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,

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

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