The smoke from the brush fires contains fine particulates that may cause serious trouble for the young, elderly and those with respiratory and heart disease.
"People are being exposed to the particulate matter, the carbon, that's being burned off of trees and other plant matter. That can irritate the lungs and cause a lot of bad symptoms, particularly respiratory ones," said Dr. Seth Oskie, an emergency room physician at California Hospital Medical Center.
Oskie says the best advice for those most at risk is to stay indoors.
Tens of thousands of acres of land have burned in the past several days.