Draeger gas pum and test tube for ozone (C) D FriedmanUse of Ozone Generators for Mold / Odor Removal
Hazards, False Claims, Overdosing & Oxidation Smells

  • OZONE MOLD / ODOR TREATMENT WARNINGS - CONTENTS: Warnings About Using Ozone for Treatment of Indoor Air. What is Ozone - O3 Problems Encountered Using Ozone to Get Rid of Skunk, Indoor Odors, or Mold Smells?Ozone Hazards & Ozone Toxicity Levels. Oxidation of Building Carpets, Contents, Coatings from Ms-Use of Ozone Generators. Materials that May be Oxidized & Become Odor Sources from Excessive Ozone Treatment. Using Ozone Left a Smell in our Home. Use this Smell Patch Test to Track Down Ozone-Oxidation Problems. General Use of Ozone Generators nor Ozone Shock Treatments as a "Cure" for Building Mold or Odors is Not Recommended
  • POST a QUESTION or READ FAQs about using ozone generators to kill odors or mold: dangers & false claims & about how to get rid of odors caused by ozone overdosing
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Ozone air treatment warnings:

Ozone has been widely used as a disinfection method for more than 100 years and has applications ranging from hospital disinfection to water treatment. However if ozone treatments are not properly matched to the application the results can be both ineffective and potentially harmful.

This article provides government and other authoritative warnings about using ozone generators and ozone air purifiers in buildings to "purify" indoor air or to "kill mold" in buildings. We give a definition of ozone or O3, we explain what problems can arise when using ozone generators to try to get rid of odors indoors or to try to kill mold.

We explain the problem of oxidation of building materials from excessive ozone exposure and the horrible chemical smells that may follow such mistakes. We describe how to track down which building materials were over-dosed with ozone and are now giving off a new stink, and we explain how to cure that problem. (Note: other uses of ozone as a disinfectant can be effective and are important in many applications.)

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Warnings About Using Ozone for Treatment of Indoor Air: Respiratory System

Article Series Contents

What is Ozone - O3

While there are many sound and important uses of ozone (such as for medical disinfection under controlled conditions), in general this is an idea which ranges from bad to dangerous in the home. Ozone is widely promoted by ozone generating equipment companies and cleaning services for use in indoor building environments to deodorize, disinfect, "kill" mold, and for "general health".

Ozone generators are also promoted for use to reduce the level of airborne particles, pollen, animal dander, and allergens, ostensibly to improve indoor air quality for asthmatics and people with allergies.

Ozone or O3, or "trioxygen" is a molecule made of three oxygen atoms. In this form, and referred to as an "allotrope" of oxygen, ozone is an unstable gas - that means it breaks down into oxygen molecules.

While ozone is helpful in the upper atmosphere (filtering out UV light rays), in lower atmosphere, or in buildings, it is an air pollutant that is harmful to humans and other animals, and a gas that can oxidize or "burn" plants or various materials found indoors.

Ozone is widely used in industry in a variety of applications and can be of significant benefit and use when applied

A separate question remains, in some cases, of whether or not building occupants have been exposed or are being exposed to harmful ozone levels.




Problems Encountered Using Ozone to Get Rid of Skunk, Indoor Odors, or Mold Smells?

But nevertheless, ozone is a highly toxic gas. Now even highly toxic substances can be encountered safely. The main concern with ozone exposure is that the ozone concentrations to which people are exposed

  1. should not average more than 0.1 ppm over an 8-hr day, and
  2. should not exceed that value by more than a factor of 2 or 3 during the exposure." [1]

Ozone Hazards & Ozone Toxicity Levels

Exposure to a level you can smell or exposure to ozone over long periods at levels greater than 0.05 ppm for 24 hours at a time is likely to be dangerous: [2], [3]Health hazards to humans and animals occur and can be severe at ozone levels used for indoor cleaning purposes.

