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?
Poland relies heavily on coal, with over 80 percent of its electricity generated from coal. Earlier this year unprecedented smog alerts were issued across the country. Environmentalists fear Poland's plans for large-scale reindustrialisation could worsen the smog problem.