The Sandy Fire in Ventura County entered its third day on Tuesday as shifting winds and rising temperatures were reported in Glendale, Pasadena, Monrovia and as far as Chino Hills.
Residents reported the air "smelled like an ashtray," though experts caution that smell alone is not a reliable indicator of air quality.
Fires in Riverside County - the Bain and Verona fires - as well as the Santa Rosa Island Fire in Santa Barbara County are also contributing to pollution levels across the region.
Air quality officials say residents should monitor conditions through mobile apps or the South Coast Air Quality Management District's website rather than relying on their senses.
"It's not always possible to see or smell smoke," said Scott Epstein of the South Coast Air Quality Management District. "Also, there are pollutants that we worry about in Southern California that you can't smell, like ozone or smog pollution."
Experts recommend limiting exposure when air quality worsens.
"If you are in an area with poor air quality, you want to minimize the amount of pollution that you breathe, so the best way to do that is to try to avoid outdoor strenuous activity where you breathe in a lot of air," Epstein said.
Residents are advised to remain indoors with doors and windows closed and avoid using devices that draw smoky air inside, such as whole-house fans or swamp coolers.
Health officials warn that while anyone can feel the effects of pollution, certain groups are more vulnerable.
"These are people who have pre-existing heart and lung issues, elderly people, pregnant women and young children are particularly susceptible, and they need to be extra careful to not breathe in high levels of air pollution," Epstein said.
Weather conditions are compounding the problem. High temperatures and intense sunlight can accelerate the formation of ground-level ozone. Intense sunlight acts as a chemical catalyst, speeding up the reactions between emissions, nitrogen oxide and volatile organic compounds.
With ongoing winds, heat and multiple active fires, officials say the combination is creating a heightened risk for unhealthy air across the region.