The Chinese capital, Beijing, has been experiencing the worst air pollution for more than two decades.
Out of the 74 cities in China being monitored, 33 showed readings of "hazardous" this weekend. Over the past 24 hours, the air in Beijing has been 22 times the World Health Organisation's recommended average for the amounts of the smallest particles in the air, under 2.5 micrograms: the most dangerous size of pollutants.
Living in Beijing, especially with children, you soon acquire a nerdy knowledge of air quality measurement.
Late last night in Beijing, the air pollution was measured at almost twice the maximum levels on the World Health Organisation scale. The cause: Beijing's five million cars and China's almost total reliance on coal-fired power, combined with cold weather and no wind.
Outside Beijing's largest children's hospital, I spoke to parents, many wearing face masks. One mother, Mrs Yu, told me she has to bring her daughter to see the doctors twice a month because of respiratory problems.
"I am worried about my daughter, the air pollution is very bad and it definitely affects my daughter's health", she said. Other parents told me, "I have to keep my son at home every day. He's so bored, but the air is so bad for children".
"Beijing is developing so fast there are so many cars on the streets, I would suggest if people don't have to drive they should not use their cars".
Official medical statistics suggest that 9,000 people died in four cities, including Beijing, from diseases brought on by air pollution in 2012. It's also been reported that over the last decade there's been a 600,000 increase in deaths related to respiratory disease, without a corresponding rise in smoking.
The US embassy has infuriated the Chinese authorities by publishing online every hour the latest air quality measurements - the readings expose that the official figures are not giving a true picture.
However, at times this weekend, both the Americans and the Chinese have been in rare agreement: the pollution has been off the scale.
The Chinese are now taking air pollution much more seriously, and improving the monitoring in many cities. The current situation was even top story on last night's CCTV main bulletin at 7pm - as the air becomes more murky the Government is showing more transparency.
However, the state media continues to refer to the pollution as "fog" rather than the more accurate "smog".
In Shandong province, in eastern China, flights were cancelled because of low visibility and travel has disrupted in many regions. There are now more than 180 airports in China, more than half a billion passengers every year. China is also the world's largest car market.
Rapid development has made China the world's second largest economy, but at what cost to its people and the environment?