At least some people can smell levels of ozone down to 0.05 ppm. This odor-detection level is already half-way to the recommended limit. If you are generating ozone indoors, even at "low" levels a problem may be present. People become desensitized to odors in a short time, perhaps 20 minutes. So if you do not smell it, the ozone level could still be hazardous. Problems include:[4]

Use of ozone to "remove" or "kill" mold is ineffective, not recommended, and may be dangerous. Even if ozone were applied at a concentration and for a duration sufficient to "kill" every mold spore in a building (which is a very dubious claim), depending on the mold genera/species present there is a good chance that the process leaves toxic and allergenic particles in the building.

Ozone Toxicity: How toxic is ozone, general background, levels of ozone gas toxicity, ozone gas applications

The following information about Ozone is quoted from "Health Hazards of Some Gases" [8]

"Ozone is a kind (called an "allotrope") of oxygen . It is formed in the ionosphere by the action of ultraviolet radiation from sunlight on oxygen. Lightning strokes are another natural source of ozone and the characteristic odor of that material can often be noted during and after a thunderstorm.

When pollutants are emitted into the air either by man or nature, almost all are eventually removed by one or more of several processes including reaction under the influence of ultraviolet radiation. One series of such reactions results in the formation of ozone as a "secondary" (formed by reaction in the air) air pollutant, often in rather high concentrations (several tenths of a part per million).

"As ozone can be formed by nature's sparks (lightning), it can also be formed by man's. Whenever an electrical spark or corona occurs in air, some ozone is formed. This accounts for the characteristic odor noted near an operating electric motor such as an electric shaver.

"Because ozone is found in so many places, its toxicity (ability to injure a living organism by other than mechanical means) has been investigated extensively since the early 1900s. Experimentation has shown that the odor of ozone can be detected and identified by most people at a concentration of from 0.02 to 0.05 ppm (parts ozone per million parts air + ozone). As the concentration increases to a few tenths of a part per million, the first effect noted is likely to be a feeling of dryness in the back of the throat. If a concentration on the order of 0.2 or 0.3 ppm is inhaled more or less continuously for several hours to a few days some lung irritation may result.

"Higher concentrations can produce several kinds of toxic effects if exposures are sufficiently prolonged. Eye irritation (despite newspaper and TV accounts seemingly indicating otherwise) occurs only at concentrations high enough to result in other, more severe, toxic effects.

"Ozone is a very reactive substance. It will readily react with just about any material capable of being oxidized, and with many that are not. The material with which it reacts may be a gas or vapor, a particle floating in the air (a mold spore, for example), or a solid (or liquid) surface. For this reason, when ozone is present in most enclosed spaces its concentration declines quite rapidly with time. Of course, if ozone is being generated more rapidly than it is destroyed by reaction, its concentration can build up. This is the main reason why devices that produce relatively large amounts of ozone are safe only in relatively large enclosures and why the ozone generation rate should be reduced in small enclosures.

"Ozone is well known for its ability to eliminate certain odors. How this is accomplished is controversial. At concentrations just above the odor threshold, some odors do seem to vanish. The main reason for this may be ozone's ability to desensitize the olfactory apparatus so that the odors can no longer be perceived. Some evidence indicates that this may be the case at least occasionally. Other evidence indicates that ozone may react with the odor-causing substances, eliminating them from the air (this is probably the only mechanism that operates when concentrations are below the odor threshold).

"Finally, some people have insisted that even if ozone does not paralyze the olfactory sense, its odor is such that it "masks" other odors. Perhaps all three mechanisms operate, each in its own area of effectiveness.

"As with all other materials, ozone has a dose-effect relationship with a threshold. That is, once the threshold dose has been exceeded, toxic effects are proportional to dose. For inhaled gases, dose is proportional to both time and concentration. If the duration of exposures cannot be controlled (as is usually the case), then the concentration must be kept low enough so that no injury will occur even from prolonged and repeated exposures. For ozone, that "threshold" concentration is 0.1 ppm.

