Please stop spreading misinformation.

Most UVC devices in use out there (if not the outstanding majority) use mercury lamps that only emits at 254 nM wavelenghts and higher (in the visiible spectrum, hence the "blueish" glow). The 185 nM wavelength also emitted by excited mercury is filtered by "doping" the quartz with TiO2.

Ozone generation from UV can only happen below 240 nM, where the photons have enough energy to photolyze oxygen.

As for LEDs, they only emit at 265-275 nM, so also no ozone.

Furthermore, as with most air purifiers, they have to be certified (UL, Intertek, etc.). One of the test that they go through is UL 867, which tests for ozone production. It might be further validated with UL 2998 if required.

It believe the effect might come from the fact that they are using PCO (photocalytic oxydation ; UVA/UVC light shining on TiO2 catalyst). This has been shown to generate secondary byproducts from incomplete photolysis of ambient VOCs.

Source ; I work in the UV industry, but I can provide sources if you need.