In Beijing, the air is polluted. The air is 17 times worse than the limits. Highways close. People must stay indoors.
The pollution gets worse in the winter months. It is cold, and people turn their heating systems on. There is no wind, so the smog stays in the city. It does not move away.
Difficult words: polluted (something that is so dirty that it is dangerous), limit (the greatest amount which is OK), heating system (a system which make your home warmer).
