Question

What is carbon dioxide that humans breathe out produced by

Answer 1

It is produced by breathing in one carbon atom and two oxygen atoms.

Answer 2

The food and drink we eat is one of the by-product  of carbon compounds when it reacts with oxygen in the cells it produces carbon dioxide and water plus energy. We use the energy and the carbon dioxide is breathed out as gas.

The oxygen and glucose are produced in mitochondria of cells. Glucose contains carbon. That is where the carbon in carbon dioxide comes from. Glucose plus oxygen produces carbon dioxide, water and energy. When this process stops, the cell dies.