空气污染。

You can use it as a noun or a subject. If you want to say the air is bad, say 空气污染很严重. It works just like the English phrase. You will hear it on the news and in casual chats. It is a very practical piece of vocabulary for living in big cities.

Use it when checking the weather report with friends. Use it when explaining why you are wearing a mask outside. It is perfect for complaining about the lack of blue sky. You can also use it in school or work presentations. It is a very versatile and necessary term.

Do not use it for a smelly room or a bad odor. If your friend forgot to take out the trash, that is not 空气污染. That is just a 臭味 (bad smell). Also, avoid using it for indoor smoke from cooking. Keep this term for the outdoor environment and general climate issues.

In China, talking about the air is as common as talking about tea. Many people check the AQI (Air Quality Index) every single morning. It is a major topic of conversation in cities like Beijing or Shanghai. You might hear people jokingly call themselves 'human vacuum cleaners.' Mask-wearing and air purifiers are standard parts of the modern urban lifestyle.

You will often hear the shorter word 雾霾 (wùmái). This specifically refers to the thick, gray smog or haze. While 空气污染 is the scientific name, 雾霾 is what people say when they look out the window. Another common one is PM2.5, which people use even when speaking Chinese!