50 to 85% of the oxygen on Earth comes from the ocean, mainly from phytoplankton.
The noble gas Xenon can be used as a near-perfect general anesthetic. It works fast, has minimal side effects, protects your brain, is environmentally friendly, and can be used at normal air pressure. The only downside is it's expensive and can't be synthesized.
When helium is cooled to almost absolute zero, the lowest temperature possible, it becomes a liquid with surprising properties. It can flow against gravity and will start running up and over the lip of a glass container.
Breathing 100% pure oxygen for more than 16 hours can lead to irreversible lung damage, damage to the retinas, and eventually, death.
Radon (a radioactive gas that can cause lung cancer) baths exist in the town of Khmilnyk, Ukraine. They advertise to treat a host of medical conditions and around 50,000 people a year use them.
Too much carbon dioxide in a crowded room can make you dumber.
Carbon Monoxide is used in the USA to make meat appear fresher. This practice is banned in Canada, Japan, Singapore, and the European Union.
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The air used in bags of chips is actually Nitrogen, which keeps the chips from going stale.
In 2014, Chlorine gas was released in a Hyatt Regency hotel, Illinois hosting a FurFest convention, injuring 19 people. The apparent chemical attack is unsolved, and the source of the chlorine is still unknown.
When Radon gas in American homes was discovered, nuclear plant employees were tested for radiation and one of them was more radioactive when he came to work in the morning than when he left in the evening.