The Dangers of Indoor Ozone

If ozone is injected at a depth of 20 (twenty) feet in water you will get almost 100% absorption. No spa is 20 feet deep though. While mixing chambers, and injecting it into long runs of pipe help, there is STILL a fair amount of ozone that DOES NOT get absorbed into the water. Spas that have ozone injected right at the jet have VERY LITTLE absorption... almost all the "kill" happens right at the surface of the ozone bubble where it's in contact with the water. "Most" spas (not all), have a fair amount of off-gassing. It doesn't take long for the ozone to fill the air space between the bottom of the spa cover and the top of the water, then start "spilling" out of the spa.

Ozone is O³, three atoms of oxygen combined. It's very unstable and has a "half life". In hot water the half life is about 20 minutes, in air about 2 hours (temperature has an effect on the half-life time). HALF LIFE: in air, every 2 hours 1/2 of the ozone molecules break down in to one O² molecule AND 1 oxygen atom. More danger exists from this single atom than from ozone. A single oxygen atom is called a "free radical". In simple terms, free radical atoms do not exist. They HAVE to attach to, or combine with something else. Now many would like you to believe that this oxygen atom will combine with another oxygen atom and form O² (oxygen). Problem is, there aren't really many other single oxygen atoms to combine with. 80% of our atmosphere is nitrogen and is most likely what the single oxygen atom will combine with. The result is NOx, or nitroxides . As you can tell, NOx are produced as by-product almost every time when ozone is produced from air. NOx is toxic and one of the signs of toxicity is throat irritation. Secondly,  the sensitivity to NOx is very individual. It is estimated that approximately 10-15% of population is sensitive to these substances to a point that they can not tolerate even very tiny amounts.

The American Academy of Allergy, Asthma and Immunology (AAAAI)  has concerns that high ozone levels result in the detrimental affects on the respiratory system including:

• irritation of the respiratory system causing coughing and irritation in the throat and chest
• a reduction in lung function, making breathing shallow and labored
• inflammation and temporary damage to the lining of the lung

There have been experiments in which a mouse was killed in 45 minutes at about a 100 PPM concentration of ozone...on the other hand, there have been experiments conducted on turkeys in which a small flock was raised for six weeks in a controlled environment in which their food was ozonated, their water was ozonated and their air was constantly maintained at 0.28 - 0.30 ppm ozone. Their survival and health was notably superior to two control groups, the first of which had no ozone at all, and the second had their food and water ozonated but no air ozonated. The survival rate of the first group was about 60%. The second group was about 80%. The totally ozonated group was 100% with about a 10% weight gain advantage.

One other down side to ozone is it's corrosiveness to certain plastics, most notably plastics used for electrical wire insulation and printed circuit boards.

Outdoors ozone is rarely a problem since the concentrations are very small and dissipate quickly.

I have followed, with interest, a few of the recent threads that deal with the general subject of "ozone".

I guess it is appropriate for my first post on this forum to focus on ozone - a topic that I have nearly 30 years of experience with ranging from 3 million gallon per day high purity water systems (semiconductor) to drinking water systems many times that size to table-top carafes treating a liter at a time to hot tubs. Regarding hot tubs, one of my first posts on the rhtubs.com site concerned ozone - and, over the years, there have been several dozen on that site that followed that original post.

For this post, I wanted to not get into specific applications of ozone to hot tubs; what brand does or doesn't have a contactor, etc. Rather, I thought I would go through the basics of ozone - both as an oxidizer and as a sanitizer / disinfectant.

I should say that the following represents my opinion - backed by a considerable amount of experience and actual testing of ozone in hot tubs.

General:

First, ozone, like chlorine and bromine, is both an oxidizer and a sanitizer / disinfectant. Ozone functions first as an oxidant and secondarily as a sanitizer (I will use the term sanitizer vs. disinfectant; while ozone is a disinfectant, its use, in hot tubs, would, under optimum conditions, function as a sanitizer).

