The sky is our atmosphere, and without it, we wouldn't be here. The lower the atmosphere is to the surface of the earth, the thicker it is, and that's what we breathe and where we live. Pollution is there, but there is also the normal pressures and temperatures to support life. The farther away from the earth's gravitational pull, the thinner it gets. People who live in high altitude, such as in the Rocky Mountains, are used to a thinner atmosphere (slightly), so they have extra red blood cells. When those of us who live at lower altitudes travel to a place where the elevation is thousands of feet higher, we will sometimes feel short of breath for a day or so until our bodies produce more red blood cells in response to needing more oxygen to the cells. That's what some athletes do before large competitions. They train in cities like Denver or Mexico City, which are high in the mountains, so their bodies produce more RBCs, enabling more oxygen to be carried to their muscles faster..and it is legal.