Air quality in Shanghai has plummeted since the World Expo left town, sliding from “excellent” to “polluted” and now—with levels of airborne particulates up to three times beyond what the government considers safe—downright frightening.

But even as the smog thickens, the question remains whether Shanghai can match a new standard of Sino-smokiness recently set by Beijing—a standard that (as one U.S. authority recently suggested) stretches the bounds of sanity.

According to a report in today’s China Daily, Shanghai’s air pollution index has broken 100 on eight days so far this month, at one point reaching 370—the worst reading in 10 years.

Scary stuff, no doubt. But compare that to last week in Beijing, when air pollution readings from a monitoring station at the U.S. Embassy soared past 500—a level so astonishing it prompted the embassy’s automated air pollution Twitter feed (which usually rates Beijing’s air quality on a scale from “Good” to “Hazardous”) to add a completely new rating: “Crazy Bad.”

According to an investigation last week by the Guardian’s Jonathan Watts, the “Crazy Bad” designation — which was quickly taken down and replaced with the more bureaucratic “Beyond Index” — was a joke written into the embassy’s monitoring software that had been triggered by the off-the-charts reading .

“It was an inadvertent humorous moment,” Watts quoted U.S. embassy spokesman Richard Buangan as saying. Buangan then went on to express pleasure that, far from being excoriated for the blunder, the “Crazy Bad” miscue had seemed to make “heroes” out of the embassy staff.

Inadvertent or not, the new designation helped focus attention on just how awful the Chinese capital’s air had become. How bad is crazy bad? Try 15 times the U.S. Environmental Protection Agency’s standard for acceptable concentrations of particulate matter.

By China’s less stringent standards, the cutoff for “polluted” is an index reading of 100. At 300, or “heavily polluted”–roughly one-half the highest reading in Beijing last week–even healthy people suffer “strong irritations and other symptoms,” according to China Daily.

The horrifying numbers out of Beijing and Shanghai have sent experts and residents alike scrambling to explain the sudden surges in pollution. In a detailed analysis of the “crazy bad” phenomenon posted yesterday, the English-language blog livefrombeijing settles on a surprisingly mundane answer: the weather. Certain types of wind, the post notes, can trap pollution against the mountains outside Beijing, causing it to build up quickly.

Authorities explain Shanghai’s pollution surge in much the same way, blaming an increase in sandstorms and winter cold waves that carry polluted air in from inland provinces.

But seasonal weather isn’t the whole story. Air quality has been declining nationwide in recent years, thanks largely to a car boom and a surge in heavy industry tied to the country’s post-financial crisis stimulus efforts. According to a report (in Chinese) from the Ministry of Environmental Protection, pollution in China in the first half of this year was the worst it’s been since 2005.

A map released by NASA earlier this year provides a Technicolor bird’s-eye view of how China’s air quality compares to the rest of the world. Closer to the ground, a photo diary from the Asia Society called “Clearing the Air” documents in disturbing detail the daily fluctuations in visibility experienced by residents of Beijing.

While China’s efforts to cool the economy may end up tamping down some of the industrial pollution, the country’s automobile craze shows few signs of slowing down.

One consolation for those stuck living in Beijing, Shanghai or any other Chinese city with chewable air? At least now there’s a common term to describe it.