So long as concentrations are kept at or below that level, injury is not expected even in the most sensitive workers so long as their exposure durations coincide reasonably well with or are less than the 8 hr/day, 40 hr/wk regimen. This "threshold" level is accepted by the American Conference of Governmental Industrial Hygienists (and is called the Threshold Limit Value by that organization) and by the Occupational Safety and Health Administration, OSHA.

The TLV or OSHA's Permissible Exposure Level (PEL) is not a fine line between safe and non-safe. Instead, it represents the best judgment of a group of experts of the highest concentration that can be inhaled repeatedly by a population of workers with no resulting injury. Higher concentrations may or may not have any particular effect on a specific individual.

"Ozone is a highly toxic gas but even highly toxic substances can be encountered safely. The main concern with this material is that concentrations to which people are exposed do not average more than 0.1 ppm over an 8-hr day, and do not exceed that value by more than a factor of 2 or 3 during the exposure."

Odors from Ozone-Oxidation of Building Carpets, Contents, Coatings from Misuse of Ozone Generators

A second class of problems when ozone is misapplied indoors is the creation of lingering odors due to the oxidation effects of the highly reactive ozone gas while it was present.

In our OPINION, following ozone use as a "deodorant" if there is no lingering odor from oxidized materials in the enclosed space (a building, car, boat, RV, etc), and considering that ozone itself is so volatile that it does not hang around in the building, then its application probably did not create a problem for the building.

Ozone is never recommended as a "mold killer" since that strategy is fundamentally flawed in the first place. Details are

Ozone Oxidation Problems: Second-Cause Odors in Buildings Following use of an Ozone generator

At OZONE AIR PURIFIER WARNINGS we include an example report of horrible building odors that were caused by attempts to get rid of an indoor odor using an ozone generator. when high levels of ozone have been produced in an enclosed space, we find that other materials in the space become partly oxidized, subsequently giving off horrible, often chemical-like odors. We have traced odors to painted surfaces, furniture, upholstery, curtains, carpeting, carpet padding, and other materials.

Common Materials that May be Oxidized & Become Odor Sources from Excess Ozone Treatment

Here are some examples of material we've found giving off horrible smells after misuse of an ozone generator. (Misuse means using the ozone generator to try to kill mold, or running an ozone generator too long at too high a setting in too small a space - overdoing it).

Using Ozone Left a Smell in our Home - How do I Get Rid of It? - Using Ozone Indoors to Cure Skunk Smells, Mold Odors, and other Stinks

This quote from a reader's email pretty well sums up what happens if you overdo it when using an ozone generator indoors to try to "kill off" odors:

It's a long story, but I used a high powered ozone generator in our house, to get rid of skunk smell. Now I can't get rid of the left over nitric oxide, or whatever odor or gases, that linger in our house. I have been leaving the windows open every day, with running the heat on high (85 degrees) at night, to try to force off-gas the odors/gases.

We have investigated a number of cases of misapplication of ozone generating machines both to "kill mold" (no good, you're leaving toxic or allergenic particles, and you haven't corrected the reason for mold growth in the first place). We have also investigated several cases of excessive ozone-use to try to remove odors from buildings, including fire or fireplace smells, mold smells, pet or animal smells, skunk odors, smoking odors, etc.

Here is another similar case:

Our dog was sprayed by a skunk and then ran through our house. The skunk smell was terrible. We hired servpro to get rid of the odor. They used the ozone machine and although is helped to get rid of the skunk smell, we now have a lingering chemical smell.

We have had our walls, ceiling, furniture, rugs, clothes, bedding all professionally washed but the smell still remains. What do you recommend? BTW, we live in eastern massachusetts. Thanks for your time and for this service you provide. - S.M.

Find & Eliminate Odor Sources Using This Simple Smell Test Procedure

To track down the source of post-ozone-treatment smells, try making a smell-patch test to determine just which building component has been oxidized. Following this procedure we can often narrow down the source of post-ozone-treatment smells to a single material that can then be removed or remedied, such as carpet padding or a specific piece of furniture.

