Mold in Your Home: Causes, Health Effects, Cleanup & Prevention

Mold vs. Mildew

Molds are fungi. They reproduce by releasing microscopic spores that float through indoor and outdoor air all the time; there is no such thing as a spore-free house. Spores only become a problem when they land somewhere with enough moisture to germinate and something organic to eat, such as drywall paper, wood, dust, fabric or the soap film in a shower.

“Mildew” is not a separate organism so much as an everyday label for a certain look: flat, powdery, gray or white growth sitting on the surface of a damp material. It wipes off tile grout, window sills and shower curtains with little effort. What people call “mold” is usually fuzzier or slimy, often black, green or brown, and has grown into a porous material rather than sitting on it. That distinction matters for cleanup: surface mildew on hard, non-porous material is a quick job, while mold that has penetrated drywall or insulation generally means the material has to come out.

Humidity Is the Root Cause

Every mold problem is a moisture problem. Sometimes the moisture is obvious: a roof leak, a burst pipe, a basement that floods, a window that lets rain in. Just as often it is invisible water vapor. When indoor air holds too much humidity, it condenses on the coolest surfaces it touches, such as exterior walls behind furniture, window frames, uninsulated pipes, closet corners and basement slabs, and gives mold exactly the film of water it needs.

The EPA's guidance is to keep indoor relative humidity below 60 percent and ideally between 30 and 50 percent. Held under 50 percent, a home with no active leaks is a very poor host for mold. The lever is measurement: an inexpensive digital hygrometer in the dampest room, usually a basement, bathroom or bedroom with an exterior corner, tells you whether you actually have a humidity problem instead of guessing. Common sources of excess indoor humidity include:

• Showers and cooking without exhaust fans that vent outdoors
• Clothes dryers vented indoors or with disconnected ducts
• Unvented gas or kerosene space heaters (which also raise carbon monoxide risk)
• Damp basements and crawl spaces with exposed soil or poor drainage
• Oversized air conditioners that cool quickly without running long enough to dehumidify
• Large numbers of houseplants, aquariums, or drying laundry indoors

Where Mold Hides

Visible mold on a bathroom ceiling is the easy case. The growth that affects air quality most is usually out of sight. Check, or have a professional check, these spots when you smell a musty odor without an obvious source:

• Behind and under things: the back of drywall after a leak, under sink cabinets, beneath vinyl flooring or carpet pad that got wet, behind wallpaper
• Around water: window sills and frames, refrigerator drip pans, washing machine gaskets, the base of toilets, water heater closets
• HVAC systems: the evaporator coil and condensate pan, dirty filters, and duct insulation that has been wet
• Attics and roofs: the underside of roof sheathing where bathroom exhaust fans dump into the attic instead of outdoors
• Basements and crawl spaces: the bottom edge of drywall, stored cardboard boxes, rim joists, and any area that floods seasonally

A persistent earthy or musty smell is a reliable signal even when you cannot see anything. Follow the smell and the moisture, not the color.

Health Effects: What the Evidence Supports

Mold exposure is a genuine health issue, but the internet has stretched it well past what is known. The CDC and EPA describe the following effects:

• Allergic reactions are the most common response: sneezing, runny or stuffy nose, red or itchy eyes, skin irritation, and coughing. People with mold allergies react more strongly.
• Asthma. Mold can trigger attacks in people who already have asthma, and there is evidence linking damp, moldy homes to the development of asthma in children.
• Respiratory irritation in otherwise healthy people, especially in heavily contaminated spaces.
• Infections in people with weakened immune systems or chronic lung disease, who should avoid moldy environments and cleanup work entirely.

On “toxic black mold”: some molds can produce mycotoxins under some conditions, but the CDC is clear that the phrase is not accurate, that color does not indicate toxicity, and that the recommended response to any indoor mold is the same regardless of type: remove it and fix the moisture. Routine air sampling and species identification are generally not necessary for homeowners and do not change the fix. If you see it or smell it, you have enough information to act.

DIY Cleanup and Its Limits

The EPA's rule of thumb is that a moldy area smaller than about 10 square feet, roughly a 3-by-3-foot patch, is reasonable for a homeowner to clean. Within that limit:

1. Fix the water problem first. Cleaning mold while the surface stays damp is wasted effort.
2. Protect yourself. Wear an N95 respirator, non-porous gloves and goggles without vents. Open windows and run a fan blowing outdoors.
3. Scrub hard surfaces with detergent and water, then dry them completely. The EPA does not recommend routine use of bleach or other biocides for mold cleanup; killing mold is less important than removing it, because dead spores are still allergenic.
4. Discard porous materials that are moldy: ceiling tiles, carpet pad, insulation, and drywall that has been soaked. They cannot be cleaned reliably.
5. Bag waste before carrying it through the house to avoid spreading spores.
6. Do not paint or caulk over mold. Paint over moldy surfaces peels, and the growth continues underneath.

When to Hire a Professional

• The affected area is larger than about 10 square feet, or you suspect a lot more behind walls or under floors
• Mold is inside your HVAC system or ductwork (and do not run the system until it is addressed, or you will spread spores through the house)
• The water came from sewage or floodwater, which brings bacteria and other contaminants along with mold
• Mold keeps returning after cleanup, which means the moisture source has not actually been found
• Anyone in the home has asthma, a compromised immune system, or is experiencing symptoms that track with being at home

Look for contractors with experience specifically in mold remediation and ask whether they follow the EPA's mold remediation guidelines or an industry standard such as the IICRC S520. Be wary of anyone who leads with air testing and fogging rather than finding and fixing the moisture source.

Prevention

Once the growth is gone, prevention is mostly moisture discipline:

• Dry wet materials within 24 to 48 hours. That is roughly the window before mold takes hold after a leak or spill.
• Vent moisture outdoors. Run bathroom fans during and for 20 minutes after showers, use the range hood when cooking, and make sure dryer and bath fans exhaust outside rather than into the attic.
• Hold humidity between 30 and 50 percent. In humid climates and basements a dehumidifier is usually the only way to get there in summer; size it to the space and empty or drain it reliably.
• Insulate cold surfaces such as pipes, exterior walls and windows to stop condensation.
• Manage water outside. Keep gutters clear, extend downspouts away from the foundation, and grade soil to slope away from the house.
• Keep air moving. Leave a gap between furniture and exterior walls, and open closet doors periodically.
• Maintain HVAC. Change filters on schedule and keep the condensate drain clear.

Where an Air Purifier Fits

A True-HEPA purifier captures airborne mold spores along with dust, pollen and pet dander, and in a bedroom that can noticeably ease allergy and asthma symptoms. Be clear about what it does not do: it does not lower humidity, does not dry a wet wall, and does not remove growth from surfaces. Treat it as the last step after the moisture is controlled and the visible mold is gone, not as a substitute for either.

Related Indoor Air Hazards

The damp, below-grade spaces where mold thrives are also where radon collects, so a basement moisture project is a good moment to run a radon test as well. Fuel-burning appliances in those same utility rooms are the usual source of carbon monoxide. For the broader picture on ventilation, filtration and source control, see our guide to improving indoor air quality.