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.
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?
At least 19 people have frozen to death in Poland since last week, with temperatures going down to minus 25 Celsius (-13 F). Another 24 people have died since then as a result of carbon monoxide poisoning. Exhaust fumes, extreme cold and a lack of wind have caused smog in many places. For this reason, public transport has been made free in several cities, such as Warsaw.