Chemistry has fed billions of people and poisoned millions. Sometimes the same discovery, or the same person, did both. This level looks at four cases from the last century and at the ideas people now use to weigh chemical benefits against chemical harms.

Bread from air

Plants need nitrogen, and although air is nearly four fifths nitrogen, its triple bond makes it useless to most plants. Around 1900 farmers relied on manure, guano and nitrate mined in Chile, and in 1898 the British chemist William Crookes warned that the world would soon run short of wheat. In 1909 the German chemist Fritz Haber showed how to combine nitrogen from air with hydrogen, under high pressure and with a catalyst, to make ammonia. Carl Bosch at the company BASF turned the lab method into an industrial process, and the first plant opened in 1913. High pressure helps because, as Le Chatelier's principle predicts, it pushes the reaction toward the side with fewer gas molecules.

The Haber-Bosch process makes ammonia from the nitrogen in air, and the fertilizer it supplies helps feed roughly half of humanity, by common estimates. It also has costs. It uses a small but significant share of the world's energy, and excess fertilizer washes into rivers and coastal waters, feeding algae that strip oxygen from the water. And ammonia makes explosives as easily as fertilizer: Haber-Bosch let Germany keep producing munitions in the First World War after a blockade cut off Chilean nitrate.

Gas

Haber was a fervent patriot. He led Germany's chemical weapons program, and on 22 April 1915 at Ypres, German troops released chlorine gas that drifted over Allied lines. His wife, Clara Immerwahr, one of the first women to earn a doctorate in chemistry in Germany, took her own life days after he returned from overseeing a gas attack; her reasons remain debated. Haber received the 1918 Nobel Prize in Chemistry for ammonia synthesis.

After the war, revulsion at gas led to the 1925 Geneva Protocol. The 1925 Geneva Protocol banned the use of chemical weapons in war, but not their production or stockpiling. Nations kept building them. The Chemical Weapons Convention, in force since 1997, bans the development, production and stockpiling of chemical weapons and requires their destruction, checked by inspectors of the Organisation for the Prohibition of Chemical Weapons. The organisation won the Nobel Peace Prize in 2013. By 2023 every declared stockpile had been destroyed, though chemical attacks in Syria and poisonings with nerve agents in recent years show the ban is not self-enforcing.

Lead in the tank

In 1921 Thomas Midgley, an engineer at General Motors, found that adding tetraethyl lead to gasoline stopped engines from knocking. In 1924 workers at plants making the additive went mad and died of lead poisoning, and critics warned about spreading lead through cities. The industry argued that lead was safe at the levels involved. Over the following decades scientists showed that lead had spread across the world, and that even low levels of lead harm the developing brains of children. Countries began phasing out leaded gasoline in the 1970s. Algeria, the last country selling leaded gasoline for cars, stopped in 2021.

A hole in the sky

In 1928 Midgley's team developed chlorofluorocarbons, or CFCs, as refrigerants. They were nontoxic and did not burn, a real improvement on the dangerous gases in early refrigerators. Their stability turned out to be the danger. The stability of CFCs let them drift intact up to the stratosphere, where ultraviolet light breaks them apart and releases chlorine. In 1974 Mario Molina and Sherwood Rowland explained the threat: a single chlorine atom can destroy many thousands of ozone molecules, because it acts as a catalyst and emerges unchanged from each cycle. The ozone layer screens out ultraviolet light that causes skin cancer and harms crops. In 1985 scientists of the British Antarctic Survey reported a deep seasonal loss of ozone over Antarctica. Nations responded with the Montreal Protocol of 1987, which phased out CFCs. The ozone layer is now slowly healing.

Weighing risk

How should societies handle chemicals whose harms may appear decades later? One answer is the precautionary principle: when an activity threatens serious harm, a lack of full scientific certainty should not be a reason to delay protective measures. Its defenders point to lead and CFCs, where waiting for proof cost dearly. Critics reply that strict caution can block useful products and that a banned chemical's replacement may be worse. A more recent approach, set out by Paul Anastas and John Warner in 1998, is green chemistry: designing products and processes to avoid hazardous substances in the first place, rather than cleaning up afterward.

Worked example: one inventor, two inventions

Thomas Midgley solved two real problems. Knocking engines were inefficient, and early refrigerators leaked toxic gases into kitchens. Both his solutions were praised in his day, and both caused harm on a scale he never foresaw. One historian later remarked that he had more impact on the atmosphere than any other single organism in Earth's history, a judgment that is debated but makes the point.

The two cases teach different lessons. With lead, the danger was known in outline from the start, and warnings were overridden by commercial interest. With CFCs, the danger was genuinely hidden in the very stability that made them attractive. Good chemistry policy needs both honesty about known hazards and humility about unknown ones, with special care for those least able to protect themselves, such as children and people not yet born.