Air pollution is leading to the premature deaths of some 3.3 million people around the world every year – and is on track to claim twice as many lives annually by 2050 at the current rate – according to a new study.

Researchers at the Max Planck Institute for Chemistry in Germany have revealed that China suffers the worst impact of air pollution globally, with an estimated 1.4 million people dying each year from inhaling pollutants in the outdoor atmosphere. This is followed by India, with a death toll of 650,000 annually, then Pakistan with 110,000.

Of the global toll stemming from air pollution, three-quarters of deaths are due to strokes and heart attacks, and a quarter due to respiratory diseases and lung cancer. The biggest surprise to the researchers, however, was finding out where some of the worst toxic pollutants were coming from.

"It is generally assumed that industry and transport are the worst air polluters. But that is evidently not the case on a global scale," said Johannes Lelieveld, director at the Max Planck Institute for Chemistry, in a press release. Rather, in India and China, much of the airborne smog is actually due to people lighting fires in their homes.

"Although these are low-key activities, they add up, particularly if the majority of the population uses them," said Lelieveld. In total, it's estimated that more than a million deaths each year are due to people simply lighting household fires for things like cooking and heating.

Another surprise is how much pollution stems from agriculture in modernised countries. According to the researchers, the ammonia used in fertilisers in places like Europe, Russia, Turkey, Japan, and the US routinely undergoes subsequent chemical reactions and forms ammonium sulphate and ammonium nitrate. In airborne form, these harmful substances account for one-fifth of premature pollution-based deaths worldwide.

Some countries have it worse than others due to their geographical position. Germany, for example, in the centre of Europe, is surrounded by air pollution from several neighbouring countries in addition to that which it produces itself. All up, the European Union suffers 180,000 deaths a year from pollution, with 35,000 of those being in Germany.

As alarming as the numbers are, they're actually slightly lower than previous figures provided by the World Health Organisation (WHO) last year, which estimated outdoor pollution causes 3.7 million deaths every year, with household pollution accounting for another 4.3 million deaths annually.

Taken together, this makes pollution the single biggest environmental health risk faced by people around the world. Let's do something about this, people!

The findings are published in Nature.