What kills mold, and why killing it is not enough

The wrong question

People ask what kills mold because they want it gone for good. But mold is not like an infestation that ends when the last one dies. Spores are in the air of every home, indoors and out, all the time. They settle on everything. They grow wherever they find moisture.

So two things decide whether mold comes back:

1. Whether the material stays damp
2. Whether the growth was physically removed

A product that kills what is there, on a wall that is still wet, buys a few weeks.

Dead mold is still a problem

Mold affects people mainly through allergy and irritation, and dead spores and fragments cause both just as live ones do. A patch that has been sprayed and left is still on the wall. It can still be disturbed, still stain, and still affect someone who is sensitive. This is why public health guidance talks about removing mold, not killing it.

What Health Canada recommends

For small areas on washable surfaces, Health Canada’s guidance is to scrub with a cloth and an unscented soap solution or a little baking soda, wipe clean and dry the surface quickly. It says plainly not to use bleach to clean up mold.

Protection for the person cleaning: safety glasses or goggles, a well-fitting N95 respirator and household gloves.

It treats three or fewer patches with a total area under one square metre as small. It advises a professional for a patch larger than three square metres, mold that keeps returning, and mold hidden inside walls or above ceilings.

Common products, honestly assessed

Soap or detergent and water. Works. The scrubbing does the job, lifting growth off the surface so that it can be wiped away.

Bleach. Widely used, and not recommended. It can lighten the stain, which makes the patch look gone. It is harsh on lungs, skin and eyes, damages many surfaces, and must never be mixed with ammonia or acids. On porous materials such as drywall and wood, it does not reach growth below the surface, and the water in it adds moisture.

Vinegar. Mildly acidic and often suggested online. It can help with light surface growth, and the smell fades. It is not a substitute for scrubbing and drying. Do not mix it with bleach.

Baking soda. Mentioned in Health Canada’s guidance as a cleaning option. Mild and safe on most surfaces.

Hydrogen peroxide. Can lighten stains on some surfaces. Test first, as it bleaches fabrics and finishes.

Tea tree oil and other essential oils. Popular, expensive and strongly scented. Scents can irritate sensitive people.

Borax. Sometimes used on wood. Keep it away from children and pets.

Commercial mold and mildew sprays. Most are bleach or a disinfectant in a bottle, with the same limits. Follow the label.

“Mold-killing” primers and paints. These resist growth on their own surface. They are useful after proper cleaning and drying. Painted over active mold, they fail.

Machines: ozone, foggers, UV and air purifiers

Ozone generators. Sold as mold killers. Health Canada advises against using ozone generators in homes. Ozone irritates the lungs, and at levels people can tolerate it does not remove mold from materials.

Foggers and “mold bombs”. They spread a chemical through a room. They do not remove growth, do not dry anything and do not reach inside walls. They can leave residue on everything.

UV lamps. Can inactivate organisms on a surface directly exposed for long enough. They do nothing for growth inside materials or in shadow.

Air purifiers. A HEPA unit captures spores from the air, which can help someone who is sensitive. It does not touch the source.

Dehumidifiers. The one machine that addresses the cause. Keeping humidity between 30 and 50 percent removes what mold needs.

Does mold die when it dries out?

It stops growing. Without moisture, mold becomes dormant, not harmless. Spores survive dry conditions for a long time and start again when the material gets damp. Dried mold is also brittle and releases spores more easily when disturbed. So drying is essential, but dry mold still needs removing.

By material

Hard, non-porous surfaces: tile, glass, metal, sealed countertops, tubs. Scrub, wipe, dry. Fully cleanable.

Semi-porous: painted drywall with light surface growth, finished wood, concrete. Usually cleanable if the growth is on the surface and the material is sound.

Solid wood framing. Cleaned by HEPA vacuuming, scrubbing or sanding, then dried. See mold on wood.

Porous: drywall that is moldy through, ceiling tiles, carpet and underpad, insulation, upholstery, mattresses, cardboard and paper. Remove and discard. No product makes these clean again.

What professionals do differently

Professional remediation relies on the same principle, removal, with better control:

• The moisture source is found and fixed
• The work area is sealed off and kept under negative pressure with HEPA-filtered air, so that spores are not spread
• Moldy porous materials are cut out and bagged
• Remaining surfaces are HEPA vacuumed and physically cleaned
• The structure is dried and checked with a moisture meter

Antimicrobial products may be used as a supplement on cleaned surfaces. They are not the method.

Safety when cleaning

• Wear gloves, eye protection and an N95 respirator
• Open a window and run an exhaust fan
• Dampen the patch lightly before wiping, to keep spores down
• Never mix cleaning products
• Bag cloths and sponges and take them out
• Keep children, pets and anyone with asthma or allergies away

If it comes back

Mold that returns after cleaning is telling you the moisture is still there. Look for a leak, condensation on a cold surface, poor ventilation or damp inside the wall. That is the point to get help.

More: mold removal and remediation.