Ozone as an Oxidizer:

An oxidizer serves to "oxidize" or break down both organic and inorganic "contaminants" in your spa's water. If you use chlorine, some of the more common contaminants are chloramines - shocking with chlorine or MPS will effectively break down those compounds; theoretically, ozone would do the same. Other contaminants are from body waste (sweat, skin flakes, etc.), natural contaminants (pollen, insects, dust, etc.), contaminants in your water (both organic carbon as well as inorganic species such as iron, manganese, etc.). Oxidation breaks these contaminants down into simpler compounds. In the case of organics, the ultimate breakdown product is likely to be carbon dioxide or some simple form of organic acid. In the case of inorganics, it will be some oxidized form of the inorganic (i.e. a metal oxide).

If you have a tub with an ozonator installed and, if you have some "clear" tubing downstream of the Mazzei injector site, take a look at the tubing after a few weeks or months of use - most likely it will be reddish colored. That is the result of the ozone oxidizing the iron in your water from the reduced, ferrous state to the oxidized, ferric (rust) state. The fact that you see this immediately downstream of you ozonator is a visual representation of what I said above - oxidizers, in this case, ozone, functions first as an oxidizer (hence the color immediately after the ozone is introduced).

Ozone as a Sanitizer:

If the oxidative demand of the water in the tub or in the "contact chamber" is met, then, theoretically, ozone can function as a sanitizer. However, simply "injecting" ozone will not make this happen. To have ozone act as a sanitizer, you have to have ozone in the dissolved, or aqueous form, vs. the gaseous form that we all equate as "ozone" (i.e. the bubbles that you see and smell). All hot tub CD ozonators use air as the makeup gas - the ratio of air to ozone coming out of the ozonator is typically well over 95% and well less than 5% ozone. To get ozone into the water the best approach is to use a venture type of injector - such as the Mazzei (a brand) used in many hot tubs. This makes use of a pressure drop within the injector to "suck" in the air/ozone gas from the ozonator and to then diffuse it into the water flowing through the infector in an effort to transfer the gas into the liquid. Ideally, you want very, very small bubbles - so that for a given volume of gas you will have a maximum ratio of surface area of the bubbles to allow the ozone to enter the aqueous form. To do this effectively requires a long contact chamber and sufficient time for the phase transition to occur.

As indicated, once the ozone / air bubble is produced, it has a very limited time to transfer the ozone to the water. By the time you see bubbles rising in your tub, it is too late. Anyone who says that you can put a spa blanket on the water and "hold" in the ozone is simply wrong (my opinion).

If you succeed in getting ozone into the dissolved or aqueous phase, it doesn't stay there long. Ozone, in pure water, at a temp of 20'C (68'F) has a half-life of about 22 minutes (estimates vary a bit); meaning that after 22 minutes you have 50% of what you started with, after another 22 minutes you have half of that, etc. Hot tub water is not "pure water" and it is not at 68'F. The half-life of ozone in typical hot tub water is likely to be in the single minutes - perhaps even seconds (I have not seen studies on this, but comparable ones for other water types would indicate these numbers to be in the right ball park).

Heat is a big enemy of ozone half-life - so under the best of conditions (ozone generator, injector...) you are fighting an uphill battle in hot tubs. In my HotSpring Grandee (2001), the ozone is "injected" on the heater line - meaning that, if the heater is on, the ozone is being injected into water that is about 120'F - not good at all for efficient introduction of ozone. There are other problems associated with ozone - including such factors as pH and general water quality.

OK, but can ozone work as a sanitizer in a hot tub? Sure, IF you can achieve a dissolved ozone concentration and if that can be maintained for a long enough time to meet the CT (Concentration x Time) "kill" factor required for the target microbes. Ozone is one of the most powerful sanitizers (actually, in this sense, disinfectant and ozone is second only to fluorine) known and is effective, given enough CT, against essentially all microorganisms. BUT, to get a CT of above "0" you have to have both the C and the T above "0" or the product is "0" and you won't sanitize.

