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This article explains the hazards associated with use of ozone gas indoors as a "mold remedy" or as an "air purifier" or as a "mold killing agent". While there are established uses of ozone in industrial applications and in certain sterilization applications, environmental testing experts and authorities such as the US EPA do not recommend use of ozone for mold treatment.
Use of ozone indoors can be hazardous and can cause oxidation or other adverse reactions with other materials and chemicals indoors.
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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".
And while the evidence is that ozone as a mold "treatment" is questionable and while sometimes ozone "mold killing or deodorizing by ozone" causes more problems than it "solves", there are indeed many other appropriate and effective uses of ozone. Indeed dissolved ozone is used in some laundry systems as a disinfectant, typically at levels of 1.5 to 3.0 ppm. .
See OZONE AIR PURIFIER WARNINGS
That said, This article describes the dangers of using ozone gas indoors or in vehicles or other enclosed spaces as a deodorant or as a mold treatment.
Other articles in this series give details about ozone gas exposure limits & standards, ozone air purifier warnings, use of ozone as a mold or mold odor treatment, how to test for ozone, ozone toxicity, and authoritative references on the use of ozone in various deodorizing, sterilization and purification applications.
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 associated with ozone gas exposure include
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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.
A "dead" (or non-viable) mold spore may not grow but it can still be a health concern. The operative proper word for mold remediation is "clean" or "remove", not "kill." In 1997, Dr. Karin K. Foarde of Research Triangle Institute, tested the ability of ozone to decontaminate fungi on building materials.
At ozone levels of 9 ppm for a 23-hour exposure, ozone was found to be ineffective.  (Notice that this is 90-times higher than permitted ozone exposure. Exposure at these "deodorizing" levels would be considered extremely toxic to humans.)
This ozone treatment procedure is not recommended by the NY City Department of Health Guidelines on the Assessment and Remediation of Fungi in Indoor Environments. Jim Holland's article on Ozone as a "mold remediation step" is available online  and is a good summary of this point. Jack E. Peterson's 1987 excellent work "Health Hazards of some Gases" also addresses ozone hazards but it may be harder to find so I have quoted from it at the end of this paper.
Deodorization and cleaning claims are questionable: The apparent deodorization at high ozone levels may be simply the effect of a general desensitization to odors in the nose of building occupants rather than actual removal of an odor source. Ozone has been used following building fires to "reduce" smoke odors but even in this application it does not remove soot.
The answer is, maybe.
If, for example, there is a persistent odor source (such as a dead animal, flood damage, mold in building wall and ceiling cavities), no amount of "air treatment" of any kind will remove the problem source. There is no substitute for the actual physical effort to find and remove the offending source. Cleaning or removing the problem source is proper and effective. Professional use of ozone, at concentrations and durations which the applicator guarantees will not damage building materials or cause other outgassing, may be helpful as one step in a cleaning procedure where mold is not involved.
Ozone has been used successfully in water treatment and in disinfection of cooling towers and possibly wastewater. However it is not a durable, reliable treatment in that O3 molecules are highly reactive and volatile and thus treated substances do not remain so.
Use of ozone may oxidize and damage materials and increase odor levels:> If ozone is no longer being generated in a building the presence of ozone will diminish quite rapidly. However, other odors may remain or may even be increased.
Because ozone is a very powerful oxidant, it may react with (oxidize) many materials found indoors, including carpets, carpet padding (especially rubber), other floor coverings, furniture, furniture cushion foam, and even surface paints and finishes. A common example is ozone-oxidized rubber carpet backing or padding. We gather from research and other studies indicate that any material that will oxidize may be expected to react with ozone, especially cross-linked organic molecules, especially rubber.
Use of ozone may produce dangerous airborne byproducts: In other words, attempts to use high levels of ozone to "clean" or "deodorize" building interiors may in fact generate a second generation of unpleasant and even dangerous outgassing which may remain, persistent indoors, after the ozone "treatment." Examples include increased levels of indoor formaldehyde, formic acid and other acid gases, toluene, or other toxic chemicals.
Use of ozone may increase sub micron particulates: Attempts to use high levels of ozone to "clean" or "deodorize" building interiors may also increase the level of extremely small sub-micron particles which themselves can be severe respiratory irritants.
