It's called the Air Quality Index, and on bad days, it looks like a stoplight in traffic.
Thirty-eight air quality monitors feed data on ozone, particulate matter and other pollutants into an algorithm that converts the pollution concentrations into a single number, called an AQI value. The scale ranges from zero to an eye-watering 300.
It's rare, but not unprecedented, to see an AQI value above 200, indicated by a purple color on the map. But every day since last Friday, when the heat wave began, the South Coast Air Quality Management District has reported at least one hour of purple every day at either its Crestline or Redlands stations. In a purple area, everybody should stay indoors.
Red is more common. In these areas, where the AQI values are above 150, pollution is so bad it can affect anyone exercising or spending significant time outside.
In orange regions, pollution concentrations are only bad enough to affect people with respiratory problems. And in yellow areas, all but the most sensitive people should be fine.
Green areas are best, and usually found along the coast or inland after a winter rain -- but the map can be misleading. For example, in the snapshot above, taken at 3:30 p.m. on Monday June 20, South LA appears to be an oasis of pure air in a sea of moderate to severe pollution.
But in reality, the particulate matter sensor at that station was broken. Once it came back online, it began registering concentrations of diesel soot and other small particulates again, bumping South LA back into the yellow zone.
That's disappointing. I was hoping for a discrete pocket of clean air.— paige (@paige) June 21, 2017
Paige Newman, the Twitter user cited above, lives in Malibu and "literally drives into the smog" when she commutes to Burbank. She likes the map because it's a visual reminder of what she sees every day coming over the Sepulveda Pass.
Kenny Uong just found out about the map. The high school senior in Glendale said he'd rather get his information from social media and text alerts, and would like to sign up for AQMD's customizable Air Quality alerts. The agency can let you know when air is bad in your neighborhood.