A complete guide to tracking down odors in buildings is at ODORS, Smells, Gases in Buildings-Diagnosis & Cure.The smell patch test procedure and its use to track down building odors caused by over-dosing with ozone is described here.

Smell test setup on carpeting (C) Daniel FriedmanBefore Using Ozone or Gutting a Building to Get Rid of Odors, Try the Smell Capture Patch Test to Pin Down a Specific Indoor Odor Source

Our friend and fellow forensic investigator Jeffrey May suggested a smell or odor source track-down procedure for pinning down a specific odor test in buildings - it has worked remarkably well for us where ozone had caused an indoor smell that could not be tracked down as well as for general odor emitting source identification.

The odor source pinpointing procedure uses simple materials readily available: paper towels, masking tape, aluminum foil, and a person with a good sense of smell.

However, essential for success are the steps and their sequence, and the choice of who is going to do the sniffing, as we describe in detail in our adaptation and illustrations of Jeff's idea, now found
at SMELL PATCH TEST to Track Down Odors.

We have had very good results with this procedure when used to narrow down odor sources in an ozone-treated building, and in a field study we obtained roughly 95% odor source identification reliability when we used additional smell test patches.

How to Use the Smell Patch Test to Track Down Ozone-Oxidation Problems

Look First at These Prime Suspects for Ozone Oxidation

If you have aired out the building and days or more have passed and you still smell a "new" chemical or plastic or other odor that was not there before you tried using your ozone generator, you'll need to determine just what materials were oxidized by the high levels of ozone in the building.

It's been our experience that once you identify and dispose of the new-smelly material you'll probably be fine.

However, by nose alone, it is very difficult to track down a specific indoor material to the odor source in this case. Jeff May suggested[1b], and I've more extensively explained and documented an inexpensive means to track down odor sources to indoor materials or furnishings:


The procedure works best if you have as the "smeller" a person with a good sensitive ability to discriminate among odors. The smeller can briefly go indoors to become familiar with the odor whose source you are seeking. But they then have to stay outdoors breathing clear air long enough to regain their original smell sensitivity. (When we are exposed to an odor for some time, our brain starts to tune it out.)

So typically you bring in the smeller, let her sniff and agree that she will recall the objectionable odor, then give her a few days off while you prepare the test we describe above. You use the foil, tape, and paper towel procedure I describe at the link above.

Watch out: People become desensitized to odors in a short time, perhaps 20 minutes. So if you do not smell it, the ozone level could still be hazardous.

Reader Question: We left the ozone machine running and are now left with a strong chemical smell

(May 6, 2014) L.K. said:

We purchased second hand furniture for my sons room that smelled like smoke. A friend gave us an ozone generator to eliminate the smell. Unfortunately, we did not know much about ozone. We left it running on high in his room for many hours and now are left with a strong chemical smell. Upon doing research, we now realize that ozone is unsafe and have sealed off the room and had windows open and fan running for a week. However, the smell still lingers and gives us a headache to even walk into the room.

I am concerned for my 8 year old son to move back into that room and he is concerned for all his belongings! We have small children as well as a newborn. How do we keep them all safe once this toxic gas is already in the house? How do we eliminate it? No one seems to know much about ozone or how to get rid of it once it is present. Please help! We are very concerned. - L.K. [reader anonymity protected]


Sorry to read you're another victim of over-dosing a room with ozone.

The ozone itself is very volatile and will be long gone if you turned off the equipment and aired out the room.

The smell that remains is probably from oxidized materials, possibly carpeting, padding, foam cushions or something else. That outgassing odor tends to continue for a long time. The solution is usually to identify exactly what is giving off the odor and dispose of it.

See our SMELL PATCH TEST procedure linked to throughout this article.

for an inexpensive and easy way to track down the offending material.

Keep us posted.

Reader Follow-up:

Thank you so much for your response! We had contacted home inspectors, EMT, Poison Control, doctors etc and no one knew anything about ozone! I am glad we found someone who is knowledgeable in the area, as I have been very concerned.

Here are the circumstances of our situation.

