Calculating air pollution from wildfires and other events has become more complicated. That doesn’t mean it’s necessarily more accurate.

The Air Quality Index, or AQI, is the primary system used to warn the public when air pollution is dangerous. It’s based on data collected from 5,000 air monitors across the country. The index ranges from 0 to 500 and uses a color coded system:

Air quality in wildfire-stricken Los Angeles improved on Thursday from hazardous peaks hit earlier this month, as tiny soot particles that can penetrate the lungs and brain dropped to moderate levels,

With another surge of fierce Santa Ana winds, residents were asked to remain vigilant. An Air Quality Alert has been extended for many portions of LA County through Wednesday evening.

Just for perspective, we've now burned down [an area] like the entire city of San Francisco.” The experts said that even after the fires are extinguished, health risks from ash, smoke and toxic materials that linger in the air can persist for months or years — especially in or near burn areas.