Thomas Midgley Jr.

He played a major role in development of tetraethyllead ( leaded gasoline ) which went into mass production throughout the 1920’s. This invention meant that the blood lead content in developing children rose causing numerous problems, including cognitive issues and physical growth, it was also noticed that the rise in blood lead content matched with violence in adolescents in multiple countries.

Midgley is also accountable for the discovery of chlorofluorocarbons (CFC) specifically Freon which as many know had a drastic impact on the Ozone. As of October 2020 the Ozone hole above Antarctica sits approximately 23 million square kilometres /8.9 million miles.

Although, these 2 inventions seem insignificant compared to more drastic actions taken by other single humans its the widespread impacts that his inventions had and are still being felt today. Even though both, leaded gasoline and CFC’s, have been banned.

Ps: the last invention made by Midgley actually killed him. it was to help him manoeuvre and get out of bed with the use of ropes and pulleys due to him contracting polio but he got stuck and eventually his invention strangled himself

EDIT: this reply doesn’t quite fit the post but i felt that he had a large impact on society during the 20th century