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.
To curb air pollution, Madrid now has some of the strictest restrictions for vehicles in Europe. Older cars are banned from entering the inner city, visitors will need special permission. Sidewalks are being widened, and the speed limit has been set at 30 km/h in many places. The decision has proved to be highly controversial — but the left-wing city government remains committed.
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.