"An inversion layer is keeping the smoke close to the ground, limiting visibility and reducing helicopter air operations, " Berry said.

CNN: Firefighters make headway in California wildfire

While the ozone layer in the upper atmosphere plays an important role in limiting harmful ultraviolet light reaching the Earth's surface, low-level ozone is a pollutant that is harmful, causing respiratory problems in animals as well as well-documented cases in humans.

BBC: Invasive plant 'increases ozone pollution'