Mold after water damage: when to worry and when to get help

Mold is a moisture problem first. EPA advises drying water-damaged areas and items within 24-48 hours to prevent mold growth. If a moldy area is smaller than about 10 square feet (roughly a 3 ft by 3 ft patch), EPA says you can usually clean it yourself. Larger areas, mold after sewage or other contaminated water, and mold you suspect in the heating and cooling system call for experienced professionals. Fixing the water source matters more than identifying the mold. EPA, CDC and the California Department of Public Health (CDPH) all say testing is usually unnecessary when you can see or smell mold.

Safety first

• Call 911 for a gas smell, sparking, water near the electrical panel, a sagging ceiling or an injury.
• Who should stay out: people with asthma, allergies, weakened immune systems or chronic lung disease should stay out of the area during cleanup. CDC notes that people with asthma or mold allergies may have severe reactions, and that immune-compromised people and people with chronic lung disease can get lung infections from mold.
• Health questions: EPA advises consulting a health professional before starting cleanup if you have health concerns. This guide can’t tell you whether mold is affecting anyone’s health; a doctor can.

Why the first 24-48 hours matter

Mold needs moisture. Once materials such as drywall, wood, carpet and pad stay damp, growth can start. EPA’s household guidance is to dry water-damaged areas and items within 24-48 hours. Its flood-cleanup guidance warns that mold can grow on wood, drywall, carpet and furniture that stay wet for more than 24 hours. These are guidelines, not a countdown. Warm rooms and materials that stay wet speed things up.

That is why the water you can’t see matters most. EPA lists common hiding places: the back side of drywall, wallpaper or paneling, the top side of ceiling tiles, the underside of carpets and pads, and inside walls around leaking pipes. A surface that looks dry can sit in front of a wet wall cavity.

Signs worth taking seriously

• Visible growth: spots, fuzz or discoloration, especially where there was water.
• A musty or earthy smell, even if you can’t see anything. CDPH treats mold odor as a sign of a mold or moisture problem.
• Water damage that went unnoticed for days, such as a slow leak under a sink or behind a dishwasher.
• Warped baseboards, bubbling paint or peeling wallpaper near a past leak.
• Materials that were wet for more than a day or two and were never dried with measurements.

CDPH says building dampness and mold are linked to increased risk of respiratory effects, including asthma and allergies, and that the more extensive the dampness, the greater the risk. It also says you don’t need to know whether it’s “toxic mold” or “black mold”: any mold you see or smell should be fixed. CDC agrees that no matter what type of mold is present, it needs to be removed.

Small areas: EPA’s 10-square-foot guideline

For a patch under about 10 square feet, EPA’s guidance for homeowners is:

1. Fix the water problem first. Otherwise the mold comes back.
2. Protect yourself with an N-95 respirator, gloves and goggles without ventilation holes.
3. Scrub mold off hard surfaces with detergent and water, and dry them completely.
4. Throw away porous materials that are moldy, such as ceiling tiles and carpet. They may not come clean.
5. Don’t paint or caulk over mold. Clean and dry the surface first, or the paint will likely peel.
6. Skip routine bleach. EPA doesn’t recommend bleach or other biocides as a routine part of mold cleanup. A background level of spores will remain, and they won’t grow once the moisture problem is fixed. If you do use a disinfectant, ventilate the area to the outdoors and never mix bleach with other cleaners.

When to bring in a professional

• Larger areas: mold covers more than about 10 square feet, or there was a lot of water damage. EPA points to its guide for professional remediation in these cases.
• Contaminated water: the damage was caused by sewage or other contaminated water. EPA says to call in a professional.
• HVAC: you suspect mold in the heating and cooling system. EPA says not to run a system you know or suspect is contaminated, because it could spread mold through the building.
• Hidden mold: you smell mold but can’t find it, or it’s inside walls, under floors or in the attic.
• Higher-risk household: someone in the home is at higher risk.

Choosing a contractor: EPA advises making sure the contractor has experience cleaning up mold, checking references, and asking them to follow EPA’s remediation guidance or other professional guidelines. Professional work typically includes containment to keep spores from spreading, removal of moldy porous materials, HEPA vacuuming and cleaning, drying confirmed with moisture readings, and correcting the moisture source.

What mold testing can and can’t tell you

• EPA: “In most cases, if visible mold growth is present, sampling is unnecessary.” It adds that no EPA or other federal limits have been set for mold or mold spores.
• CDC: “CDC does not recommend mold testing.”
• CDPH: there are no recommended “unhealthy levels” of spores or mold types, and “no mold tests or measurements can show when remediation efforts have been successful.” CDPH’s clearest sign that the job is done is a fixed moisture source, damaged materials cleaned or removed, and remaining materials that are dry, with no visible mold or mold odor.

Testing still comes up sometimes, for example in a real-estate transaction, a dispute, or at a doctor’s suggestion. If you decide to test, consider hiring an assessor who is independent of the company that will do the removal, so the person measuring isn’t the person selling the fix. Agree in advance on what the results will be used for.

Be wary of anyone who promises a home with no mold at all. EPA notes that it’s impossible to get rid of all mold and mold spores indoors, because some are always in the air and in house dust. The realistic goal is no mold growth, and that comes from controlling moisture.

Keeping it from coming back

• Fix leaks promptly and dry wet materials within 24-48 hours.
• Keep indoor humidity below 60%, ideally 30-50%, per EPA.
• Run the bathroom fan when showering and exhaust fans when cooking or running the dishwasher. Vent the clothes dryer outdoors (EPA).
• Replace porous materials that were moldy. CDPH notes that previously moldy porous materials left in place are even more likely to grow mold again.

This guide is general information, not medical advice. If you have health concerns, talk to your doctor.