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.
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.
Heavy smog and air pollution have put at least 23 Chinese cities on 'red alert.' Shijiazhuang, the capital of industrial Hebei province, registered airborne pollutants 100 times over WHO guidelines on Monday. Hundreds of factories have stopped their production and schools have closed.