China had warned residents across a large part of its north on Friday to prepare for a wave of severe smog arriving over the weekend, the worst of which would hit Beijing, prompting the capital to issue its second ever "red alert".

A red alert is triggered when the government believes air quality will surpass a level of 200 on an air quality index that measures various pollutants for at least three days.

The US government deems a level of more than 200 "very unhealthy".

But on Saturday morning, the Beijing Municipal Environmental Monitoring Centre was showing an air quality reading of 104.

"Today I have to work. And where is the smog?" posted one Beijing resident on Weibo, China's Twitter equivalent.

"The smog is not so bad. Why do they have driving restrictions?" said another Weibo user in the capital.

The Beijing city government issued its first "red alert" last week following criticism that previous bouts of smog had failed to trigger the highest warning level.

In Beijing, a red alert means around half the vehicles are removed from the roads with an odd-even licence plate system enforced. Schools are recommended to close and outdoor construction is banned.

The Beijing environment bureau had said the red alert would last from 7 am Saturday to midnight on Tuesday.

But with the haze not as severe as predicted, the National Meteorological Centre only issued a yellow alert for the city and many surrounding areas on Saturday. Yellow is the second level on China's four-tier colour-coded pollution alert system.

The weather observatory said that areas including southern Beijing, central Hebei, north Henan and west Shandong would be affected by heavy smog, which would start to dissipate from Wednesday. It advised the public to take protective measures and those that suffer from respiratory diseases were urged to stay indoors or wear face masks if they needed to go out.