Indoor Air Quality Alert: California Flooding

What is the location of the flooding?

As of December 25, 2025, California is experiencing a life-threatening atmospheric river flooding event affecting large portions of the state. The flooding is associated with a powerful Pacific storm system delivering prolonged heavy rainfall, intense runoff, and debris flows.

The event is impacting coastal, valley, and mountainous regions across Northern, Central, and Southern California, with particular severity in areas near the Sierra Nevada, the Transverse Ranges, and wildfire burn scars.

Major geographical references include the San Gabriel Mountains, Sierra Nevada range, Interstate 5, Interstate 80 near Donner Pass, and urban drainage systems in the Los Angeles Basin (1).

Which cities or areas are affected by the flooding?

The following cities and areas have reported direct impacts from flooding, debris flows, or flash flood warnings:

• Alhambra
• Burbank
• Los Angeles
• Redding
• Sacramento
• San Diego
• San Francisco
• Santa Barbara
• Ventura
• Wrightwood

Mountain communities and transportation corridors, including Donner Pass and the Angeles Crest Highway, are also heavily affected (2).

What is the current containment status of the flooding?

As of December 25, 2025, the flooding is not fully contained. Rainfall intensity has fluctuated, with brief periods of easing followed by additional storm waves.

Emergency crews are actively conducting water rescues, clearing debris, and monitoring rivers and urban drainage systems. Saturated soil conditions continue to increase the risk of renewed flash flooding and mudslides, particularly in burn scar areas (3).

Wind map showing the Pacific storm system impacting California on December 25, 2025. Source: IQAir.

Are there any evacuation orders or alerts in place?

Multiple evacuation orders, shelter-in-place advisories, and flash flood warnings remain in effect across impacted regions.

• Mandatory evacuations issued in parts of Los Angeles, Orange, Ventura, San Bernardino, and Riverside regions (4).
• Shelter-in-place orders enacted for Wrightwood due to impassable roads and debris flows.
• Flash flood warnings active across much of Southern California.
• State of emergency declared to mobilize resources and emergency response teams (5).

Residents in flood-prone and burn scar areas are urged to avoid travel and comply with local emergency instructions (6).

How can I protect myself from mold after flooding?

It’s important to take steps to prevent mold after a flood.

• Before entering a flooded area, make sure it is safe and free from electrical hazards or structural damage.
• Use protective gear: wear gloves, goggles, and a KN95/FFP2 mask to minimize exposure to mold spores.
• If there is still standing water, remove it as quickly as possible. Use pumps, wet-dry vacuums, or buckets to extract the water.
• Open windows and doors to facilitate air circulation and use fans, dehumidifiers, and HVAC systems to help dry the space. This step is crucial within the first 24-48 hours after flooding to prevent mold growth.
• Remove and discard any items that have been severely damaged by floodwater and cannot be thoroughly cleaned and dried. This includes carpets, upholstered furniture, mattresses, and any other porous materials.
• Thoroughly clean all hard surfaces with a detergent or cleaner suitable for mold remediation. Use a solution of 1.5 cups of bleach mixed with one gallon of water to disinfect nonporous surfaces. Never mix bleach with ammonia or other cleaning products, as the fumes are toxic.
• Maintain an indoor relative humidity (RH) of 35-50%. High humidity promotes mold growth, so consider using dehumidifiers in damp areas.
• Run a high-performance air purifier for mold, both during and after the mold removal work.
• If the flooding and mold damage are extensive, consider contacting professionals specializing in mold remediation to ensure proper cleanup and restoration.

Remember, mold can pose health risks, so it's important to take these steps promptly and thoroughly. If you experience persistent mold-related health issues or have concerns about extensive mold growth, consult with a medical professional or a mold remediation specialist.