Spare the Air alerts were issued for Sunday and Monday in the Bay Area because of an excessive amount of smog expected in the region, air quality officials said.
A high-pressure system over the region, triple-digit temperatures, light winds and smoke from Oregon wildfires are expected to contribute to unhealthy ozone levels in the South and East Bay regions, according to the Bay Area Air Quality Management district.
There is no free transit Monday, and there is no wood-burning ban in place, air quality officials said.
The alerts are the eighth and ninth such alerts in 2017.
"Hot temperatures and tailpipe exhaust from Bay Area traffic are expected to cause unhealthy air quality this weekend," air district executive officer Jack Broadbent said in a statement.
During Spare the Air days, air quality officials advise people to limit outdoor activities during the hottest part of the day and to take public transit or carpool instead of driving alone.
A forecast for high temperatures in the San Francisco Bay and Monterey Bay areas has also prompted the National Weather Service to issue excessive heat warnings for some areas.
A warning is in effect until 9 p.m. Monday for the Santa Lucia Mountains, the East Bay Hills and Diablo Range, and interior Monterey County, including Pinnacles National Park and the southern Salinas Valley.
Weather officials said high temperatures of 98 to 112 degrees are expected Sunday and Monday. Lows will mostly be in the 60s and 70s but may not get below the low 80s in the hills.
Another warning goes into effect at 11 a.m. Sunday and ends at 9 p.m. Monday for the North Bay Mountains and the Santa Cruz Mountains.
Highs will reach 95 to 105 degrees both days. Lows will mostly be in the 60s in the valleys and 70s to lower 80s in the hills.
A third warning has been issued for the inland valleys of the East Bay. At 5:05 p.m. Sunday, weather officials upgraded a heat advisory for the area to an excessive heat warning, which went into effect at 11 a.m. Sunday and ends at 9 p.m. Monday.
An excessive heat warning means heat illnesses are likely because of the hot temperatures.
Weather officials suggest residents and visitors drink plenty of fluids, stay in air conditioning, stay out of the sun and check in on relatives and neighbors.
Pets and livestock may require extra care during the heat. Also,the heat increases the risk of human-sourced wildfires, according to weather officials.