"Dear passengers, Beijing will start a red alert for air pollution from 8 p.m. December 16. We suggest you avoid outdoor activities and take preventive measures," announced every subway station in the capital.

On December 19, the PM2.5 reading in many cities in northern China, mainly in Hebei and Henan, exceeded 500 micrograms per cubic meter. In Shijiazhuang, some 300 kilometers away from Beijing, the reading even exceeded 1,000 micrograms per cubic meter.

Beijing and its neighboring Tianjin Municipality have both issued their first red alerts for air pollution this year with heavy smog sweeping northern China on December 16.

Besides Beijing and Tianjin, the Ministry of Environmental Protection said another 21 cities including Shijiazhuang, Taiyuan and Zhengzhou are expected to issue red alerts, with nine others expected to issue orange.

The severe smog in northern China worsened on December 20, disrupting the lives of hundreds of millions of people.

A lot of netizens shared photos of the hazy skies nearby with anxious comments. "Kids can't go outside where they could in the past. They can only go to museums, indoor parks, shopping malls and supermarkets. The mask has become a label of their live before they could enjoy the blue sky," said Han Yi, in a published article Childhood with Masks on WeChat.

The Children's Hospital of Hebei Province on November 22, 2016. On average there are 550 kids diagnosed with respiratory illness each day since the beginning of November, 30 percent higher than last November last year (VCG)

Another netizen with the screen name Queyuehuyu Xiaoqiang said: "When I went to hospital today for business, the pediatric ward was a full house. During the queuing hours, all I heard from parents were pneumonia, tracheitis, pediatric respiratory-tract infection and high fever."

Some others nostalgically posted photos of the past or cities without smog. A WeChat netizen named Michelangelo recalls the past years when the sun warmed the face, even in winter, and the moon lightened the yard at night.

In Beijing, the heavy winds are no longer despised because local residents consider it as a useful tool to blow the smog away. However, the netizen Wanmohan said, "I always think that the sky turns blue when the wind comes. It's only a self-deception."

Besides online comments, some people, including parents, created surveys on masks and air purifiers. They help customers select effective tools to safeguard families' health.

As for public efforts, transport authorities in Beijing and Tianjin have announced emergency vehicle controls. Vehicles will be restricted based on an odd-or-even license plate rule. In Beijing, high polluting vehicles will be ordered off the roads and Governments and state-owned enterprises are requested to cut the number of vehicles on the road by 30 percent. Offenders will be fined 100 yuan ($14).

All kindergartens, primary schools and middle schools in polluted cities including Beijing, Tianjin, Zhengzhou and Xi'an, were closed from December 19-21. Teachers offered online classes via communication apps such as WeChat and QQ, while some schools gave live classes to students at home.