Below the atmosphere, we have a little problem called global warming, or just in general high levels of pollution for you non-believers, which is the general degradation of our atmosphere and lakes and oceans due to excessive amounts of waste, brought on by agregious practices and poor waste management. In space, there's Kessler syndrome, the hypothetical scenario where, when the amount of space debris orbiting our planet becomes over-saturated, various "leftovers" from spacecraft will collide and split apart, going on to hit even more debris creating a cascade of small but dangerous shrapnel that will make travelling through low-earth orbit an unfeasible, and at the very least highly difficult, affair.
Just like below our atmosphere's limits, we need to be concerned about pollution. While space pollution has a much smaller influence (space is big, and the chances of hitting something are slim), it is still dangerous and costly, but much of the time preventing this pollution is difficult, as it requires a lot of energy to bring a empty fuel tank or decoupler back down into the atmosphere. But, for the moment, it isn't too serious, so in case any of you were actually planning a mission into the grasps of space, don't get anxious.