Quoting and/or paraphrasing further from "Ozone-Generating Air Cleaners and Indoor Air Chemistry":
Results of some controlled studies show that concentrations of ozone considerably higher than these standards are possible even when a user follows the manufacturer’s operating instructions.
There are many brands and models of ozone generators on the market. They vary in the amount of ozone they can produce. In many circumstances, the use of an ozone generator may not result in ozone concentrations that exceed public health standards. But many factors affect the indoor concentration of ozone so that under some conditions ozone concentrations may exceed public health standards.
Watch out: improper use of ozone indoors may oxidize certain plastics, leading to dangerous formation of oxides of nitrogen gases. See Nitrogen Oxides Gas for details.
The answer is, maybe.
The same worries pertain about use of ozone inside vehicles as we discussed above concerning the use or over-use of ozone as an "odor killer" inside buildings. But the risks of overdoing ozone inside of a car or vehicle may be still greater for two reasons:
Using an ozone generator inside of the much smaller space of a vehicle can expose the vehicle's interior to higher ozone concentrations, leading to higher risk of over-dosing with ozone, and oxidized materials.
Most vehicles and campers use a greater quantity of plastics and synthetic materials than may be found in most buildings. So the risk of oxidizing plastic materials may be greater.
Please see our full article about odors in cars and other vehicles, found
Watch out: as we warned earlier, improper use of ozone indoors may oxidize certain plastics, leading to dangerous formation of oxides of nitrogen gases. See Nitrogen Oxides Gas for details.
After breathing a high dose of Ozone, is it possible to absorb it and urinate it out? As after breathing it in at my workplace, I believe I could smell it in my urine. - Anthony
Ozone can indeed become dissolved in the human bloodstream, and Velio A Bocci et als point out that while there are effective medical uses for inhaled ozone when applied in a medically supervised [oxygen] gas mixture use, ozone may be "toxic for the pulmonary system during prolonged inhalation, continuing ... when it is chronically inhaled, [ozone] is highly toxic for the pulmonary system because the enormous alveolar surface, unprotected by sufficient antioxidants, is exposed to the cumulative ozone dose, which causes a chronic inflammation." .
As for ozone being dissolved in urine, presumably removed from blood and excreted through the kidneys, the same authors point out risks of kidney damage from ozone. But we have not found an authoritative source indicating the olfactory detection of ozone in human urine and we suspect that the ozone level would have to be very high, probably dangerously so, for that to be the case. [Queries for citations for this point are pending -  - Ed.]
What do you do if you think you and your cat are suffering symptoms of ozone exposure in your home? I think it is from an air purifier I removed, but sumptoms persisting the next day. - Anon 7/31/11
Consult your doctor and
Anon, high levels of ozone could be a respiratory irritant and at quite high levels more dangerous. But ozone gas itself is so highly volatile that once the ozone source is removed, you and your cat would no longer be exposed to ozone in that home - in minutes to hours the ozone will have become depleted - gone.
Therefore if you and kitty are suffering it makes sense to see your doctor who may in turn refer you to a pulmonologist.
In my OPINION the potentially harmful effects of any indoor air quality hazard may be greater for pets like your cat than even for most humans - that's because the pet
- has smaller body mass
- has a higher respiration rate
- may be inhaling dust and irritants found closer to floor level
So kitty may merit a trip to a catpulmonologist - or at least a veternarian who has experience with animals suffering from respiratory distress
Ozone is more of a threat to current businesses who depend on ozone-less products such as chemicals or standard purification than it is to humans. If used properly, O3 is very safe. Theoretically it CAN be harmful. Fact is, there has yet to be proven incident in the past 100 years of O3's use to my knowledge. Obviously you wouldn't run it all day and share the same environment with 03 just as you wouldn't spray fragrance or a chlorine based substance in the air and hang out in it. I will agree for household air purification it can be touchy due to misuse. As a gas o3 should be exhausted form the living environment. However, For cleaning surfaces, meat, produce, toilets, hands, etc..... OZONE is perfectly safe. If contained in water, it is perfectly sensible to be used around the house, in a restaurant, food processing facility etc. And. it can be used for a prolonged period of time. - Anonymous 8/1/11
Ozone at high levels is more than theoretically harmful - take a look at the authoritative citations earlier in this article and also at the bottom of this and our other Ozone articles. Those sources are clear about the hazards and cite sources such as the U.S. EPA who cannot be considered " ...businesses who depend on ozone-less products such as chemicals ..."
