The atmosphere of Earth is really thin!

Earth drawn to scale. Human life is only possible within the thin light blue line...

The atmosphere of Earth is often shown in illustrations (that are not drawn to scale) as a vast space, suggesting that it is a practically endless resource. In reality though, the zone that is potentially habitable by humans is very limited, between sea level and 8000 meters. (That is the extreme limit, where the "death zone" begins. Actually, for most people it's difficult to acclimatize already above 2100 meters.)

So avoid pollution and take care of the atmosphere – it's our only one, and we all share it.

Two examples from the web, where Earth's atmosphere is not shown in its actual proportions. (These were intended to illustrate the layers of the atmosphere.)