Air quality is a major concern for parts of China.
Industry and cities, largely fueled by coal-fired power plants, are growing at a rapid rate in China
Those power plants may not be as clean burning as ones in the United States, where power companies adhere to government rules aimed to keep air quality impact to a minimum.
For the most part, air quality is good across the United States on any give day in most areas.
In China the rapid growth means more pollution into the air.
Weather conditions can sometimes trap that pollution over populated areas, which is what happened in Harbin, China this week.
Calm winds and cold air helped to lock the pollution over Harbin and the surrounding area.
The pollution build up came after the city turned on the city-wide coal-powered heating system to battle the fall chill.
Farmers in the area were also burning off debris following harvest, adding more particulate matter (dust) to the air.
The city of more than 10 million people was shut down because of high levels of pollution.
Chinese authorities grounded planes, shut down schools, and closed major roadways in an effort to keep people out of the pollution and inside.
These efforts also reduced travel in the area, which adds to the pollution levels.
NASA Earth Observatory says "measurements taken in the city on October 20, 2013, scored the air quality index (AQI) at 500, the highest possible reading. Levels above 300 are considered hazardous to human health."
The below image was taken by a NASA satellite on Monday October 21st.
NASA Earth Observatory says it shows "extensive haze and fog throughout the region. The brightest areas are fog, which has a tinge of gray or yellow due to the air pollution. Other cloud-free areas have a pall of gray and brown smog that entirely blots out the city and surrounding towns below."