Children who live and go to school in Beijing in China say the thick smog is stopping them playing outside and doing some sports.
Some cities across China have been put on high alert by the Government because the air has become very polluted.
It has become so bad in the past few days that school children are being warned to stay inside more.
If they do go out, most children wear face masks.
Coal is used to power some Chinese factories and since it's cold now, more people are using it to heat their homes, so there is more pollution.
These pupils, at a school in Beijing, sent Newsround this special report about the smog.
They say it's having a big impact on their lives and they use special air filters in the classroom to make sure they can breathe clean air.