In the past I have challenged anyone to provide defensible results showing that dissolved ozone concentrations are present in EITHER the contact chamber of, more importantly, the main hot tub water. To do so, the method used must be an ozone-specific method. Many of the kits sold and "used" are DPD based - and will pick up any type of oxidizer present (chlorine, oxygen, etc.). There are ozone-specific methods such as indigo trisulfonate. I have tried measuring ozone myself - in my hot tub, in my contact chamber, in a few other hot tubs and have been unable to get any measurable (my detection level is 0.03 ppm) levels of dissolved ozone.

If you don't have a dissolved ozone residual - you will not get any "kill".

Why don't hot tub manufacturers make powerful enough units? I'll admit, it's been a couple of years since I've really looked into this and it is possible that someone has. But the age-old problem is that since the transfer process of the ozone from the gas (bubble) phase to the aqueous (dissolved) phase is so poor that to get any ozone in the water you would generate so much off-gas that the units would fail federal limits. I have read of some "off-gas destruct" units that would theoretically take care of some of this - but I have yet to see any manufacture of tub or ozonator publish defensible, empirically generated data on dissolved ozone concentrations in tubs. A couple of years ago, my personal communication with a very, very large tub manufacturer and the largest manufacturer of ozone generators for factory installed units (in hot tubs) confirmed that it is "unlikely" to be able to achieve and measurable ozone level in the hot tub water itself. That may have changed and I would welcome any input on that.

But, keep in mind, even if you get a residual in the contact chamber and, after satisfying the oxidative demand, you get some antimicrobial (killing) action going on, since you don't have anything close to "plug-flow" in a tub, there is no way that the residual will be able to be maintained in the main tub or that all the contents of the main tub will be exposed to the contact chamber before being "recontaminated" (keep in mind the half-life and temperature related problems).

Bottom Line:

There is much, much more I could write on ozone - but my goal was to give basic information on what ozone is, how it works, how it is applied to hot tubs and to hopefully give a basic understanding of what it would take for ozone to work as a sanitizer in hot tubs.

Ozone is widely used in the drinking water, food, semiconductor and other industries as a sanitizer / disinfectant. It is a popular option for many hot tubs and, in my opinion, functions as an oxidizer and, as a result, may slightly reduce the need for your normal oxidizer / sanitizer. I have yet to see any data to support any claim that ozone can function as a stand-alone sanitizer in hot tubs or, for that matter, that it can exert any "killing" effect on microbes in hot tubs.

Do I have it on my hot tub? Yes! Will I keep it? Yes! Is there any scientific reason for that? No - at least not based on any numbers I have been able to generate or that I have seen in the literature! But I enjoy watching the bubbles!

Effects of Ozone
When ozone is inhaled, it travels throughout the respiratory tract. Because ozone is very corrosive, it causes damage to the lungs bronchioles and alveoli  (air sacs that are important for gas exchange.) Repeated exposure to ozone can inflame lung tissues and cause respiratory infections.

Ozone exposure can aggravate existing respiratory conditions such as asthma, reduce lung function and capacity for exercise and cause chest pains and coughing. Young children and the elderly are most susceptible to the high levels of ozone encountered during the summer.

In addition to effects on humans, the corrosive nature of ozone can damage plants and trees. High levels of ozone can destroy agricultural crops and forest vegetation.

What is ozone?
Ozone is a gas that occurs both in the Earth's upper atmosphere and at ground level. Ozone can be "good" or "bad" for your health and the environment, depending on its location in the atmosphere.

How Can Ozone Be Both Good and Bad?
Ozone occurs in two layers of the atmosphere. The layer closest to the Earth's surface is the troposphere. Here, ground-level or "bad" ozone is an air pollutant that is harmful to breathe and it damages crops, trees and other vegetation. It is a main ingredient of urban smog. The troposphere generally extends to a level about 6 miles up, where it meets the second layer, the stratosphere. The stratosphere or "good" ozone layer extends upward from about 6 to 30 miles and protects life on Earth from the sun's harmful ultraviolet (UV) rays.