Using ozone as a "magic bullet" to "cure" toxic mold problems is an incorrect approach that risks not only the consumers' money but when mis-used, creates additional problems by oxidizing certain building materials and causing worse odors than before.
We agree that there are safe and appropriate uses of ozone, properly selected and applied. It is not a mold remedy nor a magic bullet.
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I personally use an ozone generator in my marine saltwater tank i dose approximatly 35 mg per Hr. for 4 hrs. per week with a calcium reactor dosing at 20 Mg per Hr. I use large charcoal fines in a sock filter over the overflow canister off of the protein skimmer to eliminate the odor. (IT WORKS GREAT)
By the way screw the FDA they will approve any drug on the market only to take it off of the market in 5 years or less with major law suites. - Tuffguy 9/26/11
Tuffguy, in a marine tank using ozone is a completely different process than using an ozone generator to "clean" indoor air. I appreciate that we may not always be happy with the level of FDA enforcement and the difficulties faced by that agency, but nevertheless, there is an absolutely stunning abundance of authoritative research on this topic and not much room left for wild arm-waving.
Can a house that was overexposed to ozone approximately four years ago in an attempt to kill an animal odor still be subject to air quality issues today? If so are there tests that can be run regarding this? - DP 10/18/11
DP, I don't know. All of the ozone over-oxidation issues I've investigated were so bothesome to building owners/occupants that they were dealt with in weeks to a month or so by finding and removing oxidized materials that could not otherwise be salvaged, cleaned, or re-coated with a suitable sealant.
However I can report that people with whom I've consulted on this matter have asserted that once certain materials have been over-dosed with ozone sufficient to damage the material (apparently by a form of oxidation), when bad smells ensued, they did not appreciably diminish until the problem was solved.
So I agree that one could imagine that a home overdosed with an ozone generator several years ago, if that is indeed what happened, may still have odor sources if nothing was done in the mean time to address the problem.
After breathing a high dose of Ozone, is it possbile to absorb it and unrinate it out? As after breathing it in at my workplace, I believe I could smell it in my urine. - Anthony 12/19/11
Anthony, see Can we detect dissolved ozone in human urine? in this article.
And if you have a concern for personal exposure to high levels of ozone,
4 years ago my neighbor hauled raw sewage sludge. my house still reeks horrid. we vomited & dirreah for 9 months. our eyes & throat still burns. how can we find out what happening to our beautiful farm home. is there any advice u could give us to help same our home. We were forced to move off our farm. any advice. - Karen Ellerbach 2/26/2012
Karen it sounds serious enough that you should find an expert to inspect, test if needed, and diagnose the problem as well as to specify the cleaning or cure for your home.
From the brief comment you've offered I suspect that the home was not properly cleaned and that at this point professional investigation, probably some demolition and cleaning and sanitizing are needed.
You should also consult your doctor if you have not already done so.
If treating an apartment with ozone to remedy a odor, can it be a hazard to unit above or below? - Brian 1/20/13
No one can give a definitive answer to your question with what basically amounts to no data whatsoever about the method of ozone treastment, its extent, levels reached, air movement between apartments, apartment contents, exposure duration and other factors that would be involved such as cross-movement of air between the two spaces by various means including convection currents, pressurization differences, or common-shared HVAC ducting.
However it seems unlikely. Only if occupants of an adjacent area actually smelled ozone during the treatment time would the question seem likely to arise.
I am hoping you can help me with my problem. I am quite desperate because there is little information to be found on the topic, and most people I contacted have never heard of my problem. Here is what happened:
A few days ago I had an ozone shock treatment done in one room of my apartment. I was gone during the process but when I returned I noticed a very strong electrical/chemical odor. The odor is concentrated in the room I had the treatment done but some of it can be smelled in other rooms. I opened all windows for about 4 hours with little success. After a while my airways started to become irritated, and I left the home to stay with family. I spoke to the contractor who did the shock treatment, and he told me he left the machine inside the small room for 20 hours, which seems excessive to me. I also think the room was not aired out until I returned to the home, which was 24 hours after the machine was removed. I read on your website that overdosing can cause furniture to oxidize, producing harmful chemicals.
