Move your mouse across the satellite image above to view the comparison between January 14 and January 3.
Thick, dangerous smog and air quality warnings were becoming a daily fact of life for residents in Beijing and many other China cities in mid-January, as the Asian nation faced one of its worst air quality periods in recent history. The Chinese government ordered factories to scale back emissions, while hospitals saw spikes of more than 20 to 30 percent in patients complaining of respiratory issues, according to news reports.
Several factors -- including China's notoriously congested traffic and its heavy reliance on coal for electricity -- have combined to produce this month's eye-popping measurements of PM2.5 particulates in the air, which recently exceeded 700 micrograms per cubic meter in parts of Beijing.
PM2.5 particulates are tiny bits of matter that can penetrate deep into the lungs, and measurements of their density (or absence) in the air is used as a gauge for air quality. The World Health Organization considers less than 25 micrograms per cubic meter and below to be safe.
By moving your mouse over the image above from NASA's Earth Observatory, you can see the difference in the air over Beijing between January 3 and January 14, 2013.
The brightest areas in the January 14 image 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.)
The Associated Press and NASA Earth Observatory contributed to this report.