An Ozone Advisory is in effect for much of Los Angeles, Riverside, Orange and San Bernardino counties through Thursday at 8 p.m. in what the South Coast Air Quality Management District is calling a "multi-day extreme ozone (smog) event."

The SCAQMD has warned that the Air Quality Index will reach "Unhealthy" levels in parts of the Southland while air quality that is "Moderate" and "Unhealthy for Sensitive Groups" will be more widespread.

Ozone levels are expected to be lower in coastal areas.

In Los Angeles County, the worst air quality is expected in the San Gabriel, San Fernando and Santa Clarita valleys. In some areas in those regions, the air quality is expected to hit Unhealthy levels by Thursday.

When air quality is in the "Unhealthy for Sensitive Groups" range (101-150), the general public is unlikely to be affected. But people with heart or lung disease, children, older adults, and people who are active outdoors may be sensitive and therefore at greater risk. People in the sensitive groups should reduce prolonged or heavy exertion, according to the SCAQMD.

Forecasters predict the heat wave will reach its peak on Wednesday and Thursday, with inland valleys experiencing temperatures between 95 to 103 degrees. Desert areas are expected to be even hotter, reaching between 100 and 105 degrees, while inland coastal regions, including downtown Los Angeles, will see highs approaching 90 degrees.