Authorities in Beijing have stepped up their health warnings to residents, as thick smog blanketed the city and large parts of the country for a third consecutive day.
The US embassy's air quality index reading for Beijing stood at 301 at 5pm local time on Wednesday. It rates a reading over 150 as "unhealthy" and above 300 as "hazardous".
People are being told to close their windows, eat a "balanced diet" and drink plenty of water.
The municipal government has also advised the elderly, young and those with health problems to stay indoors, or wear protective masks if going out.
The warnings, on Internet news sites and microblogs and in regular bulletins by state broadcaster China Central Television (CCTV), were issued as emergency measures were announced to counter the pollution.
Authorities also announced the closure of 103 factories and ordered 30 percent of official cars off the road on Tuesday.
Visibility in central Beijing was reduced to 300 metres, according to local media, causing 36 flights to be cancelled.
This is the fourth serious bout of toxic air in China in recent weeks.
The winter of smog has sparked an Internet outcry and repeated calls from state media for action.
A campaign for clean air legislation by real estate tycoon and Internet blogger Pan Shiyi - who has 14 million followers on Sina Weibo (China's version of Twitter) - is gathering pace.
By Wednesday afternoon, more than 43,000 people had voted in favour of new laws to tackle the smog in a survey posted by Mr Pan.
He had led a campaign in 2011 to force Beijing to release transparent details on levels of tiny air particles known as PM2.5.
Reform-minded investor Xue Manzi, who has 10 million followers on Weibo, backed Mr Pan's campaign.
"I have lived in Beijing for four years and I have not seen it this bad before," said domestic cleaner Jiang Hua, who is originally from the central province of Henan.
"It just seems so prolonged."
Pedestrians also expressed anger at having to wear protective face masks.
"Wearing a face mask is annoying. As soon as I want to take a photo, I have to take the mask off because my glasses fog up," said Liu Lili, a tourist from the southern province of Guangdong.
CCTV also showed images of the eastern province of Jiangsu - around 1,000 kilometres south of Beijing - covered in a thick blanket of smog.
China's pollution problems are blamed on rapid urbanisation and dramatic economic development.
While Beijing residents struggle with the city's toxic air, one man has taken advantage with a unique marketing ploy.
Chinese billionaire Chen Guangbiao, who made his fortune in the recycling business, has begun selling 'canned air' on the streets of Beijing
"I'm selling this clean air to remind everyone to protect our environment," Mr Chen said.
"Selling this air I'm using an exaggerated method, a principle to tell everyone that if we don't start protecting the air in our environment, in ten years our descendants will all be wearing gas masks."