Ozone kills both mold and mold spores. Ozone generators can be used on mold-infested rooms and even the recesses inside ductwork. Any moisture problems should be resolved before bothering to use ozone to kill mold though; otherwise, the ozone will not be a permanent fix.
The great benefit of using ozone is its thoroughness. You can replace a moldy wall, but if you miss a little mold on the studs behind, you can have mold growing through your drywall again in half a year. The ubiquity of air--and therefore of the ozone in the air--prevents this.
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In a Room
Step 1
Purchase or rent a high-powered ozone generator. The milligram-per-hour ozone output should be at least 2,000, and preferably 6,000. The higher the output, the better.
Step 2
Remove moisture and humidity from the room, e.g. by fixing any leaks and running a dehumidifier. Ozone kills mold best in dry environments.
Step 3
Give the ozone some help by first removing all visible mold with water and bleach, or with a boric acid-hydrogen peroxide mix. Remember, the merit of ozone is not that it kills all mold, no matter how layered, but that it reaches the invisible mold you can't see or reach that would otherwise lead to a return of the problem several months down the line.
Step 4
Set the timer on the generator for two hours. Increase this number for large rooms or larger amounts of mold. Leave the room after turning it on.
Step 5
Keep the room unoccupied for two hours longer than the timer runs.
In Ducts
Step 1
Purchase or rent a high-powered ozone generator. The milligram per hour ozone output should be at least 6,000 if you will gas each duct vent one by one, and around 28,000 if you will gas one duct vent to fill the whole duct system.
Step 2
Have the ducts professionally cleaned. Professionals use special equipment, like a spinning-head vacuum, and the cost is only around $150—small compared to the price of the ozone generator. Don't forget to cover furniture around the vents when you first turn on the air system again, as dust will fly out for a little while.
Step 3
Cover every vent in the house and any air intake of the system with cardboard. Tape the edges with duct tape to prevent ozone from leaking.
Step 4
Uncover one vent a little, enough to thread a hose from the ozone generator in. Tape around the hose to the cardboard in such a way as to keep ozone from escaping through the cardboard.
Step 5
Run the generator for a couple of hours. Keep the vents covered for a couple hours afterward. The covers can then be removed since ozone degrades quickly. If you're treating each duct individually, move the hose and generator around to hit every duct for a couple of hours each.
Warning
Because ozone is hard to produce and degrades quickly, very few ozone generators produce the concentration needed to kill off all of the mold in the ducts of a house. The ozone generating rate is therefore very important to know. Square-footage estimates on generator packaging instead of mg-per-hour rates may be covering up that the product has weak output.
Ozone generators that are strong enough to kill mold in a whole room are not meant to be used as air purifiers. The rooms must be unoccupied during the ozone generation, and up to a few hours after turning the machine off.
Ozone will not travel inside walls. In fact, if you pump it into a hole in a wall, the ozone will accumulate between studs and crossbeams, staying confined to a small compartment. Instead the proper method for applying ozone inside walls is to open them up, taking down the drywall before ozone generation.