meteorite. His samples kept coming out contaminated, however carefully he cleaned. In 1953 he had his answer, about 4.5 billion years, and a second question: where was all this lead coming from?

We are as contaminated as the laboratory. (CLAIR PATTERSON, CALTECH ORAL HISTORY, 1995)

In 1965 he published that Americans carried about a hundred times the natural amount of lead in their bodies, most of it from industry, above all from leaded petrol in the air. Later he measured it in the snow of Greenland, layer by layer back through time: modern snow held more than a hundred times the lead of snow from before the industrial age.

"Wham!" he recalled. "They stopped my research." He lost the funding he had from the oil companies.


---

*10 · Unleaded*

*1973 TO 2021*

## Taking it back out.

In 1973 the United States Environmental Protection Agency ordered the lead in petrol cut, step by step. The industry sued and lost. From 1 January 1996, leaded petrol was banned for American cars. Between the late 1970s and the early 1990s, the average level of lead in Americans&\#39; blood fell by 78 percent.

A 2022 study estimated that more than 170 million Americans alive in 2015, over half the country, had high levels of lead in early childhood, and that lead had cost Americans alive in 2015 a combined 824 million IQ points. Most of that lead came from petrol.

Country after country followed. In July 2021 Algeria, the last country still selling it, stopped selling leaded petrol for cars. It had been on sale for 98 years.

> **STILL LEADED**: The worldwide end was for road vehicles. Petrol for small piston aircraft is still leaded. The 824 million counts lead from every source, not only petrol.


---

*11 · The cold*

*1928 TO 1930*

## A gas that would not burn.

Electric refrigerators of the late 1920s ran on ammonia, sulfur dioxide or methyl chloride: gases that were toxic, or flammable, or both. In 1928 Kettering asked Midgley for a new one. Midgley went to his periodic table, which he carried with him everywhere, and decided the answer had to contain fluorine.

With Albert Henne and Robert McNary he made dichlorodifluoromethane: one carbon atom, two chlorine, two fluorine. It nearly failed. Of the first three or four small batches, only the first was harmless to the test animals; the others killed them. An impurity, it turned out, not the gas.

When he presented it in 1930, Kettering later wrote, he filled his lungs with the vapour and breathed it out over a candle. The candle went out. It was safe to breathe and would not burn. It was sold as Freon.

> **THE MYTH**: Freon is often said to have been invented after a refrigerant leak killed more than a hundred people at the Cleveland Clinic in 1929. That fire was burning X-ray film, and the work on Freon had begun the year before.


---

*12 · Up*

*THE STRATOSPHERE*

## One atom, over and over.

Freon went into refrigerators, air conditioners, aerosol cans and foam. Its great virtue was that nothing could break it. So it rose, unbroken, for years, until it reached the ozone layer, about 15 to 35 kilometres up, the thin shield that stops most of the Sun&\#39;s harmful ultraviolet light.

Up there the ultraviolet light is strong enough to break it, and it lets go of its chlorine. A chlorine atom takes an oxygen atom from an ozone molecule, then hands it to a free oxygen atom, and is free to do it again. According to the US Environmental Protection Agency, one chlorine atom can destroy over 100,000 ozone molecules before it is removed.

> **THE NUMBER**: CFC-12, Midgley&\#39;s gas, lasts about 102 years in the air. All the ozone in the atmosphere, brought down to the pressure at the ground, would make a layer about 3 millimetres thick.


---

*13 · The hole*

*HALLEY, ANTARCTICA, 1985*

## A hole in the sky.

In June 1974 two chemists, Mario Molina and Sherwood Rowland, published the warning: CFCs would rise to the ozone layer and their chlorine would wear it away. Midgley had been dead for nearly thirty years.

In May 1985 three scientists of the British Antarctic Survey, Joe Farman, Brian Gardiner and Jonathan Shanklin, reported that every spring the ozone over Antarctica was falling far below normal. NASA&\#39;s satellite maps soon showed the loss spread across the whole continent.

