Originally posted to the Ferret Mailing List.  Although this was written from the perspective of ozone's effects on birds, the information applies to ferrets and humans as well.

Confusion over this issue is manifest. Every time it comes up people provide links to sites "explaining" ozone. Invariably the sites are set up by the manufactures of ozone generators. The reference to a site in the last digest was one of these. Do you believe the cigarette vendors claim that smoking is SAFE?  If you make a product are you going to say "DON'T USE IT IS NOT SAFE"? These sites have a bias.

The problem is that ozone does work. It removes odors. Used in controlled situations it is safe. Used in a home environment it creates the possibility for health effects that the manufacturers do not mention. Attached is the full article I wrote for a bird list. At the bottom are citations provided by a toxicologist from Canada. The article is about parrots. Birds have very sensitive respiratory systems. The possible danger from ozone exposure is much greater for them. However, this danger is still possible for humans and ferrets. I apologize for the length but I do not want to post it piece meal.

Ozone generators are air cleaners, not air filters. An air filter draws air through a filter of some kind. The filter material allows the air to past through but catches the air borne pollutants in the filter material. Ozone air cleaners remove nothing from the environment. Bacteria are neutralized and all airborne pollutants are electrically charged so that they are attracted to the floors and walls of the room. The pollutants will continue to build up on the floors and walls till they are removed by some other means.

To understand Ozone generators you must firsts understand what Ozone is and what its effects are. Ozone is a very special kind of oxygen molecule. A molecule is a group of atoms that are electrically connected to each other. Normally oxygen exists as O2. This symbol stands for 2 atoms of oxygen bound (electrically connected) together to form a molecule. This combination of oxygen atoms is stable. In other words O2 tends to stay O2 and not change into some thing else. When we say we are breathing oxygen we are breathing O2. The air we breathe contains O2 along with many other gases. Ozone is composed of three atoms of oxygen (written as O3) bound together. Ozone is not stable. It is very reactive with other elements (O3 very much wants to become O2). Ozone is described (Encyclopedia Britannica) as an irritating, pale blue gas that is explosive and toxic, even at low concentrations. It is used as a bleaching agent for organic compounds; as a strong germicide to sterilize drinking water as well as to remove objectionable odors and flavors.

As stated above ozone is toxic. In controlled exposure studies (Encyclopedia Britannica) it reduced the ventilory capability of healthy people in concentrations as low as 0.12 parts per million. Ozone is also toxic to plants at levels of 0.01 to 0.05 parts per million with an exposure time of several hours. Ozone can also cause breathing disorders such as bronchitis. Parts per million is a way of measuring the concentration of a gas. An ozone concentration of 0.01 parts per million means that for every 100 million particles there will be 1 ozone molecule. This is not very much ozone and it is still harmful to plants.

Ozone does exist in the atmosphere. Everyone has heard discussions of the problems with the ozone layer. The ozone layer in the atmosphere exists at a very large distance from the earth. We do not breathe this ozone. The atmospheric ozone shields us from some of the Sun's radiation. Ozone is also generated when an electrical spark passes an atmosphere containing oxygen. This happens when lightening strikes the earth or an electric motor runs (Yes your laser printer generates some ozone). The key thing to remember is the concentrations that you breathe from lightning and electric motors are very low and of a short duration.

Ozone generators use an electrical mechanism to generate Ozone gas. Ozone generators will clean the air. Bacteria in the air are neutralized. Ozone reacts with most airborne particles and the particles are attracted to floor and walls. The particles are not in the air for you to breathe but they do build up on your floors and walls. People that claim that Ozone generators will reduce air borne pollutants are correct. The problem of removing the residue that builds up on the floors and walls is ignored.

The problem is the Ozone cleaners are dangerous. Ozone is dangerous to humans and plants even at very low concentrations. The study in Encyclopedia Britannica found that levels as low as 0.12 parts per million have a negative effect on human respiratory systems. None of the ozone cleaner manufactures I found quoted the ozone they generate in part per million so there is no way to compare their output to "safe" levels. Recent media articles have indicated that at least some Ozone generators do generate dangerous ozone levels. You will read testimonials from Ozone cleaner users who say that the units have changed their lives, asthma suffers can breathe again etc. These are true. Initially the Ozone cleaners would remove all of the air born pollutants making it easier for asthma suffers to breathe. However, the continued exposure ozone could affect them in a very negative way unless the ozone levels are controlled very carefully. None of the ozone cleaners I have encountered have controls calibrated in parts per million. How do you set a safe level? Remember that very low levels of Ozone have been shown to be harmful.

There is no standard for the level of ozone that is safe for parrots. We do know that avian respiratory system is much more sensitive than the human system. The corollary is that the "safe" ozone level would be much lower than the safe ozone level required for humans. How low is safe enough? This is a question that does not have an answer. The only safe thing to do is not use Ozone air cleaners. Standard HEPA air filters are a much better choice. They do not neutralize bacteria. They only remove them from the environment. You must change/clean the filters regularly. However, they do not generate poisonous gases.

OZONE CLINICAL EFFECTS

• Damage has occurred at concentrations as low as 0.5 PPM for 2 hours of exposure.
• Patchy damage of the ciliated cells of the upper airway may be seen after exposure to 0.2 to 0.5 PPM for 7 days, to 24 hours per day, in various experimental animal species.

MINIMUM LETHAL EXPOSURE

• The lowest published lethal concentration via inhalation in humans is 50 ppm for 30 minutes (RTECS, 1996).
•  Damage has occurred at concentrations as low as 0.5 ppm for 2 hours of exposure (Stephens et al, 1974)
• Type 1 pneumocytes may be killed at 0.8 ppm for 12 hours of exposure.
• Clara cell damage occurs at 0.5 to 0.8 ppm for 90 to 180 days of 8 hours per day exposure (Boorman et al, 1980).
• Patchy damage of the ciliated cells of the upper airway may be seen after exposures to 0.2 to 0.5 ppm for 7 days, 8 to 24 hours per day, in various experimental animal species (Menzel, 1984).

MAXIMUM TOLERATED EXPOSURE

• The actual concentration at which ozone exerts toxic effects in any individual depends on many factors, including morphology of the airway passage, age, sex, dietary and hormonal status, pre-existing disease states such as asthma, and capacity of cellular protective systems based on superoxide dismutase, glutathion peroxidase, and catalase (Mehlman & Borek, 1987; Clayton & Clayton, 1994).
• The extent of lung damage is generally a function of concentration of ozone rather than duration of exposure (Stephens et al, 1974b).
• Heavy exercise increases sensitivity to ozone for forced respiratory function (Goldstein et al, 1985).
• In one study, repeated exposure to 0.2 ppm ozone did not produce cumulative effects on respiratory function, exposure to 0.35 ppm produced greater decrements in FEV on day 3, but not days 4 and 5 of exposure, and effects were greater with 0.5 ppm, but were not present by the fourth day of exposure. The authors concluded that the respiratory effects of short-term exposure to ozone are respiratory effects of short-term exposure to ozone are cumulative, but that a period of adaptation follows after improvement in pulmonary function,  but also fewer subjective complaints (Folinsbee et al, 1980).
   

Effects of Ozone in Humans - Occupational Exposures (Beard, 1982):