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

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,

The fresh air came after days of acrid smoke from wildfires, but officials warned that conditions could rapidly worsen again.

Wildfire ash carries many carcinogens that could cause short- and long-term health problems, including asthma attacks and difficulty breathing.

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:

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.