- YEAR: 1979 to 2006
- OZONE HOLE: 0.1 to 26.6 MILLION KM²

> **Readouts**: Its mean area each spring, from 0.1 million square kilometres in 1979 to 26.6 million in 2006, NASA Ozone Watch

> **THE DATA**: Each year&\#39;s hole is NASA Ozone Watch&\#39;s average for 7 September to 13 October, drawn at its true area; its shape on the globe is illustrative. There is no satellite record for 1995, so the readout passes from 1994 straight to 1996. The air shells are drawn far above the surface so they can be seen.


---

*14 · Montreal*

*16 SEPTEMBER 1987*

## The world said yes.

Two years after the hole was found, governments signed the Montreal Protocol to cut CFCs; strengthened again and again, it phased them out. On 16 September 2009 it and the Vienna Convention became the first treaties in the history of the United Nations to be ratified by every country. In 1995 Molina, Rowland and Paul Crutzen shared the Nobel Prize in Chemistry.

Production of nearly 99 percent of the banned substances has stopped. The hole still opens every southern spring, but the chlorine is falling. The 2022 assessment by the World Meteorological Organization and the UN Environment Programme projects the ozone over Antarctica back to its 1980 level around 2066.

- YEAR: 2006 to 2025
- OZONE HOLE: 26.6 to 18.7 MILLION KM²

> **Readouts**: Its mean area each spring, from 26.6 million square kilometres in 2006 to 18.7 million in 2025, NASA Ozone Watch

> **WHY IT DIPS**: Small holes like 2019&\#39;s come from warm stratospheric winters, not a sudden cure. The trend is the slow fall in chlorine.


---

*15 · The harness*

*WORTHINGTON, OHIO, 1944*

## A lot of living in a little while.

Midgley never knew what Freon did to the sky. He was showered with honours: the Perkin Medal, the Priestley Medal, the presidency of the American Chemical Society in 1944. In 1940, aged 51, he caught polio and lost the use of his legs. He reckoned the odds of it as drawing one particular card from a stack of cards as high as the Empire State Building. "It was my tough luck to draw it."

He built himself a harness of ropes and pulleys to get in and out of bed. In September 1944 he ended an address with a verse he called his epitaph:

This one did a lot of living in a mighty little while. (THOMAS MIDGLEY JR., SEPTEMBER 1944)

On 2 November 1944 he was found strangled in the harness. He was 55. The first reports called it an accident. His death certificate said suicide.

> **WHAT IS KNOWN**: Suicide carried a heavy stigma in 1944, and his family and colleagues did not speak of it in public; books about him mentioned it only decades later. Why he died is not known. It cannot have been the ozone layer: nobody knew of the harm until 1974.


---

*16 · The air*

*THE AIR*

## It belongs to everyone.

Two inventions, both made to solve a real problem, and both did. The harm that lasted was in the air, and the air belongs to everyone.

In 2021 the last leaded petrol for cars was sold. The ozone hole still opens every southern spring; the chlorine behind it is falling, and the ozone over Antarctica is projected back around 2066.

The historian J. R. McNeill wrote that Midgley "had more impact on the atmosphere than any other single organism in earth history."

It took the whole world to stop.

