On January 13, The New York Times reported that air pollution levels in Beijing, China were elevated to a point "beyond indexing". The readings help form the Air Quality Index (PDF), which is used to determine levels of harmful particulate matter, and topped out at 755 ppm at a monitoring station at the U.S. Embassy (on a scale of 500).
The folks at NASA have posted two satellite images that show what the entire Beijing region looks like with and without a shroud of smog:
From Earth Observatory:
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired these natural-color images of northeastern China on January 14 (top) and January 3, 2013. The top image shows extensive haze, low clouds, and fog over the region. The brightest areas tend to be clouds or fog, which have a tinge of gray or yellow from the air pollution. Other cloud-free areas have a pall of gray and brown smog that mostly blots out the cities below. In areas where the ground is visible, some of the landscape is covered with lingering snow from storms in recent weeks. (Snow is more prominent in the January 3 image.)
At the time that the January 14 image was taken by satellite, ground-based sensors at the U.S. Embassy in Beijing reported PM2.5 measurements of 291 micrograms per cubic meter of air.
Most outlooks suggest that China's demand for energy will continue to increase in the coming decades. If left unchecked, it's reasonable to expect that air quality will continue to degrade as there are more manufacturing, personal vehicles, and power plants in China burning fuel.
A question is: how long can this go on? My guess is that human health will be one of the most important drivers in China cleaning up its energy sector. It's hard to do much else if people don't have clean water to drink or air to breathe.