A general view of the CCTV towers, headquarters of China Central Television, in Beijing 24 hours later, on January 13, 2013. Over the weekend, the air quality index registered as high as 755 in Beijing. New York City, by comparison, registered a level of 45 on Monday. - 

The last time the air was this bad in Beijing, the air monitor attached to the U.S. embassy pumped out a reading which was labeled by a staff member as "crazy bad." That was two years ago.

This time, it’s worse. “This is emergency level air pollution,” said Alex Wang, an environmental law professor at University of California-Berkeley, who specializes in China's environmental regulations.

One of Beijing’s biggest hospitals, Peking University Hospital, reported treating twice as many heart attacks over the weekend. The Beijing children’s hospital said a third of its outpatient visits in the past two days were due to lung ailments.

Last year, China saw an uptick in protests from the country’s growing middle class over environmental concerns; concerns that Wang says could now be reaching a tipping point.

"For a while now, the government has been aware that environmental pollution issues very much have the potential to trigger instability," says Wang,  "So they’ve responded to this gradually, but it seemingly has not gotten better."

Beijing has moved factories outside the city and tightened vehicle emissions standards, but Zhou Rong of Greenpeace Asia says this time around, the smog is coming from smaller cities outside the capital -- places where local governments have done little to curb pollution in their quest for more economic growth.

"Especially for the local government, I think, should really need to balance the GDP growth and the people’s health," says Zhou.

In the end, better air would help the economy. The World Bank found that China’s air pollution causes damage equal to 5 percent of the country’s GDP, or $360 billion, each year.

Follow Rob Schmitz at @rob_schmitz