What is Happening to the "Good" Ozone Layer?
Ozone is produced naturally in the stratosphere. But this "good" ozone is gradually being destroyed by man-made chemicals referred to as ozone-depleting substances (ODS), including chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, methyl bromide, carbon tetrachloride, and methyl chloroform. These substances were formerly used and sometimes still are used in coolants, foaming agents, fire extinguishers, solvents, pesticides, and aerosol propellants. Once released into the air these ozone-depleting substances degrade very slowly. In fact, they can remain intact for years as they move through the troposphere until they reach the stratosphere. There they are broken down by the intensity of the sun's UV rays and release chlorine and bromine molecules, which destroy the "good" ozone. Scientists estimate that one chlorine atom can destroy 100,000 "good" ozone molecules.

Even though we have reduced or eliminated the use of many ODSs, their use in the past can still affect the protective ozone layer. Research indicates that depletion of the "good" ozone layer is being reduced worldwide. Thinning of the protective ozone layer can be observed using satellite measurements, particularly over the Polar Regions.

How Does the Depletion of "Good" Ozone Affect Human Health and the Environment?
Ozone depletion can cause increased amounts of UV radiation to reach the Earth which can lead to more cases of skin cancer, cataracts, and impaired immune systems. Overexposure to UV is believed to be contributing to the increase in melanoma, the most fatal of all skin cancers. Since 1990, the risk of developing melanoma has more than doubled.

UV can also damage sensitive crops, such as soybeans, and reduce crop yields. Some scientists suggest that marine phytoplankton, which are the base of the ocean food chain, are already under stress from UV radiation. This stress could have adverse consequences for human food supplies from the oceans.

What is Being Done About the Depletion of "Good" Ozone?
The United States, along with over 180 other countries, recognized the threats posed by ozone depletion and in 1987 adopted a treaty called the Montreal Protocol to phase out the production and use of ozone-depleting substances.

EPA has established regulations to phase out ozone-depleting chemicals in the United States. Warning labels must be placed on all products containing CFCs or similar substances and nonessential uses of ozone-depleting products are prohibited. Releases into the air of refrigerants used in car and home air conditioning units and appliances are also prohibited. Some substitutes to ozone-depleting products have been produced and others are being developed. If the United States and other countries stop producing ozone-depleting substances, natural ozone production should return the ozone layer to normal levels by about 2050.

What Causes "Bad" Ozone?
Ground-level or "bad" ozone is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC.

At ground level, ozone is a harmful pollutant. Ozone pollution is a concern during the summer months because strong sunlight and hot weather result in harmful ozone concentrations in the air we breathe. Many urban and suburban areas throughout the United States have high levels of "bad" ozone. But many rural areas of the country are also subject to high ozone levels as winds carry emissions hundreds of miles away from their original sources.

How Does "Bad" Ozone Affect Human Health and the Environment?
Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. "Bad" ozone also can reduce lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue.

Healthy people also experience difficulty breathing when exposed to ozone pollution. Because ozone forms in hot weather, anyone who spends time outdoors in the summer may be affected, particularly children, outdoor workers and people exercising. Millions of Americans live in areas where the national ozone health standards are exceeded.

Ground-level or "bad" ozone also damages vegetation and ecosystems. It leads to reduced agricultural crop and commercial forest yields, reduced growth and survivability of tree seedlings, and increased susceptibility to diseases, pests and other stresses such as harsh weather. In the United States alone, ground-level ozone is responsible for an estimated $500 million in reduced crop production each year. Ground-level ozone also damages the foliage of trees and other plants, affecting the landscape of cities, national parks and forests, and recreation areas.

What Is Being Done About "Bad" Ozone?
Under the Clean Air Act, EPA has set protective health-based standards for ozone in the air we breathe. EPA, state, and cities have instituted a variety of multi-faceted programs to meet these health-based standards. Throughout the country, additional programs are being put into place to cut NOx and VOC emissions from vehicles, industrial facilities, and electric utilities. Programs are also aimed at reducing pollution by reformulating fuels and consumer/commercial products, such as paints and chemical solvents, that contain VOC. Voluntary programs also encourage communities to adopt practices, such as carpooling, to reduce harmful emissions.