## Sources & notes

1. [William Kovarik, "Charles F. Kettering and the 1921 discovery of tetraethyl lead", SAE paper 941942 (1994), web edition: Midgley set on knock in 1916, iodine, aniline, selenium and tellurium, 9 December 1921 and the silenced engine, the compound counts from 143 to 33,000 and the 24 tested compounds, the missing notebooks, the name Ethyl, Midgley's letters of 1922 and 1923 ("my lungs have been affected", "almost impossible", "fanatical health cranks")](https://environmentalhistory.org/people/charles-f-kettering-and-the-1921-discovery-of-tetraethyl-lead/) (secondary)
2. [William Kovarik, "Ethyl-leaded gasoline: how a classic occupational disease became an international public health disaster", International Journal of Occupational and Environmental Health 11 (2005): the plant deaths and Kovarik's higher count, the House of Butterflies, Joseph Leslie and who in his family knew, the press conference of 30 October 1924 as reported by the New York Times, Alice Hamilton's words to the New York World, the 1926 warning that the investigation must not be allowed to lapse, Patterson and Kehoe](https://environmentalhistory.org/ijoeh-lead/) (secondary)
3. [Jamie Lincoln Kitman, "The Secret History of Lead", The Nation (20 March 2000): Pierre du Pont's letter of March 1922, Midgley's "My dear boss" letter of March 1923, 26 Broadway and "Nor would I take any chance doing that every day", Frank Howard's "apparent gift of God" from the 1925 conference transcript, Yandell Henderson, the 1926 committee and its warning, industry-funded research](https://chemistry.beloit.edu/classes/Chem117/lead/The%20Secret%20History%20of%20Lead.pdf) (reporting)
4. [Dietmar Seyferth, "The Rise and Fall of Tetraethyllead. 2", Organometallics 22 (2003): Hochwalt made the sample, Boyd fed it into the engine](https://pubs.acs.org/doi/10.1021/om030621b) (secondary)
5. [William Kovarik, lead history timeline: the deaths at Dayton, Deepwater and Bayway, plant by plant](https://environmentalhistory.org/lead-history-timeline/) (secondary)
6. [Deborah Blum, "Looney Gas and Lead Poisoning: A Short, Sad History", Wired (5 January 2013): Bayway and the New York medical examiner](https://www.wired.com/2013/01/looney-gas-and-lead-poisoning-a-short-sad-history/) (reporting)
7. [Clair C. Patterson, interviewed by Shirley K. Cohen, Caltech Archives oral history (March 1995): the contaminated laboratory, "We are as contaminated as the laboratory", "Wham! They stopped my research", lost oil company funding](https://digital.archives.caltech.edu/collections/OralHistories/OH_Patterson_C/OH_Patterson_C.pdf) (primary)
8. [George R. Tilton, "Clair Cameron Patterson", National Academy of Sciences Biographical Memoir: the age of the Earth, the 1965 paper, Greenland snow with over 100 times preindustrial lead](https://www.nationalacademies.org/read/6201/chapter/16) (secondary)
9. [C. C. Patterson, "Contaminated and natural lead environments of man", Archives of Environmental Health 11 (1965)](https://pubmed.ncbi.nlm.nih.gov/14334042/) (primary)
10. [US EPA, "EPA Requires Phase-Out of Lead in All Grades of Gasoline" (28 November 1973)](https://www.epa.gov/archive/epa/aboutepa/epa-requires-phase-out-lead-all-grades-gasoline.html) (primary)
11. [US EPA, "EPA Takes Final Step in Phaseout of Leaded Gasoline" (29 January 1996): the ban from 1 January 1996, leaded fuel still allowed for aircraft and some other engines](https://www.epa.gov/archive/epa/aboutepa/epa-takes-final-step-phaseout-leaded-gasoline.html) (primary)
12. [J. L. Pirkle and colleagues, "The decline in blood lead levels in the United States", JAMA 272 (1994): a 78 percent fall from 1976 to 1980 to 1988 to 1991](https://pubmed.ncbi.nlm.nih.gov/8028141/) (primary)
13. [M. J. McFarland, M. E. Hauer and A. Reuben, "Half of US population exposed to adverse lead levels in early childhood", PNAS 119 (2022): over 170 million people, 824,097,690 IQ points, leaded gasoline the dominant source from the 1940s to the late 1980s](https://doi.org/10.1073/pnas.2118631119) (primary)
