Fighting Smallpox with Inoculations
Ingenhousz became increasingly interested in inoculation as a defense against smallpox – the disease killed about a third of those it infected, and epidemics were common.

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Plants Convert Bad Air to Good Air
Ingenhousz did over 500 experiments in the summer months of 1779, working morning until night. He discovered that plants need sunlight to convert “bad air” to “good air”, or in modern terms, covert carbon dioxide to oxygen. Using heat from a fire in the absence of sunlight he showed that light, not heat is needed for plants to produce oxygen.

Plants Release Oxygen from Undersides of Leaves
Moreover, Ingenhousz correctly identified that sunlight causes plants to release oxygen from the undersides of their leaves. He carried out his experiments by placing leaves in water and collecting the gas bubbles they released. He tested a large number of different plants, including aquatic plants, which he found to be exceptionally good at generating oxygen.

Plants Release Carbon Dioxide in Darkness
Soon after his discovery that in sunlight green plants convert carbon dioxide to oxygen, he found that in darkness or heavy shade, they do the opposite: like animals, they take in oxygen and release carbon dioxide. Overall, the rate that plants make oxygen in sunlight is much greater than the rate they consume it in darkness."

https://www.famousscientists.org/jan-ingenhousz/