OZONE GENERATORS THAT ARE SOLD AS AIR CLEANERS:
An Assessment of Effectiveness and Health Consequences

Introduction and Purpose

Ozone generators that are sold as air cleaners intentionally produce the gas ozone. Often the vendors of ozone generators make statements and distribute material that lead the public to believe that these devices are always safe and effective in controlling indoor air pollution. For almost a century, health professionals have refuted these claims (Sawyer, et. al 1913; Salls, 1927; Boeniger, 1995; American Lung Association, 1997; Al-Ahmady, 1997). The purpose of this document is to provide accurate information regarding the use of ozone-generating devices in indoor occupied spaces. This information is based on the most credible scientific evidence currently available.

Some vendors suggest that these devices have been approved by the federal government for use in occupied spaces. To the contrary, NO agency of the federal government has approved these devices for use in occupied spaces. Because of these claims, and because ozone can cause health problems at high concentrations, several federal government agencies have worked in consultation with the U.S. Environmental Protection Agency to produce this public information document.

Table of Contents

What is ozone
How is ozone harmful
Is there such a thing as "good ozone," and "bad ozone"
Are ozone generators effective in controlling indoor air pollution
If I follow manufacturers' directions, can I be harmed
Why is it difficult to control ozone exposure with an ozone generator
Can ozone be used in unoccupied spaces
What other methods can be used to control indoor air pollution
Conclusions
Recommendation
Additional Resources
- Bibliography

WHAT IS OZONE?

Ozone is a molecule composed of three atoms of oxygen. Two atoms of oxygen form the basic oxygen molecule--the oxygen we breathe that is essential to life. The third oxygen atom can detach from the ozone molecule, and re-attach to molecules of other substances, thereby altering their chemical composition. It is this ability to react with other substances that forms the basis of manufacturers' claims.

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HOW IS OZONE HARMFUL?

The same chemical properties that allow high concentrations of ozone to react with organic material outside the body give it the ability to react with similar organic material that makes up the body, and potentially cause harmful health consequences. When inhaled, ozone can damage the lungs. Relatively low amounts can cause chest pain, coughing, shortness of breath, and, throat irritation. Ozone may also worsen chronic respiratory diseases such as asthma and compromise the ability of the body to fight respiratory infections. People vary widely in their susceptibility to ozone. Healthy people, as well as those with respiratory difficulty, can experience breathing problems when exposed to ozone. Exercise during exposure to ozone causes a greater amount of ozone to be inhaled, and increases the risk of harmful respiratory effects. Recovery from the harmful effects can occur following short-term exposure to low levels of ozone, but health effects may become more damaging and recovery less certain at higher levels or from longer exposures (US EPA, 1996a, 1996b).

Manufacturers and vendors of ozone devices often use misleading terms to describe ozone. Terms such as "energized oxygen" or "pure air" suggest that ozone is a healthy kind of oxygen. Ozone is a toxic gas with vastly different chemical and toxicological properties from oxygen. Several federal agencies have established health standards or recommendations to limit human exposure to ozone. These exposure limits are summarized in Table 1.

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IS THERE SUCH A THING AS "GOOD OZONE" AND "BAD OZONE"?

The phrase "good up high - bad nearby" has been used by the U.S. Environmental Protection Agency (EPA) to make the distinction between ozone in the upper and lower atmosphere. Ozone in the upper atmosphere--referred to as "stratospheric ozone"--helps filter out damaging ultraviolet radiation from the sun. Though ozone in the stratosphere is protective, ozone in the atmosphere - which is the air we breathe - can be harmful to the respiratory system. Harmful levels of ozone can be produced by the interaction of sunlight with certain chemicals emitted to the environment (e.g., automobile emissions and chemical emissions of industrial plants). These harmful concentrations of ozone in the atmosphere are often accompanied by high concentrations of other pollutants, including nitrogen dioxide, fine particles, and hydrocarbons. Whether pure or mixed with other chemicals, ozone can be harmful to health.

