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.
Mexico City is notorious for traffic congestion and high levels of smog and noise pollution. One effort to tackle this problem involves electric scooters. Econduce is a privately-run pioneer scooter sharing scheme. It was the first scheme of this sort in Latin America, and is one of a select few worldwide. Since its launch, it has grown quickly. But can it really help fix the city's problems?