Why is free oxygen a problem for many organisms

All living things need nitrogen to build proteins and other important body chemicals. However, most organisms, including plants, animals and fungi, cannot get the nitrogen they need from the atmospheric supply ... Nitrogen is returned to the atmosphere by the activity of organisms known as decomposers.

Oxygen is essential for respiration because the body uses it to 'burn' food molecules. Animals take in oxygen when inhaling and give off carbon dioxide when exhaling ... This is why rescue breathing works - it can supply a non breathing patient with enough oxygen to support life. Nitrogen is essential for all living things because it is a major part of amino acids, which are the building blocks of proteins and of nucleic acids such as DNA, which transfers genetic information to subsequent generations of organisms.