Thomas Midgley Jr. (May 18, 1889 – November 2, 1944) was an American mechanical and chemical engineer. He played a major role in developing leaded gasoline (tetraethyl lead) and some of the first chlorofluorocarbons (CFCs), better known in the United States by the brand name Freon; both products were later banned from common use due to their harmful impact on human health and the environment. He was granted more than 100 patents over the course of his career.

Midgley contracted polio in 1940 and was left disabled; in 1944, he was found strangled to death by a device he devised to allow him to get out of bed unassisted. It is often reported that he had been accidentally killed by his own invention, but his death was declared by the coroner to be a suicide.

While the harmful effects of CFCs were not appreciated until decades after Midgley's death, tetraethyl lead was known to be acutely toxic by those involved in the development of leaded gasoline. This included Midgley, who publicly insisted that there was nonetheless no health hazard posed by the use of leaded gasoline in internal combustion engines.

Thomas Midgley Jr. was born in Beaver Falls, Pennsylvania on May 18, 1889, the son of Hattie Louise (née Emerson) (1865 – 1950) and Thomas Midgley Sr. (1840 – 1934). His family had a history of inventing; his father was an inventor in the field of automobile tires while his maternal grandfather, James Emerson, invented the inserted tooth saw. He grew up in Columbus, Ohio and graduated from Cornell University in 1911 with a degree in mechanical engineering.

Early on, Midgley had a penchant for finding useful applications for known substances. In high school, he used the chewed bark of slippery elm trees to give baseballs a more curved trajectory, a practice professional players would later pick up. Later in life, he was known to always carry a copy of the periodic table, his main tool in the search for the substance that would mark his breakthrough invention.

In 1916, Midgley began working at General Motors. In December 1921, while working under the direction of Charles Kettering at Dayton Research Laboratories, a subsidiary of General Motors, he discovered (after discarding tellurium due to the difficult-to-eradicate smell) that the addition of tetraethyllead (TEL) to gasoline prevented knocking in internal combustion engines. The company named the substance "Ethyl", avoiding all mention of lead in reports and advertising. Oil companies and automobile manufacturers (especially General Motors, which owned the patent jointly filed by Kettering and Midgley) promoted the TEL additive as an inexpensive alternative superior to ethanol or ethanol-blended fuels, on which they could make very little profit. In December 1922, the American Chemical Society awarded Midgley the 1923 Nichols Medal for the "Use of Anti-Knock Compounds in Motor Fuels". This was the first of several major awards he earned during his career.

In 1923, Midgley took a long vacation in Miami to cure himself of lead poisoning. He said, "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." That year, General Motors created the General Motors Chemical Company (GMCC) to supervise the production of TEL by the DuPont company. Kettering was elected as president with Midgley as vice president. However, after two deaths and several cases of lead poisoning at the TEL prototype plant in Dayton, Ohio, the staff at Dayton was said in 1924 to be "depressed to the point of considering giving up the whole tetraethyl lead program". Over the course of the next year, eight more people died at DuPont's plant in Deepwater, New Jersey. In 1924, dissatisfied with the speed of DuPont's TEL production using the "bromide process", General Motors and the Standard Oil Company of New Jersey (now known as ExxonMobil) created the Ethyl Gasoline Corporation to produce and market TEL. Ethyl Corporation built a new chemical plant using a high-temperature ethyl chloride process at the Bayway Refinery in New Jersey. However, within the first two months of its operation, the new plant was plagued by more cases of lead poisoning, hallucinations, insanity, and five deaths.

The risks associated with exposure to lead have been known at least since the 2nd century BC, while efforts to limit lead's use date back to at least the 16th century. Midgley experienced lead poisoning himself, and was warned about the risk of lead poisoning from TEL as early as 1922. Midgley well knew the hazards of lead. He investigated whether the risks, both in production and use, could be managed. Testing on the exhaust was completed, which he used to support the idea that 1 part tetraethyl lead per 1300 of gasoline could safely be used. After the initial worker exposures, controls were developed to allow the process to operate safely. Leaded gasoline use grew exponentially. The cumulative chronic impacts of environmental lead were grossly underestimated.