We not only live in the air, we live because of it.
Air is about much more than breathing.
It is a transformer and a protector, though ultimately also a poison.
It wraps our planet in a blanket of warmth.
It brings us wind and rain and fire.
It sustains our bodies and at the same time it burns them up, slowly, from the inside.
In this episode, Gabrielle Walker investigates the good side - and the bad - of two components of air: carbon dioxide and oxygen.
Carbon Dioxide makes up a tiny fraction of one percent of air, yet it at once protects, transforms and threatens life on Earth.
CO2 is infamous for its contribution to the greenhouse effect that is causing global warming.
But without it we would both freeze and starve.
It is also the basis of everything we eat.
The mass of all plants and hence the creatures that feed on them comes from carbon dioxide.
Billions of years ago, as the young Sun began to warm, bacteria and primitive algae began their insulating blanket, fossilising the air as limestone, coal and chalk.
Now we are releasing that carbon to the air again, double-glazing the global greenhouse.
The greatest transformer in air is oxygen.
It is the giver and taker of life.
Without it living things cannot be vigorous - or larger than a pinhead.
Yet it is also the bringer of death.
When bacteria started releasing it as a waste gas, a billion or more years ago, it was the worst pollution incident in the history of the planet.
Life was forced to hide or evolve.
Even though we have adapted to depend on oxygen, we are playing with fire.
In a slow and mostly controlled way, oxygen burns up the food we eat.
It also chars the genes, molecules and cells within us, bringing about ageing and, ultimately death.
Producer: Martin Redfern.
Gabrielle Walker explores how our atmosphere both protects and destroys us.