This includes Topanga Canyon beach in Malibu, Malibu Lagoon at Surfrider Beach, Mother's Beach in Marina Del Rey, and Santa Monica Pier.
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The warnings were issued due to bacterial levels exceeding health standards when last tested.
Recorded information on beach conditions is available 24-hours a day on the County's beach closure hotline: (800) 525-5662.
The warning comes as an oppressive heat wave continues to blanket Southern California, with even hotter conditions returning by later in the week.
National Weather Service forecasters expect "a little better warming on Thursday, especially in the valleys, where the weaker onshore push from the south will reduce the effects of the afternoon sea breeze."
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Another potentially dangerous heat wave is expected to arrive in the region by the end of the week, sending temperatures into triple digits again through the weekend.
Forecasters said "dangerously hot conditions" will be possible, "with temperatures up to 110 possible for interior valleys, mountains and deserts and up to 105 possible for coastal valleys."
City News Service contributed to this report.