This little plug-in ozone generator is useful but quite noisy. It uses a corona discharge in a ceramic tube with air being blown through it, to produce a modest flow of ozone into the surrounding area.
The corona generation itself is quite noticeable as a continuous high frequency hiss, but that is dwarfed by the crappy fan that makes all the noises you'd associate with a fan with worn bearings.
The circuitry inside is split into two distinct sections. A well designed 12V switchmode power supply and an NE555 timer chip driven MOSFET driving a resin-potted high voltage transformer.
The ozone generation device is quite neat. It's a short length of ceramic tube with a bit of copper tape wrapped around the outside and the tube is then mounted into a suitably sized hole in the PCB and the copper tape soldered in place. The central electrodes are stainless steel pins shaped so that they press into turned pin sockets and are then positioned against the internal sides of the tube.
The high voltage, high frequency potential causes current flow back and forth between the pins and the tubes outer electrode, but because the ceramic acts as an insulator it behaves as a capacitor with an air/ceramic insulator and the energy transfer back and forth manifests as a purple corona discharge of lots of tiny little sparks. Air is pushed through the tube by a small and very noisy fan, and as it passes through the corona discharge the oxygen (O2) gets broken into separate oxygen molecules that reform randomly as O2 and O3 (ozone). Ozone is unstable so on contact with oxidisable surfaces it liberates the spare third oxygen molecule and oxidises whatever it attaches to.
You could consider ozone as an airborne gaseous bleach.
The quality of the circuitry is actually very high. It doesn't look like a cheap Chinese product, but looks more like a small quantity hand-built product. The only thing that really lets it down is the cheap and nasty fan. But that is probably a standard size and easily changed.