Now I have the following concerns and questions[ about the ozone treastment that was performed]
I am not too concerned about the odor itself, but rather about my health. I am currently 23 weeks pregnant and I want to make sure that my home is safe for me and my baby to live in. I am sincerely hoping you can help me because I don't know of many resources I could contact about this. - S.O.
I hate to give an answer that is easy for me and hard or costly for you, but pregnant .... I would prefer you stayed out of the environment untilthe odors and issues are removed. Check with your doctor and let us know what she or he says.
I want to add that the hazard would not be from ozone - which is long gone, but there could be toxic as well as irritating gases, possibly particles,in the environment, depending on the extent of oxidation that took place
Thank you very much for your prompt answer! I am so grateful that you take the time to answer my questions. I did see a doctor right after I left because I experienced some airway irritation. He said it was good I left but he could not answer any of my questions regarding the chemicals, neither could the poison control center. After reading the article and your advice, I have a few questions left:
Once I have found the items that have been oxidized, do they have to be replaced in all instances, or can cleaning them (wiping, vacuuming, steam cleaning) take care of the problem? Again, I want to be sure it’s 100% safe for me and my child to be around these items. If we have to move and replace some of the furniture in the room, then so be it.
If an item in the room has no smell to it (as determined by the sniff patch test), does that mean it is not contaminated with harmful chemicals or was not affected by the oxidation?
Do you recommend air quality testing?
Once again, thank you for helping me with my problem! It’s been quite stressful trying to figure out what to do or who to contact. Nobody seems to ever have heard of a case like mine.
Cleaning can sometimes help on hard surfaces but if you find that carpet padding, foam cushions, etc. are oxidized and smelling, they usually have to be replaced.
Perhaps you want to try to find what smells, remove or replace it, see what's left, before hiring an expert. The cost of bringing in someone who is actually competent is probably more than $1000 - you might spend that money on cleanup first. Beware of people who just stop by to collect a test - not diagnostic so not really helpful enough. Even if such a test indicates there's a problem you still won't know what it is.
Your answers have been very helpful! I have one more question though: If an item has no smell to it, does that mean it is free from toxic particles and safe to be around?
Your question is a bit too broad to make a promise but it's reasonable to suppose that if an object or material did not develop an odor from the ozone treatment it was probably not significantly oxidized.
Alright, this should be my last question: I washed some of my clothes that have been exposed.The initial bad smell came out, however, I noticed that these clothes now smell a bit different from the clothes that have not been exposed. It is a normal clothes / fresh laundry smell and I only noticed a difference by directly comparing them. I assume they smell different because the chemical make-up of the fabric has somewhat been changed. Now, could it be harmful to wear these clothes? Once again, thank you very much!
I am doubtful that there is any easy, credible, inexpensive answer to the question you pose. I am doubtful that we can even assert that the odor change you report is due to ozone treatment, though I imagine that is a possibility.
To perform a detailed comparative analysis on two fabric samples to study their chemical makeup and chemical modification before and after ozone treatment, then cleaning, with possible effects of cleaners, laundry soaps, etc., I think you'd need two to four FLIR spectographic analyses done at about $1200. each. To me that makes just no sense whatsoever. Worry itself has a health cost. If you are worried about these things in my OPINION it would be most economical to throw them away.
(May 8, 2014) JORGE said:
HOW TO TREAT A PERSON inhaled OZONE? AND THIS WITH CHEST PAIN AND LACK OF AIR 2 WEEKS?
See your post and our reply at inspectapedia.com/sickhouse/Ozone_Treatment_Warnings.htm
(Oct 20, 2014) B bennett said:
I am looking for part of an article that describes the "ozone created problems - can lead to more problems than before". A contractor used two large ozone generators in our house months ago and there is still something in the air that we cannot tolerate.
Suer Mr. Bennett.
These articles describe ozone-created problems such as the oxidization of building materials leading to additional odor complaints as well as health concerns from over-exposure to ozone.
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