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Home Energy Magazine Online November/December 1998

Ozone Out for Indoor Air Cleaners

by John Manuel


Ozone has been successfully used for decades to treat and sanitize municipal water supplies, swimming pools, and spas. HVAC professionals use ozone for duct cleaning, and the hotel industry uses it as a room sanitizer. Now ozone is being promoted as an air purifier for regular use in the home and workplace. But is ozone really safe or effective for killing airborne contaminants?
An Alpine XL 15 ozone-generating air filter, front and back.
Figure 1. The reaction pathway of ozone and alkene compounds leading to the formation of an aldehyde or ketone and an organic acid. When ozone reacts with contaminants (alkenes) in indoor air, it can form new, harmful byproducts. 
This teacher uses an ozone-generating device in her classroom, where she feels it keeps the air clean.
If asked whether ozone is good or bad for human health, one could be forgiven for not having a ready answer. Over the years, news stories have both vilified and praised this naturally occurring gas. Scientists say that ozone in the upper atmosphere is beneficial because it blocks dangerous ultraviolet radiation from the sun. They fret because the stratospheric ozone layer is disappearing, and international treaties have been signed to limit the manufacture of chlorofluorocarbons that are destroying it.

At the same time, we read that ozone in the lower atmosphere, where people can breathe it, is harmful. Sunlight can interact with automobile and other chemical emissions to produce levels of ozone that irritate the lungs, eyes, and nose, potentially causing asthma attacks and making people prone to diseases such as emphysema.

In recent years, the dual nature of ozone has come into the home with the introduction of appliances that intentionally generate ozone as a way to purify indoor air. The intensive marketing of these devices has sparked a heated debate as to their potential benefits and drawbacks, and has thoroughly confused builders and homeowners in the process. Now, after a series of studies and court battles, a host of state and federal agencies have taken a stand on ozone air purifiers. Their advice to the public: Don't use them. Ozone generators are potentially dangerous to human health, and they probably don't work as advertised.

Kills Bacteria, Harms Human Lungs Experts say ozone can be either harmful or beneficial to human health, depending upon where, how, and how much of it is present. Ozone (O3) is composed of three atoms of oxygen, one of which can reattach to molecules of other substances and thereby alter their chemical composition. Therefore, ozone can kill micro-organisms, as well as mold, mildew, fungi, and bacteria, by oxidizing them, thus changing their chemistry to a form that no longer functions.

Ozone can attack human lung tissue in the same way it attacks other organisms. The American Lung Association (ALA) says that long-term exposure to ozone can cause decreased lung function, lung tissue inflammation, and greater susceptibility to infection. The ALA and other agencies say it is essential that humans minimize their exposure to ozone.

Marketers of ozone-generating air cleaners claim that their products expose people only to safe levels of ozone. They also claim that their products are necessary in today's indoor environment because of the increased tightness (lack of rapid air changes) in today's energy-efficient buildings. This tightness increases the likelihood of exposure to foul odors, mold, mildew, and assorted indoor air pollutants. While tighter buildings need not be less healthy, indoor air quality has become a major concern of building and HVAC professionals. But is an ozone generator the best answer to the threat of indoor air pollution?

The typical unit sold for home use is a tabletop appliance that generates ozone gas by passing a spark between two electrically charged metal plates (a corona discharge unit). A fan inside the unit disperses the ozone gas into the conditioned space. The energy use of these units is minimal--equivalent to a 40W-60W lightbulb, according to one manufacturer.

Theoretically, the ozone reacts with airborne contaminants, rendering them harmless. However, researchers are increasingly question whether airborne ozone is an effective decontaminant in concentrations safe for human health. They say consumers should be able to answer three questions before they even consider purchasing an ozone-generating appliance:

  • Is it safe?
  • Is it predictable? and
  • Is it effective?
How Safe Is Safe? State and federal agencies have established strict health standards to limit human exposure to ozone. The Occupational Safety and Health Administration limits workers' exposure to an average concentration of no more than 0.10 parts per million (ppm) for eight hours. The U.S. Environmental Protection Agency (EPA) recommends exposure to outdoor air averaging no more than 0.08 ppm for eight hours. And the Food and Drug Administration (FDA) limits ozone output from indoor medical devices to no more than 0.05 ppm. Officials say that people should never allow themselves to be exposed to levels in excess of these amounts. But with many of the ozone generating units on the market today, that may be hard to control.

