Corona generators of the air purifier variety are usually measured in mg per hour. This is a measure of the molecular weight in miligrams of ozone produced by the unit in one hour. This would be a good way to measure them against each other if there was any honesty and scientific standards applied to companies that manufacture them. While one manufacturer will claim an output of 400 mg/hr it may in reality only 100 mg/hr or less. No-one is in control of this industry and manufacturers are able to get away with outrageous claims about their products.
The best products carry UL, CTL CSA or OHSHA approvals, which means they have been voluntarily subjected to and have passed some sort of test(s) to determine whether they will or will not catch fire, put out what they claim, or electricute you if used properly etc.
Paul