UL 2998 is an environmental-claim validation program that certifies an air cleaner emits essentially no ozone. Ozone is a lung irritant the EPA discourages indoors, yet some 'active' air-treatment devices — ionizers, certain UV and electronic cleaners — can generate it as a byproduct. UL 2998 is the independent check that a given device does not.
The threshold is what makes it meaningful. To earn the validation, a device must keep ozone below 0.005 ppm (5 parts per billion) — the limit of reliable detection. That is ten times stricter than the 0.05 ppm (50 ppb) ceiling allowed under the older UL 867 electrostatic-air-cleaner standard and used by California's CARB. So 'meets CARB' and 'UL 2998 validated' are not the same claim; the second is far more demanding.
For a homeowner, UL 2998 is a simple gatekeeper. If a contractor proposes any active or electronic air-cleaning device — especially bipolar ionization — ask whether it carries the UL 2998 zero-ozone validation. A yes rules out the most direct safety concern; a vague answer is a reason to stick with passive approaches that can't make ozone at all: MERV 13 or HEPA filtration, fresh-air ventilation, and source control. Cleaning the air should never mean adding a pollutant to it.