Chinese authorities shut down much of Harbin - a city of more than 10 million people - as unusually high levels of pollution shrouded the city and the surrounding region in mid-October of 2013. Measurements taken on October 20 scored the air quality index (AQI) in the city at 500, the highest possible reading. Levels above 300 are considered hazardous to human health. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite acquired this natural-color image of northeastern China on October 21, 2013. 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 blots out the city and surrounding towns.