Ozone Generators for Mold: Why We Don't Recommend Them
An ozone generator mold treatment is not something we recommend for a home. Ozone is a lung irritant, Health Canada advises against using ozone generators in homes, and the gas does not remove mold or reach growth inside drywall, wood or insulation. At best it reduces odour for a while. The mold and the moisture that feeds it are both still there when the machine is switched off.
What an ozone generator does
Ozone is a form of oxygen with three atoms in each molecule where ordinary oxygen has two. It is unstable and reacts with almost anything it touches, including odour molecules. Ozone generators make it from room air using an electrical discharge or ultraviolet light. They are sold and rented as air purifiers, odour removers and mold killers, under names such as activated oxygen or energized air.
The same reactivity that breaks down odours is what makes ozone harmful to breathe. It reacts with the lining of the airways. It is the main irritant in summer smog, and air quality advisories in Southwestern Ontario are often about ground-level ozone.
Ozone generator mold treatment: what it cannot do
- It does not remove anything. Mold that has been exposed to ozone is still on the wall, alive or dead. Dead mold can still trigger allergy and asthma symptoms, and it still shows up in a home inspection.
- It does not penetrate. Ozone reacts with the first surfaces it meets. Growth inside drywall, behind baseboards, under carpet and in wall cavities is largely untouched.
- It needs unsafe concentrations. The levels required to have a real effect on mold on surfaces are far above what is safe for people or pets to breathe, and low levels that are safer do very little.
- It does not dry the building. The damp material that grew the mold is exactly as wet afterwards.
- It masks the warning sign. Ozone dulls your sense of smell and breaks down odour compounds for a time, so a musty smell seems to be gone while the growth continues.
What Health Canada says about ozone generators
Health Canada advises Canadians not to use ozone generators in their homes. Its concern is that these devices can raise indoor ozone to levels that irritate the lungs and airways. Breathing ozone can cause coughing, chest tightness and shortness of breath, and it can make asthma and other lung conditions worse. Those are the same people who are most affected by mold, so the treatment adds a second irritant for the household members least able to tolerate it.
If someone has been in a space while an ozone machine was running and has chest or breathing symptoms, they should get into fresh air and seek medical advice.
Damage ozone can do inside a house
High ozone levels are hard on materials as well as lungs. Ozone attacks natural rubber, so seals, gaskets, elastic and some wire insulation can crack and become brittle. It can fade dyes and pigments in fabric and artwork. It also reacts with compounds already in indoor air and on surfaces, such as the scents in cleaning products and the oils in wood and carpets, and those reactions can form other irritants, including formaldehyde and very fine particles. Many people notice a sharp, metallic smell in a room after treatment. That is a sign of these reactions and of leftover ozone, and the room needs to be aired out thoroughly.
Don't restoration companies use ozone?
Some do, for odour, under controlled conditions. In fire and smoke restoration, ozone or similar treatments are sometimes used in a sealed, empty space, with no people, pets or plants present, followed by ventilation before anyone goes back in. The key point is that it comes after the source of the odour has been physically removed and cleaned. It is a deodorizing step for whatever smell is left. It is not a method of mold remediation, and the IICRC S520 standard for mold work is based on physical removal and moisture control.
A rented machine running in an occupied house with mold still on the framing is a different situation. So is a small plug-in ozone purifier left on in a bedroom. Neither is doing what the brochure suggests.
What removes mold and musty odour
- Find the moisture and stop it. That could be a leak, condensation or high humidity.
- Remove moldy porous materials such as drywall, insulation and underpad, inside a containment kept under negative air through HEPA filtration.
- Clean what stays. Framing and sheathing are HEPA vacuumed, brushed or sanded, and wiped.
- Dry the structure and prove it with a meter. We dry framing to under 16% wood moisture content and log the readings daily.
- Ventilate and filter. Fresh air and HEPA filtration clear what is left in the air without adding anything harmful.
- Keep humidity near 50% or lower in summer and around 30% in cold weather so it stays that way.
When the source is gone and the materials are dry, the musty smell goes with it, usually within days. If you already own or have rented an ozone machine, the safest choice is to leave it off. Put the effort into finding where the dampness is. A hygrometer and a look behind stored items in the basement will tell you more about a musty house than any machine that treats the air.
Common questions
How long after ozone treatment is a room safe to enter?
Ozone breaks down over time, faster with ventilation, but the safe wait depends on the machine and the space. Follow the manufacturer's instructions and air the room out well, and note that Health Canada advises against using these machines at home at all.
Are ionizers and air purifiers that produce ozone a problem too?
Some electronic air cleaners release ozone as a by-product. A purifier with a true HEPA filter and no ozone or ionizing function is the safer choice.
Is hydroxyl treatment the same as ozone?
No. Hydroxyl generators use a different process and are marketed as usable in occupied spaces, but like ozone they address odour and do not remove mold growth or dry wet materials.
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