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.
The upcoming COP24 climate summit will be held in Katowice, deep in Poland's industrial and coal mining heartland. Its air quality is among the worst in Europe. But the town is trying to clean up its act. And if Katowice can go green, perhaps anywhere can.