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ARE OZONE GENERATORS EFFECTIVE IN CONTROLLING INDOOR AIR POLLUTION?

Available scientific evidence shows that at concentrations that do not exceed public health standards, ozone has little potential to remove indoor air contaminants. Some manufacturers or vendors suggest that ozone will render almost every chemical contaminant harmless by producing a chemical reaction whose only by-products are carbon dioxide, oxygen and water. This is misleading.

• First, a review of scientific research shows that, for many of the chemicals commonly found in indoor environments, the reaction process with ozone may take months or years (Boeniger, 1995). For all practical purposes, ozone does not react at all with such chemicals. And contrary to specific claims by some vendors, ozone generators are not effective in removing carbon monoxide (Salls, 1927; Shaughnessy et al., 1994) or formaldehyde (Esswein and Boeniger, 1994).  
• Second, for many of the chemicals with which ozone does readily react, the reaction can form a variety of harmful or irritating by-products (Weschler et al., 1992a, 1992b, 1996; Zhang and Lioy, 1994). For example, in a laboratory experiment that mixed ozone with chemicals from new carpet, ozone reduced many of these chemicals, including those which can produce new carpet odor. However, in the process, the reaction produced a variety of aldehydes, and the total concentration of organic chemicals in the air increased rather than decreased after the introduction of ozone (Weschler, et. al., 1992b). In addition to aldehydes, ozone may also increase indoor concentrations of formic acid (Zhang and Lioy, 1994), both of which can irritate the lungs if produced in sufficient amounts. Some of the potential by-products produced by ozone's reactions with other chemicals are themselves very reactive and capable of producing irritating and corrosive by-products (Weschler and Shields, 1996, 1997a, 1997b). Given the complexity of the chemical reactions that occur, additional research is needed to more completely understand the complex interactions of indoor chemicals in the presence of ozone.  
• Third, ozone does not remove particles (e.g., dust and pollen) from the air, including the particles that cause most allergies. However, some ozone generators are manufactured with an "ion generator" or "ionizer" in the same unit. An ionizer is a device that disperses negatively (and/or positively) charged ions into the air. These ions attach to particles in the air giving them a negative (or positive) charge so that the particles may attach to nearby surfaces such as walls or furniture, or attach to one another and settle out of the air. In recent experiments, ionizers were found to be less effective in removing particles of dust, tobacco smoke, pollen or fungal spores than either high efficiency particle filters or electrostatic precipitators. (Shaughnessy et al., 1994; Pierce, et al., 1996). However, it is apparent from other experiments that the effectiveness of particle air cleaners, including electrostatic precipitators, ion generators, or pleated filters varies widely (U.S. EPA, 1995).

There is evidence to show that at concentrations that do not exceed public health standards, ozone is not effective at removing many odor-causing chemicals.

• In an experiment designed to produce formaldehyde concentrations representative of an embalming studio, where formaldehyde is the main odor producer, ozone showed no effect in reducing formaldehyde concentration (Esswein and Boeniger, 1994). Other experiments suggest that body odor may be masked by the smell of ozone but is not removed by ozone (Witheridge and Yaglou, 1939). Ozone is not considered useful for odor removal in building ventilation systems (ASHRAE, 1989).  
• While there are few scientific studies to support the claim that ozone effectively removes odors, it is plausible that some odorous chemicals will react with ozone. For example, in some experiments, ozone appeared to react readily with certain chemicals, including some chemicals that contribute to the smell of new carpet (Weschler, 1992b; Zhang and Lioy, 1994). Ozone is also believed to react with acrolein, one of the many odorous and irritating chemicals found in secondhand tobacco smoke (US EPA, 1995).

If used at concentrations that do not exceed public health standards, ozone applied to indoor air does not effectively remove viruses, bacteria, mold, or other biological pollutants.

