Somewhere between 1.8 million and 12,000 years ago, our ancestors mastered the craft of fire building. Anthropologists often cite this event as the spark that truly allowed us to become human, giving us the means to cook, keep warm and forge tools. But fire also marked another important first for us: the invention of man-made pollution.

Pollution, by definition, is something introduced into the environment that harmfully disrupts it. While nature sometimes produces its own damaging contaminants – wildfires send up billows of smoke and ash, volcanoes belch noxious gases – humans are responsibile for the lion’s share of the pollution plaguing the planet today.

Wherever we go, we seem to have a knack for leaving our rubbish and waste behind. Visit even the most remote outpost on the planet and you will witness this first hand. Shredded tyres and plastic bottles punctuate the vast expanse of the Gobi desert; plastic bags ride the currents in the middle of the Pacific; and spent oxygen canisters and raw sewage mar the snows of Mount Everest.

Still, the world is a big place. Might there be some last holdouts free from the taint of our pollution? Answering that question works best if we break down the environment into four realms – the sky, land, freshwater and ocean.

Sky and land

Air pollution comes in many forms. Smog is mostly composed of particulate matter and ozone – a greenhouse gas that forms when nitrogen oxides and volatile organic compounds produced by cars and industrial plants react together in the presence of sunlight. And its impact on human health and the environment can be severe. In India alone, ozone pollution causes crop losses equivalent to $1.2 billion per year. In terms of human health, outdoor air pollution costs an estimated one million lives per year, while air pollution produced in homes – usually a by-product of cooking fires – kills around two million people annually.