Mold is a moisture problem first. In Sonoma County, that moisture usually comes from winter storms: roof leaks, wet crawlspaces, and floods along the Russian River, the Laguna de Santa Rosa and the Petaluma River. Fix the water and remove what's growing, and the mold problem is solved. Miss either step and it comes back.
1. Why mold matters
CDPH says visible mold, visible moisture, water-damaged materials or mold odor in a building are clearly linked to increased risk of respiratory effects, including asthma development and attacks, allergies and respiratory infections. The more extensive the dampness and mold, the greater the risk (CDPH). Talk to your doctor if you have health concerns.
2. How to tell if you have a problem
Look for water stains, discolored surfaces, warped floors, peeling paint, rotting wood, condensation and a musty smell. Hidden mold often lives behind baseboards, under flooring, inside walls and in crawlspaces and attics. See mold inspection.
3. Testing: usually not needed
CDPH says if you can see or smell mold or moisture, you don't need to test or identify the species; there are no recommended "unhealthy levels," and DIY kits aren't recommended. Read more in mold testing: what California says.
Want a local pro to take a look?It's free for homeowners and there's no obligation.
4. DIY or professional?
The EPA says homeowners can usually handle areas under about 10 square feet. Hire help for larger areas, contaminated floodwater or sewage, or suspected HVAC contamination (EPA). Use gloves, goggles and an N-95 respirator; scrub hard surfaces with detergent and water; don't paint over mold.
5. What professional remediation includes
Containment, protective equipment, removal of porous materials that can't be cleaned, cleaning of hard surfaces, drying and, above all, fixing the moisture source. See mold remediation.
6. Choosing a provider
California doesn't license mold inspectors or remediators, according to CDPH. Look for relevant experience and certifications such as IICRC, NORMI or NAMRI, and a CSLB license if the job includes construction. Our guide to choosing a mold remediation company has questions to ask.
Dealing with this right now?Tell us what's happening and we'll connect you with an independent local provider.
7. Renters
CDPH explains that under California's housing law (Health and Safety Code section 17920.3(a)(13)), visible mold growth, as determined by health or code enforcement officers, can make a dwelling substandard, and the owner must fix it. Tenants can contact local code enforcement to request an inspection.
8. Prevent the next round
Keep indoor humidity below 60 percent, ideally 30 to 50 percent, and dry leaks within 24 to 48 hours (EPA). Keep gutters clear, direct downspouts away from the house and vent bathrooms and dryers outdoors.
9. Winter 2026–27
NOAA's Climate Prediction Center has an El Niño Advisory in effect. Its October 8, 2026 discussion says a strong-to-very strong El Niño is likely through January–March 2027 (better than an 83% chance), with impacts "larger, though not guaranteed." More rain means more chances for leaks, floods and damp crawlspaces, which is how most indoor mold starts. CPC's next update is November 12, 2026.
Read our after-the-storm drying and mold timeline.
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Sources
- CDPH: Frequently asked questions about mold
- EPA: A Brief Guide to Mold, Moisture and Your Home
- EPA: Mold cleanup in your home
- NOAA CPC: ENSO Diagnostic Discussion, Oct 8, 2026
Sources checked October 18, 2026.
Frequently asked questions
Does bleach kill mold?
CDPH doesn't recommend bleach. It says bleach can be a respiratory hazard and that thorough cleaning with soap and water is enough for non-porous materials.
Can mold be removed permanently?
Mold spores are everywhere, so the goal is removing growth and keeping materials dry so it can't return.
Who enforces mold problems in rentals?
Local code enforcement or environmental health agencies. CDPH's Air Quality Section doesn't do inspections.