With more hot weather to come, the Bay Area Air Quality Management District has issued a Spare the Air Day for Tuesday — the second this week.
District officials are encouraging people to take public transit and to reduce their energy use.
"Tomorrow, we are again forecasting unhealthy concentrations of smog," district executive officer Jack Broadbent said Monday. "With temperatures continuing to run high, it's more important than ever that residents drive less, take public transportation, reduce energy use and do their part to minimize pollution."
Motor vehicle exhaust, gasoline vapors, industrial emissions and household chemicals contain volatile organic compounds and nitrogen oxides that can combine with oxygen.
When in the presence of heat and sunlight, it can create ground-level ozone.
Long-term exposure to ozone can reduce lung function and high levels of ozone are particularly harmful to children, seniors and those with heart and respiratory conditions, district officials said.
Tuesday will be the second consecutive Spare the Air Day this week and the sixth for 2011.
Bay City News Service contributed to this report.