The regional air district is declaring the season's first air alert beginning Monday through Wednesday.
A combination of hot weather and increased back-to-school traffic are combining to trap ozone gas in the trough-shaped valley.
In 2010, the region became the first in the U.S. to be fined for failing to meet the federal deadline for reducing ozone pollution.
The valley must record three clean years in a row for the $29 million annual fine to end.
It is being paid by an assessment on businesses and through a $12 annual vehicle registration fee for drivers from Stockton to Bakersfield.