• Some data suggest that low levels of ozone may reduce airborne concentrations and inhibit the growth of some biological organisms while ozone is present, but ozone concentrations would have to be 5 - 10 times higher than public health standards allow before the ozone could decontaminate the air sufficiently to prevent survival and regeneration of the organisms once the ozone is removed (Dyas, et al.,1983; Foarde et al., 1997).
  
• Even at high concentrations, ozone may have no effect on biological contaminants embedded in porous material such as duct lining or ceiling tiles (Foarde et al, 1997). In other words, ozone produced by ozone generators may inhibit the growth of some biological agents while it is present, but it is unlikely to fully decontaminate the air unless concentrations are high enough to be a health concern if people are present. Even with high levels of ozone, contaminants embedded in porous material may not be affected at all.

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IF I FOLLOW MANUFACTURERS' DIRECTIONS, CAN I BE HARMED?

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.

• In one study (Shaughnessy and Oatman, 1991), a large ozone generator recommended by the manufacturer for spaces "up to 3,000 square feet," was placed in a 350 square foot room and run at a high setting. The ozone in the room quickly reached concentrations that were exceptionally high--0.50 to 0.80 ppm which is 5-10 times higher than public health limits (see Table 1).  
• In an EPA study, several different devices were placed in a home environment, in various rooms, with doors alternately opened and closed, and with the central ventilation system fan alternately turned on and off. The results showed that some ozone generators, when run at a high setting with interior doors closed, would frequently produce concentrations of 0.20 - 0.30 ppm. A powerful unit set on high with the interior doors opened achieved values of 0.12 to 0.20 ppm in adjacent rooms. When units were not run on high, and interior doors were open, concentrations generally did not exceed public health standards (US EPA, 1995).
  
• The concentrations reported above were adjusted to exclude that portion of the ozone concentration brought in from the outdoors. Indoor concentrations of ozone brought in from outside are typically 0.01- 0.02 ppm, but could be as high as 0.03 - 0.05 ppm (Hayes, 1991; U.S. EPA, 1996b; Weschler et al., 1989, 1996; Zhang and Lioy; 1994). If the outdoor portion of ozone were included in the indoor concentrations reported above, the concentrations inside would have been correspondingly higher, increasing the risk of excessive ozone exposure.
  
• None of the studies reported above involved the simultaneous use of more than one device. The simultaneous use of multiple devices increases the total ozone output and therefore greatly increases the risk of excessive ozone exposure.

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WHY IS IT DIFFICULT TO CONTROL OZONE EXPOSURE WITH AN OZONE GENERATOR?

The actual concentration of ozone produced by an ozone generator depends on many factors. Concentrations will be higher if a more powerful device or more than one device is used, if a device is placed in a small space rather than a large space, if interior doors are closed rather than open and, if the room has fewer rather than more materials and furnishings that adsorb or react with ozone and, provided that outdoor concentrations of ozone are low, if there is less rather than more outdoor air ventilation.

The proximity of a person to the ozone generating device can also affect one's exposure. The concentration is highest at the point where the ozone exits from the device, and generally decreases as one moves further away.

Manufacturers and vendors advise users to size the device properly to the space or spaces in which it is used. Unfortunately, some manufacturers' recommendations about appropriate sizes for particular spaces have not been sufficiently precise to guarantee that ozone concentrations will not exceed public health limits. Further, some literature distributed by vendors suggests that users err on the side of operating a more powerful machine than would normally be appropriate for the intended space, the rationale being that the user may move in the future, or may want to use the machine in a larger space later on. Using a more powerful machine increases the risk of excessive ozone exposure.

Ozone generators typically provide a control setting by which the ozone output can be adjusted. The ozone output of these devices is usually not proportional to the control setting. That is, a setting at medium does not necessarily generate an ozone level that is halfway between the levels at low and high. The relationship between the control setting and the output varies considerably among devices, although most appear to elevate the ozone output much more than one would expect as the control setting is increased from low to high. In experiments to date, the high setting in some devices generated 10 times the level obtained at the medium setting (US EPA, 1995). Manufacturer's instructions on some devices link the control setting to room size and thus indicate what setting is appropriate for different room sizes. However, room size is only one factor affecting ozone levels in the room.