14. [UN Environment Programme, "Era of leaded petrol over" (30 August 2021): Algeria, July 2021, the last country](https://www.unep.org/news-and-stories/press-release/era-leaded-petrol-over-eliminating-major-threat-human-and-planetary) (primary)
15. [Carmen J. Giunta, "Thomas Midgley, Jr., and the Invention of Chlorofluorocarbon Refrigerants: It Ain't Necessarily So", Bulletin for the History of Chemistry 31 (2006): the refrigerants of 1929, work begun in 1928, the Cleveland Clinic myth, the death certificate, the stigma of suicide](https://acshist.scs.illinois.edu/bulletin_open_access/FullIssues/bhc2006v031f2.pdf) (secondary)
16. [Charles F. Kettering, "Thomas Midgley, Jr., 1889 to 1944", National Academy of Sciences Biographical Memoirs 24 (1947): Cornell, 1916, the periodic table he carried, fluorine, the toxic batches, the candle, the medals, polio and the odds, the epitaph](https://www.nasonline.org/wp-content/uploads/2024/06/midgley-thomas.pdf) (secondary)
17. [Science History Institute, "Portrait of Thomas Midgley, Jr. (1889 to 1944)", Blank & Stoller, New York, 1930s, Williams Haynes Portrait Collection: the photograph, rights statement No Known Copyright](https://digital.sciencehistory.org/works/9s161624t) (archival copy)
18. [US Patent 1,833,847, "Heat Transfer", Midgley, Henne and McNary, filed 8 February 1930](https://patents.google.com/patent/US1833847A/en) (primary)
19. [Cleveland Clinic, history of the 1920s: the 1929 disaster caused by nitrocellulose X-ray film](https://my.clevelandclinic.org/about/history/1920s) (primary)
20. ["Milestones, Nov. 13, 1944", Time: the death reported as accidental strangulation by a self-devised harness](https://time.com/archive/6603954/milestones-nov-13-1944/) (reporting)
21. [US EPA, "Basic Ozone Layer Science": CFCs reach the stratosphere in years, one chlorine atom can destroy over 100,000 ozone molecules](https://www.epa.gov/ozone-layer-protection/basic-ozone-layer-science) (secondary)
22. [WMO and UNEP, Scientific Assessment of Ozone Depletion: 2022, Twenty Questions: the ozone layer at about 15 to 35 km, about 3 mm of ozone, CFC-12 lifetime of 102 years](https://csl.noaa.gov/assessments/ozone/2022/twentyquestions/) (primary)
23. [Mario J. Molina and F. S. Rowland, "Stratospheric sink for chlorofluoromethanes: chlorine atom-catalysed destruction of ozone", Nature 249 (28 June 1974)](https://doi.org/10.1038/249810a0) (primary)
24. [J. C. Farman, B. G. Gardiner and J. D. Shanklin, "Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction", Nature 315 (16 May 1985)](https://doi.org/10.1038/315207a0) (primary)
25. [P. K. Bhartia and R. D. McPeters, "The discovery of the Antarctic Ozone Hole", Comptes Rendus Geoscience 350 (2018): NASA's satellite confirmation](https://comptes-rendus.academie-sciences.fr/geoscience/item/10.1016/j.crte.2018.04.006.pdf) (secondary)
26. [NASA Ozone Watch, annual Antarctic ozone hole data, 1979 to 2025 (mean area 7 September to 13 October; no data for 1995)](https://ozonewatch.gsfc.nasa.gov/statistics/annual_data.html) (primary)
27. [UN Ozone Secretariat, the Montreal Protocol: universal participation on 16 September 2009, the first treaties in UN history to achieve it](https://ozone.unep.org/treaties/montreal-protocol-substances-deplete-ozone-layer/introduction) (primary)
28. [The Nobel Prize in Chemistry 1995, press release: Crutzen, Molina and Rowland](https://www.nobelprize.org/prizes/chemistry/1995/press-release/) (primary)
29. [WMO, "Ozone layer recovery is on track" (9 January 2023): the phase-out of nearly 99 percent of banned substances, recovery around 2066 over the Antarctic](https://wmo.int/news/media-centre/ozone-layer-recovery-track-helping-avoid-global-warming-05degc) (primary)
30. [J. R. McNeill, Something New Under the Sun: An Environmental History of the Twentieth-Century World (Norton, 2000), as quoted in Michael Bess's review, Journal of Political Ecology 9](https://web.archive.org/web/2004/http://dizzy.library.arizona.edu/ej/jpe/volume_9/1101bess.html) (analysis)

