Breathing

Breathing is the most basic human function required to sustain existence, as life without air is measured in moments.  Pure air is a mixture of invisible and odorless gases, mainly nitrogen and oxygen with small amounts of water vapor, argon, carbon dioxide, neon, helium and hydrogen.   In launching to space, scuba diving, climbing to the highest elevation on the globe, or searching for coal within the deepest crevices of the earth, this mixture is life's elixir. 
The one universal tool that stretches across all of these endeavors is air, and the source of that air must be carefully calculated.  In daily life most of us take our source of air for granted, inhaling and exhaling with little thought of what we are actually pulling into our bodies.  On average most adults breathe roughly 35 pounds of air per day, or around 20,000 breaths, and children breathe almost twice that amount, as they are smaller and still developing their respiratory systems.  More air enters the body and the bloodstream than any other substance. 
Though it plays such a vital role in so many facets, it is free, invisible, and unconsciously consumed and therefore is often taken for granted.  It is easy for mankind to look up at the sky and marvel at the vastness, imagining an endless air supply that stretches off into space. 
However breathing space only stretches somewhere between 5 and 9 miles above the earth's surface.  This is where the troposphere (the layer of the atmosphere than contains all things which combine to make earth's weather) ends and the stratosphere begins.  A thought to consider is that air is something that cannot be created,  it can only be recycled.  Mother Nature has played the largest role in the history of air pollution to this point, with volcanic eruptions, dust storms, and forest fires, but there is a new player on the horizon in modern times.