Oxygen depletion of the environment
All activity requires energy, which normally involves the combustion of oxygen, the indispensable element of life and nature. The present amount of human activity is such that it involves a greater combustion of oxygen than the production of oxygen by plant chlorophyll is capable of producing; at the same time water pollution and destruction of green spaces is causing the destruction of oxygen-creating agents, phytoplankton in water (which at present produces 70% of the oxygen in air) and vegetation on land (which produces 30% of air oxygen). The risk now exists of more oxygen being used than is actually produced.
The accelerating process of the destruction of oxygen is such that some scientists predict the combustion of all oxygen in the atmosphere in less than 2,000 years.