What is drawn. The engine, the molecules and the Earth are drawings made for this story, not reconstructions of particular objects. The cylinder shows how knock works: the stroke timing is simplified, and the second, knocking flame is an illustration of the end gas going off before the flame front arrives, not a simulation. The molecules are ball-and-stick models with the published shapes (tetraethyllead and CFC-12 tetrahedral around their central atom, ozone bent at about 117 degrees); the sizes of the atoms are not to scale. The row of ozone broken by one chlorine atom shows the catalytic cycle, not its scale: the US Environmental Protection Agency gives over 100,000 ozone molecules for one chlorine atom, and nine are drawn. On the Earth (NASA Blue Marble) the shells of air are lifted far above the surface so they can be seen: the ozone layer lies about 15 to 35 kilometres up. The grains of lead and the rising CFCs show where the cars and the refrigerators were, not measured concentrations. The ozone data. Each year's hole is NASA Ozone Watch's mean area from 7 September to 13 October (table updated 20 October 2025); the readout holds each year's own value, and the drawing eases from one year to the next. There is no satellite record for 1995, which is skipped. The small holes of 2002 and 2019 came from warm stratospheric winters, not from a sudden recovery. The 2026 season is not in the table yet. The recovery years are those of the 2022 WMO and UNEP assessment; the next is due at the end of 2026. Where the sources differ. How many compounds were tried before tetraethyllead: accounts range from 143 to 33,000, and the laboratory notebooks are lost. The first test is dated 9 December 1921 by most accounts, 8 December by one. The deaths at the plants: five at Bayway in the press of October 1924, seven by William Kovarik's count; eight at Deepwater by mid-1925, ten by Kovarik's; the story gives the counts reported at the time and says when they differ. The quotations of 1922 to 1925 come to us through the historian William Kovarik and the journalist Jamie Kitman, who read the letters, transcripts and newspapers. That Midgley held the bottle under his nose for about a minute is Kovarik's account of the New York Times report; the words "Nor would I take any chance doing that every day" are as Kitman quotes them, without a date or a newspaper. The candle demonstration is as Kettering's memoir of 1947 describes it (Time in 1943 said a lighted paper), and that memoir is said to have been written for Kettering by Thomas Boyd. His death. Time and the New York Times reported an accident; his death certificate, as Carmen Giunta found in the archives, gives suicide. A local paper reportedly carried the coroner's ruling of suicide a week later; we have not seen it. Why he died is not known. The numbers. The 824 million IQ points (McFarland and colleagues, 2022) count lead from every source, most of it from petrol in those decades. Leaded fuel ended for road vehicles in 2021; small piston aircraft still burn it. The Montreal Protocol shares its record of universal ratification with the Vienna Convention. J. R. McNeill's line is quoted as Michael Bess's review gives it. The story says petrol, as Awescroll does; the quotations keep the gasoline of the time.

*Photograph: Thomas Midgley Jr., Blank & Stoller, New York, 1930s, courtesy of Science History Institute (no known copyright)*

*Earth: NASA Blue Marble, public domain*

*Ozone data: NASA Ozone Watch, annual Antarctic ozone hole areas, 1979 to 2025*

*Model: Drawn for this story: the engine is a diagram of the idea, the molecules follow their published structures, and the atmosphere is not to scale*

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