In October 1992, Consumer Reports (CR) published the findings of studies on two indoor air cleaners, the Alpine 150 and the Quantum Panda Plus. The magazine stated that neither unit allows users to measure ozone output or control ozone levels in a meaningful way. In testing under a variety of conditions, they found both units almost always produced ozone levels well above the FDA's limit. CR's conclusion: Ozone generators have limited value in unoccupied spaces. But we don't think they belong where people breathe.

That same year, the State of Minnesota sued Alpine Air Products for making false claims with respect to the safety of its ozone-generating appliances. In addition to ordering Alpine to refund purchasers the $400 price of the unit, the state said Alpine must make clear in all future sales presentations that the purifiers are capable of emitting levels of ozone in excess of 0.05 ppm; that neither the operating instructions nor the purifier itself ensures against excessive accumulations of ozone; and that users should avoid operating the purifiers in the presence of human beings.

Performance May Vary In April 1997, the California Department of Health Services issued a press release warning the public against using any ozone air cleaners. People should avoid using indoor air cleaning devices that use ozone, said Jim Stratton, a state health officer. These devices can quickly produce enough ozone in a confined space to exceed the California Stage 2 and 3 smog alert levels, as well as worker health and safety standards.

It is important to note that there are many different types of ozone generator, and that their performance and degree of control will vary. Further, many factors affect the indoor concentration of ozone. These include the capacity of the unit, the size of the room in which it is placed, whether interior doors are open or closed, the amount of outside ventilation air diluting the source, and the concentration of ozone in outdoor air. Manufacturers and vendors advise users to size ozone generators properly to the space or spaces in which they will be used. However, the EPA states that some manufacturers' recommendations about appropriate sizes for particular spaces are not sufficiently precise to guarantee safety.

And while ozone generators typically provide a control setting, the EPA says the ozone output of these devices may not be proportional to the control setting; that is, setting the device at medium does not necessarily guarantee that it will generate an ozone level that is halfway between the levels of ozone at the low and high settings. Some units provide an automatic cutoff switch that is designed to turn the unit off when ozone concentrations reach unsafe limits; the EPA is currently testing the efficacy of these units. According to James Raub, health specialist with the EPA, the only practical way for a homeowner or business to know what the ozone levels are in a building is to hire an environmental testing company that has the proper equipment and know-how. The known methods for testing ozone require expensive equipment and extensive training in their use.

Along with questions of safety comes the issue of predictability or quality control. Richard Shaughnessy is program manager of indoor air research at the University of Tulsa. He has done considerable research using ozone generators, and has found that performance is not necessarily predictable from one unit to the next. The question is, can we trust these units not to exceed ozone limits? Shaughnessy says. To some extent, that depends on the individual manufacturer. We found that identical units [from the same company] did not perform the same with respect to ozone production. The output of ozone was different at the same setting with identical models.

Effectiveness Not Proven Finally, there is the question of effectiveness. Marketers of ozone generators often claim that these units will rid the air of mold, mildew, bacteria, viruses, and household odors such as tobacco smoke. Research conducted over the past seven years casts serious doubt on these claims (see Table 1). In 1995, the Federal Trade Commission (FTC) took action against three marketers of ozone generators--Living Air Corporation, Alpine Industries Incorporated, and Quantum Electronics Corporation--for false advertising. The FTC alleged that the companies did not have adequate evidence to back up claims that (1) ozone generators eliminate gases, mold, mildew, bacteria and viruses, or dust from the environment; (2) ozone generators are more effective than air cleaners using filters in cleaning and purifying indoor air; (3) the generators do not create harmful by-products; and (4) the generators prevent or provide relief from allergies, asthma, and other specified conditions.

The companies agreed to discontinue making such claims without competent and reliable scientific evidence to support them. However, Alpine Industries has allegedly continued to make unsubstantiated claims about its air cleaners and is now being sued by the FTC. The FTC is seeking a court order against the firm and its president, William J. Converse, and civil penalties up to $11,000 per order violation.

Shaughnessy says most of the research his program has conducted indicates that ozone has a very poor ability to control indoor air quality in a safe manner. We've not seen any benefit in terms of biologicals, gases, or tobacco odors, he says. Some very good research shows there is very little effect on fungal or bacterial concentrations.