The ozone generator was left on high in a small bedroom for aprox 6 hours. The door was sealed, the AC vent closed, the window opened and ceiling fan on. Upon turning off the machine, there was a terrible smell and we had to run out of the room. We got headaches instantly. We then left the ceiling fan on, door sealed, windows opened and box fan blowing out the window, for a week.

However, the room still has a sweet – like, artificial smell to it. There is no carpet in the room. Just a bed, dresser, closet full of spare pillows and blankets, and lots of stuffed animals and toys. The clothing that I retrieved from the room continue to have the same artificially sweet smell even after washing them several times. I got a headache once again from just entering for a moment even after a week of airing out, and every morning I now wake up with a sore throat.

We have small children including a newborn and I worry for their health. I keep all the bedroom windows cracked open every night.

The website was very helpful. However, after reading I am a bit confused. I have several questions.

You mentioned in your response that the ozone would be gone by now.

· Is that “sweet” smell the smell of ozone?

· If the ozone doesn’t stay around, why do I still smell it? And why do I still get a headache?

· Are the dangers of exposure only referring to the actual time when the generator is on, or are there dangers in the lingering smells from the ozone machine? Is that smell toxic as well? Why did people say they had to throw out all their belongings or leave their home, if there was no danger once the machine is off?

· How do I know the ozone level in that room? Is there a way to measure it? Is there a machine/purifier to take ozone out of the air or a way to detox the room and belongings?

· Do I need to throw out everything in the room if it continues to have this smell? Is it safe for my son to sleep in there? Safe for infant to be exposed to?

· What department does this topic fall under and why doesn’t anyone else seem to know anything about it?!


My intention suggesting the smell patch test was to try and focus on what is the source of the horrible odors ensuing from overdosing with ozone - the oxidized materials. If you can relate one or more of your smell patch tests to the odor that was bothering you in the first place you know what needs to be tossed out (as oxidized materials usually won't be much deodorized by washing or dry cleaning).

Other odor sources can usually be cleaned successfully, or cleaned then sealed.

Reader follow-up:

Is the odor caused by oxidized materials toxic as well?

At this point, the smell left behind in the pillows, blankets and clothing don't seem to bother anyone (or be noticeable to anyone) but me. So I am wondering if it would be harmful to keep my sons clothing etc once I've washed them several times and the smell is very faint - L.K.


Possibly, the odors you smell are harmful in any of a variety of ways: respiratory irritant, or even toxic. One can't say what's toxic or not with not any idea of what was oxidized.

L.K. as people vary in their sensitivity to odors and chemicals and as we're talking vague generalities here "clothing" and "faint smells" niether I nor anyone can by e-texting reliably assess risks to your family.

I would agree that if you are confident that a noxious odor remains and that you can't get rid of it by laundering or cleaning, and if by comparison with other non-ozone-exposed items made of the same material you can confirm that the odor of the offending items truly is due to the ozone treatment, then your choices are to tolerate the odor or dispose of those items.

More war stories and complaints about excessive oxzone treatment causing trouble in buildings will be found

General Use of Ozone Generators nor Ozone Shock Treatments as a "Cure" for Building Mold or Odors is Not Recommended

This topic has moved to a separate article now found at OZONE SHOCK TREATMENTS NOT RECOMMENDED

Watch out: Ozone is a highly toxic, oxidizing gas. It can be absorbed into the body via inhalation, skin or the eyes. It can also oxidize building materials. See the Ozone hazard and use warning articles listed at the end of this article.

Watch out: In-Home or "portable" ozone generators and industrial or "shock treatment" ozone generators not only fail to find and remove the source of mold or building odors, in addition ozone concentrations generated by ionic air purifiers can exceed (industrial) levels permitted by U.S. Environmental Protection Agency.

Ozone as Possible By-Product of Xenon Light Disinfection

This discussion has moved to a separate article. Please see XENON LAMP OZONE OUTPUT?


Continue reading at OZONE ODOR TREATMENT FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.





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