By measuring the bubbles in amber caused by the flapping of insects in tree sap, one can measure oxygen content over entire evolutionary expanses. We are currently at some of the lowest oxygen contents in history. I standardized to atmospheric oxygen (20%) in the late 1970s and early 1980s, but high altitude cities with pollution and thermal inversions were reporting 12 to 15% oxygen content, even then. When oxygen is depleted enough by burning fossil fuels and generating smothering carbon dioxide, there won't be enough oxygen left for efficient burning, and carbon monoxide will be the result. Carbon monoxide poisons the hemoglobulin that carries oxygen to all our tissues, CO being a deadly gas that quickly kills all mammalian species.