For Those Grappling With California Wildfires, Dangerous Air Quality Is a New Risk

“The main pollutant that comes from the fire is called particular matter, or PM. The most health-relevant particles are in sizes smaller than 2.5 microns—they’re pretty tiny particles,” says Yifang Zhu, Ph.D., professor of environmental health sciences and acting dean of UCLA’s Fielding School of Public Health. Zhu goes on to explain that the national standard for the presence of “PM 2.5” over a 24-hour period is 35 milligrams per cubic meter, which makes for an Air Quality Index of 99. Whenever the index measures 100 or over, she says, it becomes unhealthy. Today in San Francisco, the AQI is around 250.