The Bay Area Air Quality Management District issued a Spare the Air alert for Thursday, with the National Weather Service also putting out a heat advisory.
Thursday will be the third day in a row with unhealthful levels of smog in the region, and the second day covered by the Weather Service's heat advisory.
Light winds and high temperatures, along with the daily load of motor vehicle exhaust, will continue to elevate concentrations of ozone, or smog, the district said in a news release Wednesday.
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The Weather Service said south Napa could hit 98 degrees on Thursday, with the Upvalley topping 100 degrees.
High temperatures will create a situation in which heat-related illnesses are possible. People should drink plenty of fluids and take extra precautions if they work or spend time outside.