Ozone has always been present in natural outdoor air. It's how nature regulates the presence of mold, viruses, bacteria and VOCs (Volatile Organic Compounds) in air, and why enclosed spaces like cupboards and unventilated attics can get quite "musty".
It has a very distinct odour that could be described as a fresh bleach-like smell, and when scientists first identified it they thought its fresh smell was invigorating and healthful. In reality, if you can smell it then there's too much to be breathing continuously.
That's not to say that a brief burst of higher level ozone might have a benefit of killing bacterial infections in lungs. But sustained high levels will cause irritation, and really high concentrations will cause cellular damage just like chlorine would cause skin damage in a swimming pool if used at a very high concentration.
Although ozone occurs outdoors as a natural part of air, it has a very short lifespan due to its active function of oxidising stuff. So nature creates it continuously with various environmental photochemical and electrostatic effects.
Those effects are not normally present in indoor areas, so unless there's a continuous flow of fresh air from outside, the level of ozone in indoor air will gradually deplete to near zero and give a competitive edge to spores, viruses, bacteria and other airborne contaminants.
There are two ways to maintain the ozone content of the air indoors. One is to have open windows or fans giving a continuous fresh supply of active outdoor air, or in situations where a continuous flow of outdoor air is undesirable (like in peak summer and winter) it's viable to create it at a similar level by electronic means. Fortunately this can be done with extremely low power.
Indoor ozone generators have a rather exciting and extreme history, from the fantastic looking units with clusters of neon tubes with capacitive corona discharge between them producing a high level of ozone, to the notorious but technically pleasing Ionic Breeze units with their corona discharge wires and fins.
(Corona discharge refers to the crown-like appearance of electrical plasma. It has NOTHING to do with the similarly named virus.)
High power ozone generators have their uses for deodorising hotel rooms, and helping reduce the smell of smoke in used vehicles and smoke damaged premises. But they are absolutely NOT suitable for use in occupied areas. They are the equivalent of a shock chlorine treatment in a swimming pool.
One of the most common trace level ozone replenishers is probably the classic needlepoint air ioniser. These units apply a high negative voltage at safe low current (microamps) to sharp needles or carbon fibre tufts, which then create an electrostatic charge in the air. That charge causes airborne dust and spores to precipitate out to any grounded surface including floors, walls and ceilings. One unexpected side effect of the ionisation of the air is a tiny plasma glow at the tips of the needles. That creates a continuous trace level of ozone that is nowhere even near the natural outdoor level, but still at a level that makes it useful indoors.
Unfortunately, many companies made ozone generators and then called them ionisers to cash in on their popularity. Many of those use the same needle system, but have an oppositely charged ring in front of the needle. This causes a very high ionic airflow and causes a much stronger plasma discharge on the needles. The glow is often very visible as a series of purple dots in these units. Some operate at a sensible level, but others are a bit fierce in their ozone output.
A recent trend - possibly initiated by the Japanese company Sharp - is to use two emitters, with one at a positive voltage and one at a negative voltage. This results in a high ionic flow that creates a very controlled low level plasma discharge.
Because of the aggressive anti-ozone militants, the manufacturers of these units tend to avoid saying the O-word and instead create heavy marketing about hydroxyl radicals (a molecule consisting of one atom of oxygen and one of hydrogen). In reality there are a wide variety of short-lived active air molecules created.
Japan LOVES it's ionic air purifiers. They're in almost every home, workplace and public building. They have effectively been doing a long term scientific test for decades.
I firmly believe that devices that produce a trickle of ozone and other active air molecules to replenish indoor air to a level of between 1 to 10 parts per billion are essential to maintain indoor air quality.
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