In addition to adjusting the control setting to the size of the room, users have sometimes been advised to lower the ozone setting if they can smell the ozone. Unfortunately, the ability to detect ozone by smell varies considerably from person to person, and one's ability to smell ozone rapidly deteriorates in the presence of ozone. While the smell of ozone may indicate that the concentration is too high, lack of odor does not guarantee that levels are safe.

At least one manufacturer is offering units with an ozone sensor that turns the ozone generator on and off with the intent of maintaining ozone concentrations in the space below health standards. EPA is currently evaluating the effectiveness and reliability of these sensors, and plans to conduct further research to improve society's understanding of ozone chemistry indoors. EPA will report its findings as the results of this research become available.

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CAN OZONE BE USED IN UNOCCUPIED SPACES?

Ozone has been extensively used for water purification, but ozone chemistry in water is not the same as ozone chemistry in air. High concentrations of ozone in air, when people are not present, are sometimes used to help decontaminate an unoccupied space from certain chemical or biological contaminants or odors (e.g., fire restoration). However, little is known about the chemical by-products left behind by these processes (Dunston and Spivak, 1997). While high concentrations of ozone in air may sometimes be appropriate in these circumstances, conditions should be sufficiently controlled to insure that no person or pet becomes exposed. Ozone can adversely affect indoor plants, and damage materials such as rubber, electrical wire coatings, and fabrics and art work containing susceptible dyes and pigments (U.S. EPA, 1996a).

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WHAT OTHER METHODS CAN BE USED TO CONTROL INDOOR AIR POLLUTION?

The three most common approaches to reducing indoor air pollution, in order of effectiveness, are:

• Source Control: Eliminate or control the sources of pollution;
• Ventilation: Dilute and exhaust pollutants through outdoor air ventilation, and
• Air Cleaning: Remove pollutants through proven air cleaning methods.

Of the three, the first approach -- source control -- is the most effective. This involves minimizing the use of products and materials that cause indoor pollution, employing good hygiene practices to minimize biological contaminants (including the control of humidity and moisture, and occasional cleaning and disinfection of wet or moist surfaces), and using good housekeeping practices to control particles.

The second approach -- outdoor air ventilation -- is also effective and commonly employed. Ventilation methods include installing an exhaust fan close to the source of contaminants, increasing outdoor air flows in mechanical ventilation systems, and opening windows, especially when pollutant sources are in use.

The third approach -- air cleaning -- is not generally regarded as sufficient in itself, but is sometimes used to supplement source control and ventilation. Air filters, electronic particle air cleaners and ionizers are often used to remove airborne particles, and gas adsorbing material is sometimes used to remove gaseous contaminants when source control and ventilation are inadequate.

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CONCLUSIONS

Whether in its pure form or mixed with other chemicals, ozone can be harmful to health. When inhaled, ozone can damage the lungs. Relatively low amounts of ozone can cause chest pain, coughing, shortness of breath and, throat irritation. It may also worsen chronic respiratory diseases such as asthma as well as compromise the ability of the body to fight respiratory infections.

Some studies show that ozone concentrations produced by ozone generators can exceed health standards even when one follows manufacturer's instructions. Many factors affect ozone concentrations including the amount of ozone produced by the machine(s), the size of the indoor space, the amount of material in the room with which ozone reacts, the outdoor ozone concentration, and the amount of ventilation. These factors make it difficult to control the ozone concentration in all circumstances.

Available scientific evidence shows that, at concentrations that do not exceed public health standards, ozone is generally ineffective in controlling indoor air pollution. The concentration of ozone would have to greatly exceed health standards to be effective in removing most indoor air contaminants. In the process of reacting with chemicals indoors, ozone can produce other chemicals that themselves can be irritating and corrosive.

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RECOMMENDATION

The public is advised to use proven methods of controlling indoor air pollution. These methods include eliminating or controlling pollutant sources, increasing outdoor air ventilation, and using proven methods of air cleaning.

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