Ozone - FAQ
"Ozone Generators that are Sold as Air Cleaners: An Assessment of Effectiveness and Health Consequences"
(This article was produced by the EPA, Environmental Protection Agency, and can also be found on their web site.)
*For your convenience, the same information and links are listed below.
a large body of written material on ozone and the use of ozone indoors.
However, much of this material makes claims or draws conclusions without
substantiation and sound science. In developing Ozone Generators
that are Sold as Air Cleaners, the EPA reviewed a wide
assortment of this literature, including information provided by a
leading manufacturer of ozone generating devices. In keeping with EPA's
policy of insuring that the information it provides is based on sound
science, only peer reviewed, scientifically supported findings and
conclusions were relied upon in developing this document.
brands of ozone generators have EPA establishment number on their
packaging. This number helps EPA identify the specific facility
that produces the product. THE DISPLAY OF THIS NUMBER DOES NOT
IMPLY EPA ENDORSEMENT OR SUGGEST IN ANY WAY THAT EPA HAS FOUND THE
PRODUCT TO BE EITHER SAFE OR EFFECTIVE.
Please Note: EPA does not certify air cleaning devices. The
Agency does not recommend air cleaning devices or manufacturers.
If you need information on specific devices or manufacturers, one
resource you can consult is the Association of Home Appliance
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.
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.
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.
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).
EPA's Final Nonattainment Designations for 8-Hour Ozone
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.
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.
scientific evidence shows that at concentrations that do not exceed
public health standards, ozone has little potential to remove indoor air
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
is evidence to show that at concentrations that do not exceed public
health standards, ozone is not effective at removing many odor-causing
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).
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
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.
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.
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
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,
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.
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.
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
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.
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.
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.
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,
most common approaches to reducing indoor air pollution, in order of
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
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.
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.
Additional Resources section below for more
detailed information about these methods.
in its pure form or mixed with other chemicals, ozone can be harmful to
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.
studies show that ozone concentrations produced by ozone generators can
exceed health standards even when one follows manufacturer’s
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.
scientific evidence shows that, at concentrations that do not exceed
public health standards, ozone is generally ineffective in controlling
indoor air pollution.
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.
public is advised to use proven methods of controlling indoor air
methods include eliminating or controlling pollutant sources, increasing
outdoor air ventilation, and using proven methods of air cleaning.
Air Cleaning Devices: A Summary of Available Information, EPA
Document Number EPA 402-K-96-001.
Air Pollution: An Introduction for Health Professionals,
EPA Document Number EPA 402-R-94-007. American Lung Association, EPA,
CPSC, American Medical Association.
Air Quality Information Clearinghouse (IAQ INFO), PO Box 37133,
Washington D.C. 20013-7133; by phone (800) 438-4318.
Department of Health Services, Indoor Air Quality Program, 850 Marina
Bay Parkway, Suite G365/EHL,
Consumer Product Safety Commission,
Association of Home Appliance Manufacturers (AHAM) has developed an
American National Standards Institute (ANSI)-approved standard for
portable air cleaners (ANSI/AHAM Standard AC-1-1988). This standard may
be useful in estimating the effectiveness of portable air cleaners.
Under this standard, room air cleaner effectiveness is rated by a clean
air delivery rate (CADR) for each of three particle types in indoor air:
tobacco smoke, dust, and pollen.
a limited number of air cleaners have been certified under this program
at the present time. A complete listing of all current AHAM-certified
room air cleaners and their CADRs can be obtained from CADR
of Home Appliance Manufacturers (AHAM)
also provides information on air cleaners on their AHAM-certified Clean
Air Delivery Rate site at www.cadr.org
American Lung Association Fact Sheet - Air
Cleaning Devices: Types of Air Cleaning Processes
American Lung Association. 1997. Residential Air Cleaning Devices:
Types, Effectiveness, and Health Impact.
Weschler, Charles J; Shields, Helen C. 1997b. Potential Reactions
Among Indoor Pollutants. Atmospheric Environment.
To order any
of the indoor air publications, contact: Go
you can order publications directly via EPA's National Service Center
for Environmental Publications (NSCEP) (http://www.epa.gov/ncepihom/).
web site. Your publication requests can also be mailed, called or faxed
U.S. Environmental Protection Agency