Technically, it's volatile organic compounds and nitrogen oxides in the presence of sunlight, which result in the production of ozone.
Practically, it's dirty air caused by emissions from vehicles, agriculture and industry. And smog is making us sick.
Air pollution kills more than 7 million people worldwide every year. And right at the top of the list of places with unhealthy air is Delhi, also known as "the smog capital of the world." Gigantic smog-sucking towers that clean the air are now being hailed as a new solution. But they are far from a fix-all.
About five years ago, Chinese Premier Li Keqiang announced better government action on environmental issues, literally 'declaring war' on air pollution. Today, the country is still far from being a green utopia — but it has undertaken efforts to clear its smoggy skies. Here's a look at what improvements it's made and what still needs to be done.
To explain how China's shift in environmental policy came to be, as well as some of the obstacles that stand in the way of a cleaner, greener China, DW recently spoke with environmental journalist and author, Beth Gardiner. She explains how social media helped lead to greater public awareness about unhealthy air.
The need to keep the wheels of capitalism well-oiled and the lack of investment in clean technologies has taken precedence for decades. It’s led to changes in our climate and deadly levels of air pollution. Dr Michał Krzyzanowski, who’s an environmental epidemiologist, and a visiting professor at Kings College in London talks to us about the problem.