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.

Wear a tight-fitting N95 mask at the bare minimum, but a P100 is better, experts say. Wear long sleeves, eye protection, closed-toed shoes and cut-resistant gloves. Clean or decontaminate anything that’s been in contact with ash.