Yellow bands—larger, coarse particles—around the Middle East, North Africa, and western North America likely indicate dust and sand storms as the northern hemisphere reaches the driest time of year. The bright red areas in Russia and much of South America reveal the intense wildfires and some agricultural burning, which coincided with hot summers and drought in several regions. The red patches in western Canada spring from summer wildfires, while China was likely coping with a combination of fire, dust, andpollution events. It is unclear what was happening in western Africa.
Of all the particles that fill our lungs on any given day, the most dangerous are the small ones. Aerosol particles smaller than 2.5 micrometers pose the greatest risk to human health because they are small enough to be breathed deep into the lungs and, in some cases, enter the blood stream. These fine particles, about 30 times smaller than the width of a human hair, are also a major cause of poor visibility.