Indeed, two 1997 studies conducted by North Carolina's Research Triangle Institute under contract from the EPA showed that ozone concentrations far in excess of recommended limits were required for a significant kill of test organisms. The study looked at ozone's impact on one species of yeast (Rhodotorula glutinis), and three types of fungus (Penicillium chrysogenum, Penicillium glabrum, and Streptomyces) that are commonly found in indoor environments and are known to cause human health problems. In the first study, ozone concentrations of 6-10 ppm were required to achieve a significant kill of test organisms placed on glass slides. Not everyone views these findings in a negative light. Dr. Robert Olcrest, a Baltimore-based environmental toxicologist, states, RTI did find a 3-4 log reduction in the viability of the test organisms, and that, to me, is significant. We're not talking about germidical sterilization here, we're talking about reduction.

The second study involved placing Penicillium spores on actual building materials such as fiberglass duct board and ceiling tile. Here, ozone at a concentration of 9 ppm showed no effect, even after 23 hours of exposure. The researchers assume that this difference reflects the ability of porous materials such as fibrous glass duct board and ceiling tile to protect the spores deposited on their surface.

After examining a host of research done over the past six years, the EPA concludes that ozone, at levels that do not exceed public health standards, has little potential to remove indoor air contaminants. First, the EPA states that ozone generators are not effective at removing many of the most harmful chemicals--including carbon monoxide and formaldehyde--commonly found in indoor environments. Second, it states that for many of the chemicals with which ozone does readily react, ozone forms a variety of harmful or irritating by-products, including aldehydes and other organic chemicals (see Figure 1). Third, the EPA states that ozone does not remove particles (such as dust and pollen) from the air, although when used in combination with an ionizer (a negative ion generator), it may do so.

This is a significant qualification in that many air purifiers on the market include an ionizer that is designed to work in tandem with the ozone generator. Ion generators act by charging the particles in a room so that they are attracted to walls, floors, tabletops, and curtains. By causing dust and pollen to drop out of the air column, the ion generators theoretically render these materials less of a threat to human health. The EPA cites recent research that found ionizers to be less effective in removing particles of dust than either high-efficiency particle filters or electrostatic precipitators; however, it says the performance of all of these devices can vary widely.

Industry Response Home Energy contacted several manufacturers of ozone generators to determine what evidence they have to support their claims. Two manufacturers, Alpine Industries and Applied Ozone Systems, responded by phone and in writing. Both stand by their claims as to the safety and effectiveness of their products. Alpine Industries provided a four-page summary of research and testimonials by Jesse Steelman, President of Envirocon, an environmental and health consulting and analysis firm based in Fort Lauderdale, Florida, and by Dr. Robert Olcerst.

According to the summary, separate research by Steelman and Olcerst showed substantial reductions in particulates suspended in indoor air by using ozone air generators. Citing studies done with Johns Hopkins University, the summary says Olcerst found a 45% reduction in cat dander, dust mite antigens, and mold spores. The summary also asserts that Olcerst and Steelman verify that ozone is effective in reducing bacteria and other microorganisms at concentrations of less than 0.05 ppm. Finally, the summary cites the work of several physicians indicating that exposure to ozone levels well in excess of FDA guidelines causes no permanent damage to human health.

Asked why this research is not adequate to satisfy the FTC, Alpine president William Converse says, That has been the issue. We've supplied them with a large amount of material. They keep saying it's not adequate, but they don't say what exactly what would be adequate. Converse has filed suit against the FTC to allow Alpine to go forward with its advertising. The trial has not yet reached adjudication.

With respect to the scientific studies that appear to debunk ozone's effectiveness, Converse says, There are a lot of things ozone is not effective on--we don't argue that. But in combination with ionization, our devices are effective for dealing with many household problems. I challenge the research community to go out to our millions of satisfied customers and find out why the devices worked in their particular environment, rather than looking for examples of where they don't work.

Other Options Building science professionals agree that people seeking cleaner indoor air should pursue proven strategies in order of their effectiveness. First, they recommend source control, that is, eliminating or controlling the sources of pollution. Second, they recommend ventilation with outside air to dilute and/or exhaust pollutants. Finally, they recommend the use of air cleaners, but only those whose safety and effectiveness are proven by scientific research. These options include ionizers that do not produce ozone, electrostatic precipitators, and pleated air filters. Such devices may not yield the widespread health benefits claimed for ozone air generators, but they will perform a more limited function without a potential threat to human health.

