Smog is dirty air over a city — air so full of smoke and chemicals from cars, factories and burning that it hangs there like a grey or brown fog. You can see it: the buildings a few streets away go soft and pale, and the sky loses its edge. On bad days it stings your eyes and throat, and people with asthma are told to stay inside. It is a noun, and usually uncountable — "there was smog all week," not "a smog" — though older writing does say "a smog" for one particular episode. Two kinds get talked about. The old kind, sometimes called winter smog, is coal smoke trapped by cold damp air; London was famous for it, and in 1952 it killed thousands of people in a few days. The newer kind is photochemical smog: sunlight cooks car exhaust into ozone, and it looks brownish and gets worse on hot afternoons — that is the Los Angeles, Bangkok and Delhi kind. In Taiwan people mostly meet the word through news about winter air in the west of the island, though what is actually measured now is usually PM2.5 rather than smog as such. Note that everyday speech has stretched the word: many speakers now say smog for any bad air pollution you can see, even a haze that is really dust or farm burning.