If the smoke and ash you’re seeing when you look outside belie the official air quality index for your area, the explanation at least in part is that the network of detectors was not designed for tracking all the pollution from distant but massive wildfires.
Much of the grimy orange tainting the skies above the Metro LA area is caused by smoke from the major fires hundreds of miles away in northern California, said Philip Fine, the deputy executive officer for the South Coast Air Quality Management District.
“The fortunate thing is a lot of that smoke is high in the atmosphere, and we are not breathing it directly,” Fine said. As a result, it is also not detected by ground level sensors.
NBC Los Angeles, 11 September 2020