John Manuel is a freelance writer based in Durham, North Carolina, and specializing in areas related to energy, the environment, and environmental health.


Alpine Industries: Executive Summary of Efficacy Research. Unpublished document available from Ackermann Public Relations and Marketing, Tel:(423)584-0550.

Boeniger, M. Use of Ozone-Generating Devices to Improve Indoor Air Quality. American Industrial Hygiene-Association Journal 56, (June 1995): 590-598.

Foarde, K.K., VanOsdell, D.W., and Steiber, R.S. Investigation of Gas-Phase Ozone as a Potential Biocide. Applied Occupational Environmental Hygiene 12, no. 8, (August 1997): 535-542.

FTC Seeks Civil Penalties against Ozone-Generator Firm over Air Cleaning Claims in Violation of Prior Order. News release issued by the Federal Trade Commission, January 5, 1998.

Indoor Air Pollution: An Introduction for Health Professionals, 402R94007. American Lung Association, U.S. EPA, CPSC, American Medical Association. Available from the National Center for Environmental Publications and Information. Web site:; Tel:(800) 438-9198.

Marketers of Ozone-Generator Air Cleaners Agree to Settle FTC Charges. News release issued by the Federal Trade Commission, June 5, 1995.

Ozone Generators That Are Sold as Air Cleaners. EPA Web site:

Ozone Generators--Two Judged Not Acceptable. Consumer Reports, October 1992, p. 661.

State Issues Warning about Ozone Air Cleaning Devices. News release issued by the California Department of Health Services, April 27, 1997.

Steelman, H. Jesse. Making Your Environment a Better Place. Iindoor air quality study conducted for Alpine Industries. Copies available from Envirocon, P.O. Box 14484, Fort Lauderdale, FL 33302.

Thar, D. Testing of Ozone-Generating Air-Purifying Devices. Applied Occupational Environmental Hygiene 13, no. 3 (March 1998): 141-143.

Table 1. Summary of Experimental Studies on the Effectiveness of Ozone to Remove Chemicals and Odors from Air
Experimental Study Results Reference
New carpet samples were placed in chambers with and without O3 and instrumentally monitored. With ozone, only alkene compounds reacted and were converted into aldehydes, organic acids, and ketones. Total volatile organic compound concentration increased. Weschler, C., A.T. Hodgson, and J.D. Wooley: Indoor Chemistry: Ozone, Volatile Organic Compounds, and Carpets. Environ. Sci. Technol. 26:2371-2377 (1992).
Sewage sludge air was treated with ozone in wet scrubbing process. Ozone produced multiple new compounds as determined by UV spectrometry. Arnold, D.L.B.: Chemical Oxidation of Odors by Ozone. Chem. Ind. Issue 22: 899-902, 16 Nov. (1974).
Formaldehyde concentration monitored in chamber, and effect of ozone-generating device was evaluated. Ozone was not found to influence disappearance of formaldehyde in air. Esswein, E.J. and M.F. Boeniger: Effect of an Ozone Generating Air Purifying Device on Reducing Concentrations of Formaldehyde in Air. Appl. Occup. Environ. Hyg. 9:139-146 (1994). 
Panel of odor judges rated intensity of odor before and after ozone was introduced into a room. Ozone was not found to decrease odors once ozone dissipated, indicating lack of chemical removal. Witheridge, W.N. and C.P. Yaglou: Ozone in Ventilation--Its Possibilities and Limitations. ASHVE Trans. 45:309-522 (1939). 
Concentration of several organic compounds in air was monitored in presence of ozone. Ozone did not affect organic compound concentration in air, although ability to smell odorous compounds decreased in the presence of ozone. Erlandsen, A. and L. Schwarz: Experimental Investigations with Ozone in Air. Z. Hyg. Infekionskrankh 67:391 (1910).
Tobacco smoke odors instrumentally monitored and panel of judges was used to determine odor after ozone was introduced into test chamber. Ozone was not found to have effectively decreased the odor from tobacco smoke after an overnight exposure. Anonymous: Household Air Cleaners. Consumer Reports 10:657-662 (1992).
Reprinted with permission from Mark Boeniger, Use of Ozone Generating Devices to Improve Indoor Air Quality, American Industrial Hygiene Association Journal 56 (June 1995) p. 590-597.


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